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133-811: A long-range acoustic device ( LRAD ), acoustic hailing device (AHD) or sound cannon is a specialized loudspeaker that produces sound at high power for communicating at a distance. It has been used as a method of crowd control , which has caused permanent hearing damage, having an extremely high decibel capacity (up to 160 dB measured at one meter from the device). Other uses have included for negotiations in siege situations; to deal with piracy at sea; for mass notification during natural disasters or other emergencies; and by defense forces, including several navies . Acoustic hailing devices are acoustic devices capable of outputting intelligible sound at very high volumes. The distance at which acoustic hailing can be effective varies based on several factors including

266-422: A diaphragm or speaker cone (as it is usually conically shaped for sturdiness) in contact with air, thus creating sound waves . In addition to dynamic speakers, several other technologies are possible for creating sound from an electrical signal, a few of which are in commercial use. In order for a speaker to efficiently produce sound, especially at lower frequencies, the speaker driver must be baffled so that

399-466: A loudspeaker , the LRAD has an "alert" feature, which emits loud chirping or beeping sounds, which can be used at the top of the decibel range of the device. These sounds have been reported to cause pain and hearing damage. The use of the alert function for crowd control has been described as a "sound cannon" or sonic weapon , although this has been denied by the manufacturer. The sound can be targeted within

532-465: A speaker system ) is a combination of one or more speaker drivers , an enclosure , and electrical connections (possibly including a crossover network ). The speaker driver is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound . The driver can be viewed as a linear motor attached to a diaphragm which couples that motor's movement to motion of air, that is, sound. An audio signal, typically from

665-438: A transmission medium causes a deviation (sound pressure, a dynamic pressure) in the local ambient pressure, a static pressure. Sound pressure, denoted p , is defined by p total = p stat + p , {\displaystyle p_{\text{total}}=p_{\text{stat}}+p,} where In a sound wave, the complementary variable to sound pressure is the particle velocity . Together, they determine

798-435: A whizzer : a small, light cone attached to the joint between the voice coil and the primary cone. The whizzer cone extends the high-frequency response of the driver and broadens its high-frequency directivity, which would otherwise be greatly narrowed due to the outer diameter cone material failing to keep up with the central voice coil at higher frequencies. The main cone in a whizzer design is manufactured so as to flex more in

931-413: A 30–60° audio beam. This shaping is accomplished through the design of the transducers as well as various reflective horns. The focus of an AHD is typically measured at the frequency of peak directionality. This is typically in the 1–2 kHz range. Not all frequencies of sound are able to be directed equally. Lower frequencies in the bass range are difficult to form. As such, their directionality may be

1064-538: A 40 degree radius or more depending on the design of the AHD. LRAD devices are made by Genasys. As of 2022, the company claims its LRAD products meet the following specifications: LRAD devices include hand-held devices, as well as those mounted on helicopters , motor vehicles and ships. The parameter "ka", which is the wave number multiplied by the speaker radius, is often used to characterize sound source directivity. For this source, ka=19 at 2.5 kHz, and according to

1197-404: A GPD captain, said that it could be used for alerts for everything from riots to missing children to weather disasters, and that it could save lives in some cases. The Charlotte-Mecklenburg, North Carolina Police Department Crisis Intervention Team and SWAT negotiators used a long-range acoustic device to talk a man down from an energized electrical tower. The Phoenix, Arizona city council approved

1330-402: A combination of one or more resistors , inductors and capacitors . These components are combined to form a filter network and are most often placed between the full frequency-range power amplifier and the loudspeaker drivers to divide the amplifier's signal into the necessary frequency bands before being delivered to the individual drivers. Passive crossover circuits need no external power beyond

1463-808: A complete loudspeaker system to provide performance beyond that constraint. The three most commonly used sound radiation systems are the cone, dome and horn-type drivers. A full- or wide-range driver is a speaker driver designed to be used alone to reproduce an audio channel without the help of other drivers and therefore must cover the audio frequency range required by the application. These drivers are small, typically 3 to 8 inches (7.6 to 20.3 cm) in diameter to permit reasonable high-frequency response, and carefully designed to give low-distortion output at low frequencies, though with reduced maximum output level. Full-range drivers are found, for instance, in public address systems, in televisions, small radios, intercoms, and some computer speakers . In hi-fi speaker systems,

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1596-426: A crossover knob and a phase switch). These variants are known as active or powered subwoofers. In contrast, passive subwoofers require external amplification. In typical installations, subwoofers are physically separated from the rest of the speaker cabinets. Because of propagation delay and positioning, their output may be out of phase with the rest of the sound. Consequently, a subwoofer's power amp often has

1729-426: A driver; each implementation has advantages and disadvantages. Polyester foam, for example, is lightweight and economical, though usually leaks air to some degree and is degraded by time, exposure to ozone, UV light, humidity and elevated temperatures, limiting useful life before failure. The wire in a voice coil is usually made of copper , though aluminum —and, rarely, silver —may be used. The advantage of aluminum

1862-421: A dual role, acting also as a choke coil , filtering the power supply of the amplifier that the loudspeaker was connected to. AC ripple in the current was attenuated by the action of passing through the choke coil. However, AC line frequencies tended to modulate the audio signal going to the voice coil and added to the audible hum. In 1930 Jensen introduced the first commercial fixed-magnet loudspeaker; however,

1995-406: A dynamic loudspeaker, uses a lightweight diaphragm , or cone , connected to a rigid basket , or frame , via a flexible suspension, commonly called a spider , that constrains a voice coil to move axially through a cylindrical magnetic gap. A protective dust cap glued in the cone's center prevents dust, most importantly ferromagnetic debris, from entering the gap. When an electrical signal

2128-463: A high-frequency horn that sent sound through a hole in the pole piece of a 15-inch woofer for near-point-source performance. Altec's "Voice of the Theatre" loudspeaker system was first sold in 1945, offering better coherence and clarity at the high output levels necessary in movie theaters. The Academy of Motion Picture Arts and Sciences immediately began testing its sonic characteristics; they made it

2261-405: A larger magnet for equivalent performance. Electromagnets were often used in musical instrument amplifiers cabinets well into the 1950s; there were economic savings in those using tube amplifiers as the field coil could, and usually did, do double duty as a power supply choke. Very few manufacturers still produce electrodynamic loudspeakers with electrically powered field coils , as was common in

2394-463: A looped warning message about staying off the streets and not blocking the sidewalks. Rochester NY police used verbal warnings and an LRAD to help disperse disorderly crowds during the August 2019 Puerto Rican Festival. Rochester, NY police used a Long Range Acoustic Device to make announcements to protesters that they were unlawfully in the roadway and obstructing traffic on September 12, 2020. The device

2527-429: A microphone, recording, or radio broadcast, is amplified electronically to a power level capable of driving that motor in order to reproduce the sound corresponding to the original unamplified electronic signal. This is thus the opposite function to the microphone ; indeed the dynamic speaker driver, by far the most common type, is a linear motor in the same basic configuration as the dynamic microphone which uses such

2660-449: A motor in reverse, as a generator . The dynamic speaker was invented in 1925 by Edward W. Kellogg and Chester W. Rice . When the electrical current from an audio signal passes through its voice coil —a coil of wire capable of moving axially in a cylindrical gap containing a concentrated magnetic field produced by a permanent magnet —the coil is forced to move rapidly back and forth due to Faraday's law of induction ; this attaches to

2793-530: A narrow and specific range that does not impact those operating the device or standing next to it. Civil liberties organizations are concerned about its use by police forces, and its use has been contested in law courts in the United States. One concern is that police officers are not receiving enough training in its use. Law enforcement organizations and the manufacturer claim that LRAD systems are primarily designed for long-range communications; however,

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2926-419: A phase-delay adjustment which may be used improve performance of the system as a whole. Subwoofers are widely used in large concert and mid-sized venue sound reinforcement systems. Subwoofer cabinets are often built with a bass reflex port, a design feature which if properly engineered improves bass performance and increases efficiency. A woofer is a driver that reproduces low frequencies. The driver works with

3059-641: A public address system that allows you to reach thousands of people." Municipal police in Gdańsk, Poznań, Kołobrzeg and other cities in Poland used LRAD systems to deliver COVID-19 notifications. LRAD is used by the Singapore Changi Airport to disperse birds and wildlife from runways. Local police in Leganés, Spain used a vehicle-mounted LRAD to deliver alerts and notifications to residents during

3192-491: A range of settings, including civil disturbances and protests. Acoustic hailing devices differ from conventional speaker systems in three key ways: volume, clarity, and directionality. AHD manufacturers use different methods to measure their products, but a common standard has emerged. Since sound attenuates with distance, extremely high outputs are required to achieve the required range. Acoustic hailing devices have an output of 135 decibels (dB) or greater. The acoustic level of

3325-460: A reference level of 1 μPa is used. These references are defined in ANSI S1.1-2013 . The main instrument for measuring sound levels in the environment is the sound level meter . Most sound level meters provide readings in A, C, and Z-weighted decibels and must meet international standards such as IEC 61672-2013 . The lower limit of audibility is defined as SPL of 0 dB , but the upper limit

3458-414: A separate box, necessary to accommodate the size of the components used. Passive crossovers may be simple for low-order filtering, or complex to allow steep slopes such as 18 or 24 dB per octave. Passive crossovers can also be designed to compensate for undesired characteristics of driver, horn, or enclosure resonances, and can be tricky to implement, due to component interaction. Passive crossovers, like

3591-483: A single driver enclosed in a suitable enclosure. Since sound in this frequency range can easily bend around corners by diffraction , the speaker aperture does not have to face the audience, and subwoofers can be mounted in the bottom of the enclosure, facing the floor. This is eased by the limitations of human hearing at low frequencies; Such sounds cannot be located in space, due to their large wavelengths compared to higher frequencies which produce differential effects in

3724-408: A single piece, called the poleplate or yoke. The size and type of magnet and details of the magnetic circuit differ, depending on design goals. For instance, the shape of the pole piece affects the magnetic interaction between the voice coil and the magnetic field, and is sometimes used to modify a driver's behavior. A shorting ring , or Faraday loop , may be included as a thin copper cap fitted over

3857-464: A sound pressure level of 120 dB before feeling pain. AHDs are capable of 135 dB or more of acoustic energy. OSHA states that any sound pressure level over 90 dB requires hearing protection. As volume increases so does the chance of hearing loss. The effective non-lethal range of an AHD depends on the total acoustic output of the unit. Typically, this range is 50 metres (160 ft) or less. In addition to its "voice" feature, which acts as

3990-411: A system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he ultimately settled for the familiar metal horn driven by a membrane attached to the stylus. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air; he then sold the rights to Charles Parsons , who was issued several additional British patents before 1910. A few companies, including

4123-623: A type of acoustic hailing device , in 2003. Using this new technology, it became possible for naval personnel to contact approaching vessels which did not respond to radio calls from a distance of over 3,000 metres (9,800 ft), enabling them to respond appropriately in a timely manner and avert danger. Since then the technology has developed and expanded. Genasys launched its voice-based mass notification systems in 2012, and in 2019 its unified multichannel Critical Communications and Enterprise Safety system. LRAD devices have become widely used for communications, and increasingly for crowd control in

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4256-453: A way to utilize the LRADs purchased to their full potential sparked an investigation suspecting corruption behind their acquisition. National Police Headquarters spokesman Mariusz Sokolowski defended the purchase of LRAD. He also stressed that the police decided to make this investment because, "We needed good sound reinforcement equipment. With numerous demonstrations and gatherings, police need

4389-445: A wide range of frequencies with even coverage, most loudspeaker systems employ more than one driver, particularly for higher sound pressure level (SPL) or maximum accuracy. Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers (for very low frequencies); woofers (low frequencies); mid-range speakers (middle frequencies); tweeters (high frequencies); and sometimes supertweeters , for

4522-873: Is which is often considered as the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). The proper notations for sound pressure level using this reference are L p /(20 μPa) or L p (re 20 μPa) , but the suffix notations dB SPL , dB(SPL) , dBSPL, or dB SPL are very common, even if they are not accepted by the SI. Most sound-level measurements will be made relative to this reference, meaning 1 Pa will equal an SPL of 20 log 10 ⁡ ( 1 2 × 10 − 5 )   dB ≈ 94   dB {\displaystyle 20\log _{10}\left({\frac {1}{2\times 10^{-5}}}\right)~{\text{dB}}\approx 94~{\text{dB}}} . In other media, such as underwater ,

4655-478: Is a low priority. A subwoofer is a woofer driver used only for the lowest-pitched part of the audio spectrum: typically below 200 Hz for consumer systems, below 100 Hz for professional live sound, and below 80 Hz in THX -approved systems. Because the intended range of frequencies is limited, subwoofer system design is usually simpler in many respects than for conventional loudspeakers, often consisting of

4788-985: Is a small amount of passive electronics called a crossover network which helps direct components of the electronic signal to the speaker drivers best capable of reproducing those frequencies. In a so-called powered speaker system, the power amplifier actually feeding the speaker drivers is built into the enclosure itself; these have become more and more common especially as computer speakers. Smaller speakers are found in devices such as radios , televisions , portable audio players , personal computers ( computer speakers ), headphones , and earphones . Larger, louder speaker systems are used for home hi-fi systems ( stereos ), electronic musical instruments , sound reinforcement in theaters and concert halls, and in public address systems . The term loudspeaker may refer to individual transducers (also known as drivers ) or to complete speaker systems consisting of an enclosure and one or more drivers. To adequately and accurately reproduce

4921-436: Is an inverse-proportional law . If the sound pressure p 1 is measured at a distance r 1 from the centre of the sphere, the sound pressure p 2 at another position r 2 can be calculated: p 2 = r 1 r 2 p 1 . {\displaystyle p_{2}={\frac {r_{1}}{r_{2}}}\,p_{1}.} The inverse-proportional law for sound pressure comes from

5054-416: Is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it a variable electromagnet. The coil and the driver's magnetic system interact in a manner similar to a solenoid , generating a mechanical force that moves the coil (and thus, the attached cone). Application of alternating current moves the cone back and forth, accelerating and reproducing sound under

5187-703: Is defined by z ( s ) = p ^ ( s ) v ^ ( s ) , {\displaystyle z(s)={\frac {{\hat {p}}(s)}{{\hat {v}}(s)}},} where The particle displacement of a progressive sine wave is given by δ ( r , t ) = δ m cos ⁡ ( k ⋅ r − ω t + φ δ , 0 ) , {\displaystyle \delta (\mathbf {r} ,t)=\delta _{\text{m}}\cos(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{\delta ,0}),} where It follows that

5320-551: Is designed to be rigid, preventing deformation that could change critical alignments with the magnet gap, perhaps allowing the voice coil to rub against the magnet around the gap. Chassis are typically cast from aluminum alloy, in heavier magnet-structure speakers; or stamped from thin sheet steel in lighter-structure drivers. Other materials such as molded plastic and damped plastic compound baskets are becoming common, especially for inexpensive, low-mass drivers. A metallic chassis can play an important role in conducting heat away from

5453-485: Is intelligible at distance. Clarity is difficult to measure, since it is a subjective reference. However, different scales have been created to compare devices. A common measurement is the Speech Transmission Index (STI). STI ratings range from 0–1.0, with 1.0 being perfect clarity. AHDs are lastly characterized by directionality. To ensure messages are broadcast to the target, AHDs shape sound into

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5586-404: Is its light weight, which reduces the moving mass compared to copper. This raises the resonant frequency of the speaker and increases its efficiency. A disadvantage of aluminum is that it is not easily soldered, and so connections must be robustly crimped together and sealed. Voice-coil wire cross sections can be circular, rectangular, or hexagonal, giving varying amounts of wire volume coverage in

5719-471: Is louder than standing behind a jet engine taking off, or a nearby gunshot . Following the impetus for its initial development, 25 navies have adopted the use of LRADs as of 2022, to provide protection for their vessels. Navy personnel can contact approaching vessels from over 3,000 m (9,800 ft) and respond defensively if necessary. They are also used by coast guards , naval bases , commercial vessels, and in ports. Genasys offers its products for

5852-427: Is measured at a distance r 1 , the sound level L p 2 at the distance r 2 is L p 2 = L p 1 + 20 log 10 ⁡ ( r 1 r 2 )   dB . {\displaystyle L_{p_{2}}=L_{p_{1}}+20\log _{10}\left({\frac {r_{1}}{r_{2}}}\right)~{\text{dB}}.} The formula for

5985-479: Is not as clearly defined. While 1 atm ( 194 dB peak or 191 dB SPL ) is the largest pressure variation an undistorted sound wave can have in Earth's atmosphere (i. e., if the thermodynamic properties of the air are disregarded; in reality, the sound waves become progressively non-linear starting over 150 dB), larger sound waves can be present in other atmospheres or other media, such as underwater or through

6118-405: Is their clarity. Their horns and cones create sound that is distorted or out of phase. This results in the common " Charlie Brown " effect, where the message is muffled and misunderstood, analogous to the muted brass squawks representing the unintelligible voices of adults in animated Peanuts specials. AHDs create sound that is in phase. Because of this, sound emitted from acoustic hailing devices

6251-484: Is well damped to reduce vibrations continuing after the signal has stopped with little or no audible ringing due to its resonance frequency as determined by its usage. In practice, all three of these criteria cannot be met simultaneously using existing materials; thus, driver design involves trade-offs . For example, paper is light and typically well-damped, but is not stiff; metal may be stiff and light, but it usually has poor damping; plastic can be light, but typically,

6384-693: The 2012 Summer Olympic Games . It was spotted fixed to a landing craft on the River Thames . An LRAD was present, but not used, during protests of the 2004 Republican National Convention in New York City . LRAD was present and used when the New York City Police department raided and destroyed the Occupy Wall Street encampment from Zuccotti Park to disrupt and disperse the crowd before arresting or ejecting them on

6517-796: The Active Denial System ; one activist claimed that " supersonic weapons" had been used. On June 26, 2015, Czech Special Forces Police may have deployed an LRAD 500X during anti-immigration and anti-Islam protests in Brno , the second largest city in the Czech Republic . Police in Prague used a vehicle-mounted LRAD on November 17, 2020, to repeatedly alert protesters on Wenceslas Square to follow COVID restrictions and wear masks. On that day, police supervised several rallies, meetings and demonstrations. Thousands of participants gathered at

6650-750: The Hellenic Armed Forces . Japan's Coast Guard used Long Range Acoustic Devices and other methods to expel a North Korean fishing flotilla from its waters in September 2017. In February 2009, the Japanese whaling fleet operating in Antarctic waters near Australia installed LRADs on their vessels. The device was used against activists of the Sea Shepherd Conservation Society . The Japanese fleet later escalated

6783-526: The Victor Talking Machine Company and Pathé , produced record players using compressed-air loudspeakers. Compressed-air designs are significantly limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the design were used for public address applications, and more recently, other variations have been used to test space-equipment resistance to the very loud sound and vibration levels that

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6916-506: The audible frequency range. The smaller drivers capable of reproducing the highest audio frequencies are called tweeters , those for middle frequencies are called mid-range drivers and those for low frequencies are called woofers . Sometimes the reproduction of the very lowest frequencies (20–~50  Hz ) is augmented by a so-called subwoofer often in its own (large) enclosure. In a two-way or three-way speaker system (one with drivers covering two or three different frequency ranges) there

7049-639: The film house industry standard in 1955. In 1954, Edgar Villchur developed the acoustic suspension principle of loudspeaker design. This allowed for better bass response than previously obtainable from drivers mounted in larger cabinets. He and his partner Henry Kloss formed the Acoustic Research company to manufacture and market speaker systems using this principle. Subsequently, continuous developments in enclosure design and materials led to significant audible improvements. The most notable improvements to date in modern dynamic drivers, and

7182-418: The mid frequencies (between the woofer and tweeter). Mid-range driver diaphragms can be made of paper or composite materials and can be direct radiation drivers (rather like smaller woofers) or they can be compression drivers (rather like some tweeter designs). If the mid-range driver is a direct radiator, it can be mounted on the front baffle of a loudspeaker enclosure, or, if a compression driver, mounted at

7315-413: The nearfield which produces peaks 6 dB higher than the output pressure and nulls where the pressure is essentially zero. The larger the speaker, and the higher the frequency, the longer the effective nearfield. The nearfield for this device is approximately 8 metres (26 ft). An LRAD can thus emit a targeted "beam" of sound at very high volume, up to 160 dB at one meter from the device. This

7448-528: The spider , which connects the diaphragm or voice coil to the lower frame and provides the majority of the restoring force, and the surround , which helps center the coil/cone assembly and allows free pistonic motion aligned with the magnetic gap. The spider is usually made of a corrugated fabric disk, impregnated with a stiffening resin. The name comes from the shape of early suspensions, which were two concentric rings of Bakelite material, joined by six or eight curved legs . Variations of this topology included

7581-662: The suicide attack on the USS ; Cole while it was at a port in Yemen in 2000. Following this attack, the United States Navy established a requirement for an acoustic hailing device. The intent of this AHD was to provide the Navy with a means to establish the intent of an approaching vessel at a distance such that defensive measures could be taken should the vessel not heed a warning. One unique aspect of this requirement

7714-667: The 2015 settlement requirement. In September 2020, it was revealed that federal officials had explored the use of the device and the Active Denial System ("heat ray") to disperse civilians protesting outside the White House in June of that year , but had been advised that the National Guard was not currently in possession of either device. LRAD was reportedly used by the Oakland Police Department during

7847-568: The Earth. Ears detect changes in sound pressure. Human hearing does not have a flat spectral sensitivity ( frequency response ) relative to frequency versus amplitude . Humans do not perceive low- and high-frequency sounds as well as they perceive sounds between 3,000 and 4,000 Hz, as shown in the equal-loudness contour . Because the frequency response of human hearing changes with amplitude, three weightings have been established for measuring sound pressure: A, B and C. In order to distinguish

7980-405: The LRAD data sheet, the beam angle of about 30 degrees total is what is predicted for a regular loudspeaker. Small spherical "point-source" acoustic devices follow the known inverse square law , which predicts the loss of 6 decibels (dB) per doubling of distance from the source, solely due to geometric spreading . Large speakers (or large arrays), such as these, have an interference pattern in

8113-1175: The Laplace transforms of v and p with respect to time yields v ^ ( r , s ) = v m s cos ⁡ φ v , 0 − ω sin ⁡ φ v , 0 s 2 + ω 2 , {\displaystyle {\hat {v}}(\mathbf {r} ,s)=v_{\text{m}}{\frac {s\cos \varphi _{v,0}-\omega \sin \varphi _{v,0}}{s^{2}+\omega ^{2}}},} p ^ ( r , s ) = p m s cos ⁡ φ p , 0 − ω sin ⁡ φ p , 0 s 2 + ω 2 . {\displaystyle {\hat {p}}(\mathbf {r} ,s)=p_{\text{m}}{\frac {s\cos \varphi _{p,0}-\omega \sin \varphi _{p,0}}{s^{2}+\omega ^{2}}}.} Since φ v , 0 = φ p , 0 {\displaystyle \varphi _{v,0}=\varphi _{p,0}} ,

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8246-792: The Princess Anne Police Department deployed its LRAD system at the request of the Maryland State Police to disperse an unruly concert crowd on the campus of the University of Maryland Eastern Shore. The Columbus, Ohio Police Department (CPD) demonstrated a Long Range Acoustic Device to the local media on November 21, 2016. CPD expects to use the device for crowd control, barricaded suspect operations, and to communicate to residents during emergencies and natural disasters. The Cuyahoga County Sheriff's Department purchased Long Range Acoustic Devices to improve

8379-411: The addition of a felt disc to provide a barrier to particles that might otherwise cause the voice coil to rub. The cone surround can be rubber or polyester foam , treated paper or a ring of corrugated, resin-coated fabric; it is attached to both the outer cone circumference and to the upper frame. These diverse surround materials, their shape and treatment can dramatically affect the acoustic output of

8512-538: The amplitude of the particle displacement is related to that of the acoustic velocity and the sound pressure by δ m = v m ω , {\displaystyle \delta _{\text{m}}={\frac {v_{\text{m}}}{\omega }},} δ m = p m ω z m ( r , s ) . {\displaystyle \delta _{\text{m}}={\frac {p_{\text{m}}}{\omega z_{\text{m}}(\mathbf {r} ,s)}}.} When measuring

8645-704: The amplitude of the specific acoustic impedance is given by z m ( r , s ) = | z ( r , s ) | = | p ^ ( r , s ) v ^ ( r , s ) | = p m v m = ρ c 2 k x ω . {\displaystyle z_{\text{m}}(\mathbf {r} ,s)=|z(\mathbf {r} ,s)|=\left|{\frac {{\hat {p}}(\mathbf {r} ,s)}{{\hat {v}}(\mathbf {r} ,s)}}\right|={\frac {p_{\text{m}}}{v_{\text{m}}}}={\frac {\rho c^{2}k_{x}}{\omega }}.} Consequently,

8778-542: The appropriate driver. A loudspeaker system with n separate frequency bands is described as n-way speakers : a two-way system will have a woofer and a tweeter; a three-way system employs a woofer, a mid-range, and a tweeter. Loudspeaker drivers of the type pictured are termed dynamic (short for electrodynamic) to distinguish them from other sorts including moving iron speakers , and speakers using piezoelectric or electrostatic systems. Johann Philipp Reis installed an electric loudspeaker in his telephone in 1861; it

8911-495: The attack. The ship was badly damaged, with 17 U.S. Navy sailors killed and 37 injured. Following this attack, navies around the world made several policy changes, while the American Technology Corporation (which was rebranded to LRAD Corporation in 2010 and to Genasys in late October 2019) created and developed the AHD market, which included the launch of its proprietary Long Range Acoustic Device,

9044-419: The attention of the wider public when it was used at the 2022 anti-vaccine mandate Canberra protests in "loud hailer" mode for voice messages to be announced to the protesters, and there was speculation among protesters regarding purported harms from its use. However, there was no evidence that it was used as a siren. In addition, some reports appeared to confuse the LRAD with other crowd control devices, such as

9177-416: The audio signal itself, but have some disadvantages: they may require larger inductors and capacitors due to power handling requirements. Unlike active crossovers which include a built-in amplifier, passive crossovers have an inherent attenuation within the passband , typically leading to a reduction in damping factor before the voice coil. An active crossover is an electronic filter circuit that divides

9310-449: The characteristics of the speaker enclosure to produce suitable low frequencies. Some loudspeaker systems use a woofer for the lowest frequencies, sometimes well enough that a subwoofer is not needed. Additionally, some loudspeakers use the woofer to handle middle frequencies, eliminating the mid-range driver. A mid-range speaker is a loudspeaker driver that reproduces a band of frequencies generally between 1–6 kHz, otherwise known as

9443-687: The clearance of the Occupy Oakland encampment on the morning of 25 October 2011. Police in San Diego, California, used an LRAD on May 27, 2016, to order anti-Trump protesters to disperse. The Mendocino County (California) Board of Supervisors approved the purchase of a Long Range Acoustic Device for the Mendocino County Sheriff's Office on April 18, 2017. Sheriff Tom Allman said the device would aid in searches for missing persons, most often hunters and mushroom pickers, which cost

9576-486: The control of the applied electrical signal coming from the amplifier. The following is a description of the individual components of this type of loudspeaker. The diaphragm is usually manufactured with a cone- or dome-shaped profile. A variety of different materials may be used, but the most common are paper, plastic, and metal. The ideal material is rigid, to prevent uncontrolled cone motions, has low mass to minimize starting force requirements and energy storage issues and

9709-637: The country-wide COVID-19 lockdown. According to Citizen Security, Communication and local government spokesman, Oscar Oliveira, "Because the acoustic device can be heard inside of houses, we can inform older residents with mobility issues that all municipal resources are available through the Citizen Attention Service by calling 010." It was confirmed by the Ministry of Defence on May 11, 2012, that an LRAD would be deployed in London during

9842-536: The county tens of thousands of dollars. He said the LRAD might have made the intense 2011 manhunt for Aaron Bassler a little easier, and that it could be used to warn of a tsunami. MCSO found LRAD useful out of a low-flying plane, and it could help in a barricaded-person situation to communicate from the outside of the building. The Greensboro, North Carolina Police Department (GPD) purchased an LRAD 300X and demonstrated it for reporters in November 2015. Jonathan Franks,

9975-413: The crossover is an assembly of filters that separate the input signal into different frequency bands according to the requirements of each driver. Hence the drivers receive power only in the sound frequency range they were designed for, thereby reducing distortion in the drivers and interference between them. Crossovers can be passive or active . A passive crossover is an electronic circuit that uses

10108-512: The department's ability to issue dispersal orders clearly over long distances. The purchases resulted from the department's after-action report of the May 30, 2020 riot in downtown Cleveland, Ohio. The Seattle Police Department in Washington purchased an LRAD in response to claims that it did not adequately communicate orders to demonstrators during the summer 2020 protests. SPD's use of its LRAD

10241-405: The device has an extremely high decibel capacity and has been controversially used as a less-lethal weapon for crowd control. Police usually use models that are not quite as powerful as the military-grade version; however, generating levels ranging from 137 dB to 154 dB, they are capable of causing pain, disorientation, nausea , migraines , and permanent damage. The NYPD 's LRAD use

10374-419: The different sound measures, a suffix is used: A-weighted sound pressure level is written either as dB A or L A . B-weighted sound pressure level is written either as dB B or L B , and C-weighted sound pressure level is written either as dB C or L C . Unweighted sound pressure level is called "linear sound pressure level" and is often written as dB L or just L. Some sound measuring instruments use

10507-746: The driver units that they feed, have power handling limits, have insertion losses , and change the load seen by the amplifier. The changes are matters of concern for many in the hi-fi world. When high output levels are required, active crossovers may be preferable. Active crossovers may be simple circuits that emulate the response of a passive network or may be more complex, allowing extensive audio adjustments. Some active crossovers, usually digital loudspeaker management systems, may include electronics and controls for precise alignment of phase and time between frequency bands, equalization, dynamic range compression and limiting . Most loudspeaker systems consist of drivers mounted in an enclosure, or cabinet. The role of

10640-416: The drivers by moving one or more driver mounting locations forward or back so that the acoustic center of each driver is in the same vertical plane. This may also involve tilting the driver back, providing a separate enclosure mounting for each driver, or using electronic techniques to achieve the same effect. These attempts have resulted in some unusual cabinet designs. Sound pressure A sound wave in

10773-487: The drivers mounted in holes in it. However, in this approach, sound frequencies with a wavelength longer than the baffle dimensions are canceled out because the antiphase radiation from the rear of the cone interferes with the radiation from the front. With an infinitely large panel, this interference could be entirely prevented. A sufficiently large sealed box can approach this behavior. Since panels of infinite dimensions are impossible, most enclosures function by containing

10906-589: The earliest designs. Speaker system design involves subjective perceptions of timbre and sound quality, measurements and experiments. Adjusting a design to improve performance is done using a combination of magnetic, acoustic, mechanical, electrical, and materials science theory, and tracked with high-precision measurements and the observations of experienced listeners. A few of the issues speaker and driver designers must confront are distortion, acoustic lobing , phase effects, off-axis response, and crossover artifacts. Designers can use an anechoic chamber to ensure

11039-501: The ears due to shadowing by the head, and diffraction around it, both of which we rely upon for localization clues. To accurately reproduce very low bass notes, subwoofer systems must be solidly constructed and properly braced to avoid unwanted sounds from cabinet vibrations. As a result, good subwoofers are typically quite heavy. Many subwoofer systems include integrated power amplifiers and electronic subsonic -filters, with additional controls relevant to low-frequency reproduction (e.g.

11172-422: The enclosure is to prevent sound waves emanating from the back of a driver from interfering destructively with those from the front. The sound waves emitted from the back are 180° out of phase with those emitted forward, so without an enclosure they typically cause cancellations which significantly degrade the level and quality of sound at low frequencies. The simplest driver mount is a flat panel ( baffle ) with

11305-429: The first film industry-standard loudspeaker system, "The Shearer Horn System for Theatres", a two-way system, was introduced by Metro-Goldwyn-Mayer . It used four 15" low-frequency drivers, a crossover network set for 375 Hz, and a single multi-cellular horn with two compression drivers providing the high frequencies. John Kenneth Hilliard , James Bullough Lansing , and Douglas Shearer all played roles in creating

11438-626: The first half of 2020, Bad Homburg's fire brigade and city police used an LRAD 100X system more than 60 times to deliver COVID-19 information. Hellenic Police acquired two LRAD systems to be used at the Evros border. These were tested after the March 2020 migrant crisis at the border, triggered by the Turkish government. They are to be used as a deterrent against illegal border entry, as a notification system, as well as to handle aggressive actions against

11571-1052: The following categories of use on its U.S. website: defense, law enforcement , fire and rescue services, border security , protection of critical infrastructure , and maritime safety. As of 2022 its products are in use in 100 countries. The Asia-Pacific website is focused on emergency management during natural disasters such as bushfires . It is not categorized as a defense item, and does not require an export license. The technology can also be used to deter wildlife from airport runways , as at Changi Airport in Singapore , and to protect gas and oil platforms from fishing boats , birds and security threats. There are two basic ways of using LRAD devices: for voice amplification and as an alert. The technology used in " siren mode" (called an "alert tone" by Genasys), primarily for usage in emergency situations, has also been used for crowd control. In this mode, it allows sound transmission at around 2,000–4,000 Hertz , which causes maximum discomfort for

11704-433: The highest audible frequencies and beyond. The terms for different speaker drivers differ, depending on the application. In two-way systems there is no mid-range driver, so the task of reproducing the mid-range sounds is divided between the woofer and tweeter. When multiple drivers are used in a system, a filter network, called an audio crossover , separates the incoming signal into different frequency ranges and routes them to

11837-990: The inverse-proportional law: p ( r ) ∝ 1 r . {\displaystyle p(r)\propto {\frac {1}{r}}.} Sound pressure level ( SPL ) or acoustic pressure level ( APL ) is a logarithmic measure of the effective pressure of a sound relative to a reference value. Sound pressure level, denoted L p and measured in dB , is defined by: L p = ln ⁡ ( p p 0 )   Np = 2 log 10 ⁡ ( p p 0 )   B = 20 log 10 ⁡ ( p p 0 )   dB , {\displaystyle L_{p}=\ln \left({\frac {p}{p_{0}}}\right)~{\text{Np}}=2\log _{10}\left({\frac {p}{p_{0}}}\right)~{\text{B}}=20\log _{10}\left({\frac {p}{p_{0}}}\right)~{\text{dB}},} where The commonly used reference sound pressure in air

11970-525: The inverse-square law for sound intensity: I ( r ) ∝ 1 r 2 . {\displaystyle I(r)\propto {\frac {1}{r^{2}}}.} Indeed, I ( r ) = p ( r ) v ( r ) = p ( r ) [ p ∗ z − 1 ] ( r ) ∝ p 2 ( r ) , {\displaystyle I(r)=p(r)v(r)=p(r)\left[p*z^{-1}\right](r)\propto p^{2}(r),} where hence

12103-419: The large, heavy iron magnets of the day were impractical and field-coil speakers remained predominant until the widespread availability of lightweight alnico magnets after World War II. In the 1930s, loudspeaker manufacturers began to combine two and three drivers or sets of drivers each optimized for a different frequency range in order to improve frequency response and increase sound pressure level. In 1937,

12236-544: The largest protests in the Old Town Square. The police and a Prague City Hall representative used an LRAD to repeatedly call upon demonstrators to follow COVID public safety protocols and to comply with security measures. During a November 25, 2021 march from the center of Prague to the local soccer stadium for a Europa Conference League match, Prague police and regional Czech law enforcement officers utilized vehicle mounted LRADs to broadcast frequent warnings against

12369-510: The launching of rockets produces. The first experimental moving-coil (also called dynamic ) loudspeaker was invented by Oliver Lodge in 1898. The first practical moving-coil loudspeakers were manufactured by Danish engineer Peter L. Jensen and Edwin Pridham in 1915, in Napa, California . Like previous loudspeakers these used horns to amplify the sound produced by a small diaphragm. Jensen

12502-401: The letter "Z" as an indication of linear SPL. The distance of the measuring microphone from a sound source is often omitted when SPL measurements are quoted, making the data useless, due to the inherent effect of the inverse proportional law . In the case of ambient environmental measurements of "background" noise, distance need not be quoted, as no single source is present, but when measuring

12635-415: The loudspeaker diaphragm—again resulting in degradation of sound quality. This can be reduced by internal absorption using absorptive materials such as glass wool , wool, or synthetic fiber batting, within the enclosure. The internal shape of the enclosure can also be designed to reduce this by reflecting sounds away from the loudspeaker diaphragm, where they may then be absorbed. Other enclosure types alter

12768-454: The loudspeakers that employ them, are improvements in cone materials, the introduction of higher-temperature adhesives, improved permanent magnet materials, improved measurement techniques, computer-aided design , and finite element analysis. At low frequencies, Thiele/Small parameters electrical network theory has been used to optimize bass driver and enclosure synergy since the early 1970s. The most common type of driver, commonly called

12901-420: The magnetic gap space. The coil is oriented co-axially inside the gap; it moves back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside ring of the gap is one pole, and the center post (called the pole piece) is the other. The pole piece and backplate are often made as

13034-451: The minimum number of amplifier channels. Some loudspeaker designs use a combination of passive and active crossover filtering, such as a passive crossover between the mid- and high-frequency drivers and an active crossover for the low-frequency driver. Passive crossovers are commonly installed inside speaker boxes and are by far the most common type of crossover for home and low-power use. In car audio systems, passive crossovers may be in

13167-646: The morning of 15 November 2011. On December 5, 2014, the NYPD utilized an LRAD, notifying approximately 100 protestors to disperse, during the protest of the police killing of Eric Garner in Midtown Manhattan. The NYPD used a Long Range Acoustic Device during the Baltimore solidarity rally in Union Square on April 29, 2015. An NYPD pickup truck equipped with an LRAD parked near protesters and broadcast

13300-434: The noise level of a specific piece of equipment, the distance should always be stated. A distance of one metre (1 m) from the source is a frequently used standard distance. Because of the effects of reflected noise within a closed room, the use of an anechoic chamber allows sound to be comparable to measurements made in a free field environment. According to the inverse proportional law, when sound level L p 1

13433-617: The outer diameter than in the center. The result is that the main cone delivers low frequencies and the whizzer cone contributes most of the higher frequencies. Since the whizzer cone is smaller than the main diaphragm, output dispersion at high frequencies is improved relative to an equivalent single larger diaphragm. Limited-range drivers, also used alone, are typically found in computers, toys, and clock radios . These drivers are less elaborate and less expensive than wide-range drivers, and they may be severely compromised to fit into very small mounting locations. In these applications, sound quality

13566-487: The output power of some designs has been increased to levels useful for professional sound reinforcement, and their output pattern is wide in the horizontal plane, a pattern that has convenient applications in concert sound. A coaxial driver is a loudspeaker driver with two or more combined concentric drivers. Coaxial drivers have been produced by Altec , Tannoy , Pioneer , KEF , SEAS, B&C Speakers, BMS, Cabasse and Genelec . Used in multi-driver speaker systems ,

13699-1868: The particle velocity and the sound pressure along the direction of propagation of the sound wave x are given by v ( r , t ) = ∂ δ ∂ t ( r , t ) = ω δ m cos ⁡ ( k ⋅ r − ω t + φ δ , 0 + π 2 ) = v m cos ⁡ ( k ⋅ r − ω t + φ v , 0 ) , {\displaystyle v(\mathbf {r} ,t)={\frac {\partial \delta }{\partial t}}(\mathbf {r} ,t)=\omega \delta _{\text{m}}\cos \left(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{\delta ,0}+{\frac {\pi }{2}}\right)=v_{\text{m}}\cos(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{v,0}),} p ( r , t ) = − ρ c 2 ∂ δ ∂ x ( r , t ) = ρ c 2 k x δ m cos ⁡ ( k ⋅ r − ω t + φ δ , 0 + π 2 ) = p m cos ⁡ ( k ⋅ r − ω t + φ p , 0 ) , {\displaystyle p(\mathbf {r} ,t)=-\rho c^{2}{\frac {\partial \delta }{\partial x}}(\mathbf {r} ,t)=\rho c^{2}k_{x}\delta _{\text{m}}\cos \left(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{\delta ,0}+{\frac {\pi }{2}}\right)=p_{\text{m}}\cos(\mathbf {k} \cdot \mathbf {r} -\omega t+\varphi _{p,0}),} where Taking

13832-429: The patent by Rice and Kellogg is the adjustment of mechanical parameters to provide a reasonably flat frequency response . These first loudspeakers used electromagnets , because large, powerful permanent magnets were generally not available at a reasonable price. The coil of an electromagnet, called a field coil, was energized by a current through a second pair of connections to the driver. This winding usually served

13965-427: The people targeted, as this is the frequency range at human hearing is most sensitive. Although sometimes referred to as an Active Denial System , the latter is based on a different technology, in which millimetre-wave radiation causes nerve receptors in the skin to feel heat, via dielectric heating . Acoustic hailing devices have the potential to be used as non-lethal weapons . The human ear can typically stand

14098-453: The pole tip or as a heavy ring situated within the magnet-pole cavity. The benefits of this complication is reduced impedance at high frequencies, providing extended treble output, reduced harmonic distortion, and a reduction in the inductance modulation that typically accompanies large voice coil excursions. On the other hand, the copper cap requires a wider voice-coil gap, with increased magnetic reluctance; this reduces available flux, requiring

14231-535: The protesters. A document released under the Official Information Act (OIA), revealed two long range acoustic devices (LRADs) were deployed at the Parliament protest on March 2. LRAD was present, but not used because of current legal regulations during protests in Poland, including Million Marijuana March 2011 and Marsz Niepodległości (National Independence Day March) 2011 and 2012. Lacking

14364-668: The purchase of two LRAD 500X systems in November 2018 "to give clear and concise messages to groups of people whenever we find it necessary". Police utilized a Long Range Acoustic device to declare an unlawful assembly in downtown Phoenix, Arizona, on May 28, 2020, during the George Floyd protests . Multiple announcements were made in English and Spanish. The Salisbury, Maryland Police Department acquired an LRAD in October 2013 with proceeds from their speed cameras. On February 17, 2017,

14497-449: The rear radiation from the moving diaphragm. A sealed enclosure prevents transmission of the sound emitted from the rear of the loudspeaker by confining the sound in a rigid and airtight box. Techniques used to reduce the transmission of sound through the walls of the cabinet include thicker cabinet walls, internal bracing and lossy wall material. However, a rigid enclosure reflects sound internally, which can then be transmitted back through

14630-418: The rear sound radiation so it can add constructively to the output from the front of the cone. Designs that do this (including bass reflex , passive radiator , transmission line , etc.) are often used to extend the effective low-frequency response and increase the low-frequency output of the driver. To make the transition between drivers as seamless as possible, system designers have attempted to time align

14763-433: The signal into individual frequency bands before power amplification, thus requiring at least one power amplifier for each band. Passive filtering may also be used in this way before power amplification, but it is an uncommon solution, being less flexible than active filtering. Any technique that uses crossover filtering followed by amplification is commonly known as bi-amping, tri-amping, quad-amping, and so on, depending on

14896-474: The sound emanating from its rear does not cancel out the (intended) sound from the front; this generally takes the form of a speaker enclosure or speaker cabinet , an often rectangular box made of wood, but sometimes metal or plastic. The enclosure's design plays an important acoustic role thus determining the resulting sound quality. Most high fidelity speaker systems (picture at right) include two or more sorts of speaker drivers, each specialized in one part of

15029-668: The sound intensity of the wave. Sound intensity , denoted I and measured in W · m in SI units, is defined by I = p v , {\displaystyle \mathbf {I} =p\mathbf {v} ,} where Acoustic impedance , denoted Z and measured in Pa·m ·s in SI units, is defined by Z ( s ) = p ^ ( s ) Q ^ ( s ) , {\displaystyle Z(s)={\frac {{\hat {p}}(s)}{{\hat {Q}}(s)}},} where Specific acoustic impedance , denoted z and measured in Pa·m ·s in SI units,

15162-413: The sound level, directionality, and frequency of the acoustic source, the sensitivity and directionality of the receiver, and the transmission channel environment. The sound level diminishes or attenuates with distance. Consequently, as a general rule, higher source levels have greater range. Acoustic hailing devices can come in two forms; The term acoustic hailing device came into common use following

15295-402: The sound pressure created by a sound source, it is important to measure the distance from the object as well, since the sound pressure of a spherical sound wave decreases as 1/ r from the centre of the sphere (and not as 1/ r , like the sound intensity): p ( r ) ∝ 1 r . {\displaystyle p(r)\propto {\frac {1}{r}}.} This relationship

15428-445: The source is commonly expressed in terms of Sound Pressure Level or SPL. SPL is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. For reference, at a distance of 1 meter, a normal talking voice is approximately 50 dB and a jet engine at 30 meters is 150 dB. A principal weakness of common speaker systems and bullhorns

15561-496: The speaker can be measured independently of room effects, or any of several electronic techniques that, to some extent, substitute for such chambers. Some developers eschew anechoic chambers in favor of specific standardized room setups intended to simulate real-life listening conditions. Individual electrodynamic drivers provide their best performance within a limited frequency range. Multiple drivers (e.g. subwoofers, woofers, mid-range drivers, and tweeters) are generally combined into

15694-435: The stiffer it is made, the poorer the damping. As a result, many cones are made of some sort of composite material. For example, a cone might be made of cellulose paper, into which some carbon fiber , Kevlar , glass , hemp or bamboo fibers have been added; or it might use a honeycomb sandwich construction; or a coating might be applied to it so as to provide additional stiffening or damping. The chassis, frame, or basket,

15827-979: The sum of the sound pressure levels of n incoherent radiating sources is L Σ = 10 log 10 ⁡ ( p 1 2 + p 2 2 + ⋯ + p n 2 p 0 2 )   dB = 10 log 10 ⁡ [ ( p 1 p 0 ) 2 + ( p 2 p 0 ) 2 + ⋯ + ( p n p 0 ) 2 ]   dB . {\displaystyle L_{\Sigma }=10\log _{10}\left({\frac {p_{1}^{2}+p_{2}^{2}+\dots +p_{n}^{2}}{p_{0}^{2}}}\right)~{\text{dB}}=10\log _{10}\left[\left({\frac {p_{1}}{p_{0}}}\right)^{2}+\left({\frac {p_{2}}{p_{0}}}\right)^{2}+\dots +\left({\frac {p_{n}}{p_{0}}}\right)^{2}\right]~{\text{dB}}.} Inserting

15960-436: The system. At the 1939 New York World's Fair , a very large two-way public address system was mounted on a tower at Flushing Meadows . The eight 27" low-frequency drivers were designed by Rudy Bozak in his role as chief engineer for Cinaudagraph. High-frequency drivers were likely made by Western Electric . Altec Lansing introduced the 604 , which became their most famous coaxial Duplex driver, in 1943. It incorporated

16093-534: The throat of a horn for added output level and control of radiation pattern. A tweeter is a high-frequency driver that reproduces the highest frequencies in a speaker system. A major problem in tweeter design is achieving wide angular sound coverage (off-axis response), since high-frequency sound tends to leave the speaker in narrow beams. Soft-dome tweeters are widely found in home stereo systems, and horn-loaded compression drivers are common in professional sound reinforcement. Ribbon tweeters have gained popularity as

16226-520: The use of LRAD, deploying it against a Sea Shepherd helicopter carrying a camera crew. Sea Shepherd noted that they had an LRAD of their own, but as of early 2010, had not put it into use other than to play a recording of " Ride of the Valkyries " in the manner of attacking U.S. Army helicopters depicted in the 1979 film Apocalypse Now . During the dispersion of New Zealand's parliament protest in March 2022, LRAD were used by New Zealand police against

16359-526: The use of pyrotechnics by the large crowds. Prague police officers utilized a vehicle-mounted LRAD to issue more than 500 warnings to revelers not to set off pyrotechnics during downtown December 31, 2021 - January 1, 2022 New Year's Eve celebrations. A vehicle-mounted LRAD was deployed by police in Prague on March 11, 2023, to communicate to and warn away demonstrators from the National Museum. In

16492-454: The use of wide-range drivers can avoid undesirable interactions between multiple drivers caused by non-coincident driver location or crossover network issues but also may limit frequency response and output abilities (most especially at low frequencies). Hi-fi speaker systems built with wide-range drivers may require large, elaborate or, expensive enclosures to approach optimum performance. Full-range drivers often employ an additional cone called

16625-405: The voice coil; heating during operation changes resistance, causes physical dimensional changes, and if extreme, broils the varnish on the voice coil; it may even demagnetize permanent magnets. The suspension system keeps the coil centered in the gap and provides a restoring (centering) force that returns the cone to a neutral position after moving. A typical suspension system consists of two parts:

16758-399: The world by various commercial, law enforcement, and military groups. In October 2000 the USS  Cole , an American guided missile destroyer , was bombed in an attack by al-Qaeda operatives, using a small boat packed with explosives. The naval personnel on Cole were unable to be sure that their messages could be heard by the approaching boat at a sufficient distance to possibly avert

16891-471: Was alleged to have caused ear damage to a peaceful demonstrator, although an internal SPD review deemed the allegations to be unsustained. After receiving feedback from protesters who said commands from on-scene officers were not clear or loud enough, the Seattle, Washington mayor's office recommended purchasing an LRAD. Loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully,

17024-637: Was also used at the September 16, 2020 protest in front of Rochester city hall. The Washington, D.C. Metropolitan Police Department used a LRAD at the January 21, 2017 Women's March . Protesters at the June 1, 2020 Washington, D.C. George Floyd protests said police gave little or no warning before employing aggressive tactics against them. A 2015 settlement requires federal police to give large crowds several loud dispersal orders before deploying chemical irritants and other dispersion tools. Using an LRAD to issue loud protest dispersal orders would have complied with

17157-399: Was capable of reproducing clear tones, but later revisions could also reproduce muffled speech . Alexander Graham Bell patented his first electric loudspeaker (a moving iron type capable of reproducing intelligible speech) as part of his telephone in 1876, which was followed in 1877 by an improved version from Ernst Siemens . During this time, Thomas Edison was issued a British patent for

17290-481: Was denied patents. Being unsuccessful in selling their product to telephone companies, in 1915 they changed their target market to radios and public address systems , and named their product Magnavox . Jensen was, for years after the invention of the loudspeaker, a part owner of The Magnavox Company. The moving-coil principle commonly used today in speakers was patented in 1925 by Edward W. Kellogg and Chester W. Rice . The key difference between previous attempts and

17423-659: Was no use of the LRAD in public situations; most usage had been by the various police forces for communications during natural disasters or for negotiations with hostage -takers in a siege , where it was too dangerous to approach a suspect. In June 2020, during the Black Lives Matter protests in New South Wales , NSW Police used LRADs in a different way for the first time, and it has been used at other anti-racism protests in Australia. The device came to

17556-463: Was that the sound needed to be focused so that it could be clearly directed at the approaching vessel. Since their inception in 2002, acoustic hailing devices have grown into a variety of applications. AHD uses include checkpoints, crowd control , maritime shipping , mass notification, early warning systems , critical infrastructure protection , military applications and wildlife protection and control. Acoustic hailing devices are now fielded all over

17689-594: Was to be challenged in US federal court in 2020. Due to potential risks and a lack of studies on the health impact of sonic weapons, the American Civil Liberties Union recommended in a fact sheet that their use in protests be suspended. By mid-2016, most Australian police forces had acquired LRADs ( Queensland Police said that they had been using it since 2009), and concerns were raised about their use for crowd control. However, before 2020, there

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