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- Dec 03, 2016 Ultrasonic parametric speaker is phenomena when high audio frequencies (not audible) causing air to vibrate at audible spectra. On this video is shown example how.
Sound from ultrasound is the name given here to the generation of audible sound from modulated ultrasound without using an active receiver. This happens when the modulated ultrasound passes through a nonlinear medium which acts, intentionally or unintentionally, as a demodulator.
- 2Applications
- 4Products
- 5Literature survey
Parametric array[edit]
Since the early 1960s, researchers have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasound waves produced by a parametric array using heterodyning. Ultrasound has much shorter wavelengths than audible sound, so that it propagates in a much narrower beam than any normal loudspeaker system using audio frequencies. Most of the work was performed in liquids (for underwater sound use).
A very interesting mounting situation is the “Virtual speaker mode”, where the speaker is not pointing directly towards the user. Instead, the speaker is pointing towards the digital sign, (or any other object that you want to draw attention to, by using directional sound) from which the sound beam bounces and arrives at the user indirectly. Feb 07, 2004 From what I can determine, piezo speakers are rated for power handling per EIA RS-426 standard (See CTS Piezoelectric Tweeters, down near the end).This apparently specifies power as if an 8 ohm load were being driven with the same voltage as is being applied to the piezo speaker.
The first modern device for air acoustic use was created in 1998,[1] and is now known by the trademark name 'Audio Spotlight', a term first coined in 1983 by the Japanese researchers[2] who abandoned the technology as infeasible in the mid-1980s.
A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, at 100 to 110 dBSPL, to substantially change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. This technology allows a beam of sound to be projected over a long distance to be heard only in a small well-defined area;[citation needed] for a listener outside the beam the Sound pressure decreases substantially. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.[citation needed]
There are some limitations with this approach. Anything that interrupts the beam will prevent the ultrasound from propagating, like interrupting a spotlight's beam. For this reason, most systems are mounted overhead, like lighting.
Applications[edit]
Military[edit]
There has been speculation about military sonic weapons that emit highly-directional high-intensity sound; however, these devices do not use ultrasound, although sometimes thought to do so. WikiLeaks has published technical specifications of such sound weapons.[3]
Commercial advertising[edit]
A sound signal can be aimed so that only a particular passer-by, or somebody very close, can hear it. In commercial applications, it can target sound to a single person without the peripheral sound and related noise of a loudspeaker.
Personal audio[edit]
It can be used for personal audio, either to have sounds audible to only one person, or that which a group wants to listen to. The navigation instructions for example are only interesting for the driver in a car, not for the passengers. Another possibility are future applications for true stereo sound, where one ear does not hear what the other is hearing.[4]
Train Signaling Device[edit]
Directional audio train signaling may be accomplished through the use of an ultrasonic beam which will warn of the approach of a train while avoiding the nuisance of loud train signals on surrounding homes and businesses.[5]
History[edit]
This technology was originally developed by the US Navy and Soviet Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998[1] fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.
Products[edit]
As of 2014 there were known to be five devices which have been marketed that use ultrasound to create an audible beam of sound.
Audio Spotlight[edit]
F. Joseph Pompei of MIT developed technology he calls the 'Audio Spotlight',[6] and made it commercially available in 2000 by his company Holosonics, which according to their website claims to have sold 'thousands' of their 'Audio Spotlight' systems. Disney was amongst the first major corporations to adopt it for use at the Epcot Center, and many other application examples are shown on the Holosonics website.[7]
Audio Spotlight is a narrow beam of sound that can be controlled with similar precision to light from a spotlight. It uses a beam of ultrasound as a 'virtual acoustic source', enabling control of sound distribution.The ultrasound has wavelengths only a few millimeters long which are much smaller than the source, and therefore naturally travel in an extremely narrow beam.The ultrasound, which contains frequencies far outside the range of human hearing, is completely inaudible. But as the ultrasonic beam travels through the air, the inherent properties of the air cause the ultrasound to change shape in a predictable way. This gives rise to frequency components in the audible band, which can be predicted and controlled.
HyperSonic Sound[edit]
Elwood 'Woody' Norris, founder and Chairman of American Technology Corporation (ATC), announced he had successfully created a device which achieved ultrasound transmission of sound in 1996.[8] This device used piezoelectric transducers to send two ultrasonic waves of differing frequencies toward a point, giving the illusion that the audible sound from their interference pattern was originating at that point.[9] ATC named and trademarked their device as 'HyperSonic Sound' (HSS). In December 1997, HSS was one of the items in the Best of What's New issue of Popular Science.[10] In December 2002, Popular Science named HyperSonic Sound the best invention of 2002.[citation needed] Norris received the 2005 Lemelson-MIT Prize for his invention of a 'hypersonic sound'.[11] ATC (now named LRAD Corporation) spun off the technology to Parametric Sound Corporation in September 2010 to focus on their Long Range Acoustic Device products (LRAD), according to their quarterly reports, press releases and executive statements.[12][13]
Mitsubishi Electric Engineering Corporation[edit]
Mitsubishi apparently offers a sound from ultrasound product named the 'MSP-50E'[14] but commercial availability has not been confirmed.
AudioBeam[edit]
German audio company Sennheiser Electronic once listed their 'AudioBeam' product for about $4,500.[15] There is no indication that the product has been used in any public applications. The product has since been discontinued.[16]
Literature survey[edit]
The first experimental systems were built over 30 years ago, although these first versions only played simple tones. It was not until much later (see above) that the systems were built for practical listening use.
Experimental ultrasonic nonlinear acoustics[edit]
A chronological summary of the experimental approaches taken to examine Audio Spotlight systems in the past will be presented here. At the turn of the millennium working versions of an Audio Spotlight capable of reproducing speech and music could be bought from Holosonics, a company founded on Dr. Pompei's work in the MIT Media Lab.[17]
Related topics were researched almost 40 years earlier in the context of underwater acoustics.
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- The first article[18] consisted of a theoretical formulation of the half pressure angle of the demodulated signal.
- The second article[19] provided an experimental comparison to the theoretical predictions.
Both articles were supported by the U.S. Office of Naval Research, specifically for the use of the phenomenon for underwater sonar pulses. The goal of these systems was not high directivity per se, but rather higher usable bandwidth of a typically band-limited transducer.
The 1970s saw some activity in experimental airborne systems, both in air[20] and underwater.[21] Again supported by the U.S. Office of Naval Research, the primary aim of the underwater experiments was to determine the range limitations of sonar pulse propagation due to nonlinear distortion. The airborne experiments were aimed at recording quantitative data about the directivity and propagation loss of both the ultrasonic carrier and demodulated waves, rather than developing the capability to reproduce an audio signal.
In 1983 the idea was again revisited experimentally[2] but this time with the firm intent to analyze the use of the system in air to form a more complex base band signal in a highly directional manner. The signal processing used to achieve this was simple DSB-AM with no precompensation, and because of the lack of precompensation applied to the input signal, the THD Total harmonic distortion levels of this system would have probably been satisfactory for speech reproduction, but prohibitive for the reproduction of music. An interesting feature of the experimental set up used in[2] was the use of 547 ultrasonic transducers to produce a 40 kHz ultrasonic sound source of over 130db at 4 m, which would demand significant safety considerations.[22][23] Even though this experiment clearly demonstrated the potential to reproduce audio signals using an ultrasonic system, it also showed that the system suffered from heavy distortion, especially when no precompensation was used.
Theoretical ultrasonic nonlinear acoustics[edit]
The equations that govern nonlinear acoustics are quite complicated[24][25] and unfortunately they do not have general analytical solutions. They usually require the use of a computer simulation.[26] However, as early as 1965, Berktay performed an analysis[27] under some simplifying assumptions that allowed the demodulated SPL to be written in terms of the amplitude modulated ultrasonic carrier wave pressure Pc and various physical parameters. Note that the demodulation process is extremely lossy, with a minimum loss in the order of 60 dB from the ultrasonic SPL to the audible wave SPL. A precompensation scheme can be based from Berktay's expression, shown in Equation 1, by taking the square root of the base band signal envelope E and then integrating twice to invert the effect of the double partial time derivative. The analogue electronic circuit equivalents of a square root function is simply an op-amp with feedback, and an equalizer is analogous to an integration function. However these topic areas lie outside the scope of this project.
Where
- Audible secondary pressure wave
- misc. physical parameters
- SPL of the ultrasonic carrier wave
- Envelope function (such as DSB-AM)
This equation says that the audible demodulated ultrasonic pressure wave (output signal) is proportional to the twice differentiated, squared version of the envelope function (input signal). Precompensation refers to the trick of anticipating these transforms and applying the inverse transforms on the input, hoping that the output is then closer to the untransformed input.
By the 1990s, it was well known that the Audio Spotlight could work but suffered from heavy distortion. It was also known that the precompensation schemes placed an added demand on the frequency response of the ultrasonic transducers. In effect the transducers needed to keep up with what the digital precompensation demanded of them, namely a broader frequency response. In 1998 the negative effects on THD of an insufficiently broad frequency response of the ultrasonic transducers was quantified[28] with computer simulations by using a precompensation scheme based on Berktay's expression. In 1999 Pompei's article[17] discussed how a new prototype transducer met the increased frequency response demands placed on the ultrasonic transducers by the precompensation scheme, which was once again based on Berktay's expression. In addition impressive reductions in the THD of the output when the precompensation scheme was employed were graphed against the case of using no precompensation.
In summary, the technology that originated with underwater sonar 40 years ago has been made practical for reproduction of audible sound in air by Pompei's paper and device, which, according to his AES paper (1998), demonstrated that distortion had been reduced to levels comparable to traditional loudspeaker systems.
Modulation scheme[edit]
The nonlinear interaction mixes ultrasonic tones in air to produce sum and difference frequencies. A DSB-AM modulation scheme with an appropriately large baseband DC offset, to produce the demodulating tone superimposed on the modulated audio spectrum, is one way to generate the signal that encodes the desired baseband audio spectrum. This technique suffers from extremely heavy distortion as not only the demodulating tone interferes, but also all other frequencies present interfere with one another. The modulated spectrum is convolved with itself, doubling its bandwidth by the length property of the convolution. The baseband distortion in the bandwidth of the original audio spectrum is inversely proportional to the magnitude of the DC offset (demodulation tone) superimposed on the signal. A larger tone results in less distortion.
Further distortion is introduced by the second order differentiation property of the demodulation process. The result is a multiplication of the desired signal by the function -ω² in frequency. This distortion may be equalized out with the use of preemphasis filtering (increase amplitude of high frequency signal).
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By the time convolution property of the fourier transform, multiplication in the time domain is a convolution in the frequency domain. Convolution between a baseband signal and a unity gain pure carrier frequency shifts the baseband spectrum in frequency and halves its magnitude, though no energy is lost. One half-scale copy of the replica resides on each half of the frequency axis. This is consistent with Parseval's theorem.
The modulation depth m is a convenient experimental parameter when assessing the total harmonic distortion in the demodulated signal. It is inversely proportional to the magnitude of the DC offset. THD increases proportionally with m1².
These distorting effects may be better mitigated by using another modulation scheme that takes advantage of the differential squaring device nature of the nonlinear acoustic effect. Modulation of the second integral of the square root of the desired baseband audio signal, without adding a DC offset, results in convolution in frequency of the modulated square-root spectrum, half the bandwidth of the original signal, with itself due to the nonlinear channel effects. This convolution in frequency is a multiplication in time of the signal by itself, or a squaring. This again doubles the bandwidth of the spectrum, reproducing the second time integral of the input audio spectrum. The double integration corrects for the -ω² filtering characteristic associated with the nonlinear acoustic effect. This recovers the scaled original spectrum at baseband.
The harmonic distortion process has to do with the high frequency replicas associated with each squaring demodulation, for either modulation scheme. These iteratively demodulate and self-modulate, adding a spectrally smeared out and time exponentiated copy of the original signal to baseband and twice the original center frequency each time, with one iteration corresponding to one traversal of the space between the emitter and target. Only sound with parallel collinear phase velocity vectors interfere to produce this nonlinear effect. Even-numbered iterations will produce their modulation products, baseband and high frequency, as reflected emissions from the target. Odd-numbered iterations will produce their modulation products as reflected emissions off the emitter.
![Speaker Speaker](/uploads/1/2/5/0/125008079/826219904.png)
This effect still holds when the emitter and the reflector are not parallel, though due to diffraction effects the baseband products of each iteration will originate from a different location each time, with the originating location corresponding to the path of the reflected high frequency self-modulation products.
These harmonic copies are largely attenuated by the natural losses at those higher frequencies when propagating through air.
Attenuation of ultrasound in air[edit]
The Figure provided in[29] provided an estimation of the attenuation that the ultrasound would suffer as it propagated through air. The figures from this graph correspond to completely linear propagation, and the exact effect of the nonlinear demodulation phenomena on the attenuation of the ultrasonic carrier waves in air was not considered. There is an interesting dependence on humidity. Nevertheless, a 50 kHz wave can be seen to suffer an attenuation level in the order of 1 dB per meter at one atmosphere of pressure.
Safe use of high-intensity ultrasound[edit]
For the nonlinear effect to occur, relatively high intensity ultrasonics are required. The SPL involved was typically greater than 100 dB of ultrasound at a nominal distance of 1 m from the face of the ultrasonic transducer.[citation needed] Exposure to more intense ultrasound over 140 dB[citation needed] near the audible range (20–40 kHz) can lead to a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness and fatigue,[23] but this is around 100 times the 100 dB level cited above, and is generally not a concern. Dr Joseph Pompei of Audio Spotlight has published data showing that their product generates ultrasonic sound pressure levels around 130 dB (at 60 kHz) measured at 3 meters.[30]
The UK's independent Advisory Group on Non-ionising Radiation (AGNIR) produced a 180-page report on the health effects of human exposure to ultrasound and infrasound in 2010. The UK Health Protection Agency (HPA) published their report, which recommended an exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 100 dB (at 25 kHz and above).[31]
OSHA specifies a safe ceiling value of ultrasound as 145 dB SPL exposure at the frequency range used by commercial systems in air, as long as there is no possibility of contact with the transducer surface or coupling medium (i.e. submerged).[32] This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety[citation needed]. In a review of international acceptable exposure limits Howard et al. (2005)[33] noted the general agreement amongst standards organizations, but expressed concern with the decision by United States of America’s Occupational Safety and Health Administration (OSHA) to increase the exposure limit by an additional 30 dB under some conditions (equivalent to a factor of 1000 in intensity[34]).
For frequencies of ultrasound from 25 to 50 kHz, a guideline of 110 dB has been recommended by Canada, Japan, the USSR, and the International Radiation Protection Agency, and 115 dB by Sweden[35] in the late 1970s to early 1980s, but these were primarily based on subjective effects. The more recent OSHA guidelines above are based on ACGIH (American Conference of Governmental Industrial Hygienists) research from 1987.
Lawton(2001)[36] reviewed international guidelines for airborne ultrasound in a report published by the United Kingdom’s Health and Safety Executive, this included a discussion of the guidelines issued by the American Conference of Governmental Industrial Hygienists (ACGIH), 1988. Lawton states “This reviewer believes that the ACGIH has pushed its acceptable exposure limits to the very edge of potentially injurious exposure'. The ACGIH document also mentioned the possible need for hearing protection.
See also[edit]
Further resources[edit]
U.S. Patent 6,778,672 filed on 17 August 2004 describes an HSS system for using ultrasound to:-
- Direct distinct 'in-car entertainment' directly to passengers in different positions.
- Shape the airwaves in the vehicle to deaden unwanted noises.
References[edit]
- ^ ab105th AES Conv, Preprint 4853, 1998
- ^ abcYoneyama, Masahide; Jun Ichiroh, Fujimoto (1983). 'The audio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker design'. Journal of the Acoustical Society of America. 73 (5): 1532–1536. Bibcode:1983ASAJ..73.1532Y. doi:10.1121/1.389414.
- ^'LRAD technical specifications for anti-crowd, anti-pirate sound weapons, 2009'. WikiLeaks. September 27, 2009.
- ^Norris, Woody. 'Hypersonic sound and other inventions'. Retrieved 22 October 2017.
- ^'US Patent 7429935 B1'. September 30, 2008. Retrieved February 1, 2015.
- ^'Audio Spotlight Directional Sound System by Holosonics – Holosonics'. audiospotlight.com.
- ^ABC news 21 August 2006
- ^'History and background'. ParametricSound.com. n.d. Archived from the original on March 22, 2012. Retrieved February 19, 2016.
- ^Eastwood, Gary (7 September 1996). 'Perfect sound from thin air'. New Scientist. p. 22.
- ^'Best of What's New: Sound Projectors'. Popular Science. Vol. 251 no. 6. Bonnier Corporation. December 1997. p. 78. ISSN0161-7370.
- ^'Inventor Wins $500,000 Lemelson-MIT Prize for Revolutionizing Acoustics' (Press release). Massachusetts Institute of Technology. 2004-04-18. Archived from the original on October 12, 2007. Retrieved 2007-11-14.
- ^'LRAD Corporation Press Releases'. LRAD Corporation.
- ^'LRAD To Spin Off Parametric Sound, The Company Nobody Wanted – Stock Spinoffs'. Stock Spinoffs. 2010-07-19.
- ^'超指向性音響システム「ここだけ」新製品 本格的に発売開始' (Press release). 2007-07-26. Retrieved 2008-11-23.
- ^AudioBeam[permanent dead link]
- ^Audiobeam discontinued
- ^ abPompei, F. Joseph (September 1999). 'The use of airborne ultrasonics for generating audible sound beams'. Journal of the Audio Engineering Society. 47 (9): 726–731.
- ^Westervelt, P. J. (1963). 'Parametric acoustic array'. Journal of the Acoustical Society of America. 35 (4): 535–537. Bibcode:1963ASAJ..35.535W. doi:10.1121/1.1918525.
- ^Bellin, J. L. S.; Beyer, R. T. (1962). 'Experimental investigation of an end-fire array'. Journal of the Acoustical Society of America. 34 (8): 1051–1054. Bibcode:1962ASAJ..34.1051B. doi:10.1121/1.1918243.
- ^Mary Beth, Bennett; Blackstock, David T. (1974). 'Parametric array in air'. Journal of the Acoustical Society of America. 57 (3): 562–568. Bibcode:1975ASAJ..57.562B. doi:10.1121/1.380484.
- ^Muir, T. G.; Willette, J. G. (1972). 'Parametric acoustic transmitting arrays'. Journal of the Acoustical Society of America. 52 (5): 1481–1486. Bibcode:1972ASAJ..52.1481M. doi:10.1121/1.1913264.
- ^'Archived copy'. Archived from the original on 2007-12-11. Retrieved 2007-12-04.CS1 maint: archived copy as title (link). Everyday Sound Pressure Levels.
- ^ abhttp://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/safety-code_24-securite/index_e.html Guidelines for the safe use of ultrasound: Part II – Industrial and Commercial applications. Non-Ionizing Radiation Section Bureau of Radiation and Medical Devices Department of National Health and Welfare
- ^Jacqueline Naze, Tjøtta; Tjøtta, Sigve (1980). 'Nonlinear interaction of two collinear, spherically spreading sound beams'. Journal of the Acoustical Society of America. 67 (2): 484–490. Bibcode:1980ASAJ..67.484T. doi:10.1121/1.383912.
- ^Jacqueline Naze, Tjotta; Tjotta, Sigve (1981). 'Nonlinear equations of acoustics, with application to parametric acoustic arrays'. Journal of the Acoustical Society of America. 69 (6): 1644–1652. Bibcode:1981ASAJ..69.1644T. doi:10.1121/1.385942.
- ^Kurganov, Alexander; Noelle, Sebastian; Petrova, Guergana (2001). 'Semidiscrete central-upwind schemes for hyperbolic conservation laws and hamilton-jacobi equations'. SIAM Journal on Scientific Computing. 23 (3): 707–740. CiteSeerX10.1.1.588.4360. doi:10.1137/S1064827500373413.
- ^Berktay, H. O. (1965). 'Possible exploitation of nonlinear acoustics in underwater transmitting applications'. Journal of Sound and Vibration. 2 (4): 435–461. Bibcode:1965JSV...2.435B. doi:10.1016/0022-460X(65)90122-7.
- ^Kite, Thomas D.; Post, John T.; Hamilton, Mark F. (1998). 'Parametric array in air: Distortion reduction by preprocessing'. Journal of the Acoustical Society of America. 2 (5): 1091–1092. Bibcode:1998ASAJ.103.2871K. doi:10.1121/1.421645.
- ^Bass, H. E.; Sutherland, L. C.; Zuckerwar, A. J.; Blackstock, D. T.; Hester, D. M. (1995). 'Atmospheric absorption of sound: Further developments'. Journal of the Acoustical Society of America. 97 (1): 680–683. Bibcode:1995ASAJ..97.680B. doi:10.1121/1.412989.
- ^Pompei, F Joseph (Sep 1999). 'The Use of Airborne Ultrasonics for Generating Audible Sound Beams'. Journal of the Audio Engineering Society. 47 (9): 728. Fig. 3. Retrieved 19 November 2011.
- ^AGNIR (2010). Health Effects of Exposure to Ultrasound and Infrasound. Health Protection Agency, UK. pp. 167–170.
- ^'Noise and Hearing Conservation Technical Manual Chapter: Appendix I:D. Ultrasonics'. osha.gov. Archived from the original on 2016-08-09.
- ^Howard; et al. (2005). 'A Review of Current Ultrasound Exposure Limits'(PDF). The J. Occupational Health and Safety of Australia and New Zealand. 21 (3): 253–257.
- ^Leighton, Tim (2007). 'What is Ultrasound?'. Progress in Biophysics and Molecular Biology. 93 (1–3): 3–83. doi:10.1016/j.pbiomolbio.2006.07.026. PMID17045633.
- ^Safety Code 24. Guidelines for the Safe Use of Ultrasound: Part II Industrial and Commercial Applications – Guidelines for Safe UseArchived February 23, 2008, at the Wayback Machine
- ^Lawton (2001). Damage to human hearing by airborne sound of very high frequency or ultrasonic frequency(PDF). Health & Safety Executive, UK. pp. 9–10. ISBN0 7176 2019 0.
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Sound_from_ultrasound&oldid=918461618'
Digital speakers or Digital Sound Reconstruction (DSR) system are a form of loudspeaker technology. Not to be confused with modern digital formats and processing, they are yet to be developed as a mature technology, having been experimented with extensively by Bell Labs as far back as the 1920s, but not realized as commercial products.[1]
- 2Problems
- 3Improvements
- 5Future Development
Principle of operation[edit]
The least significant bit drives a tiny speaker driver, of whatever physical design is chosen; a value of '1' causes this driver to be driven full amplitude, a value of '0' causes it to be off. This allows for high efficiency in the amplifier, which at any time is either passing zero current, or required to drop the output voltage by zero volts, therefore in a theoretical ideal amplifier dissipating no power as heat at any time. The next least significant bit drives a speaker of twice the area (most often, but not necessarily, a ring around the previous driver), again to either full amplitude, or off. The next least significant bit drives a speaker of twice this area, and so on.
Other approaches are possible. For example, instead of doubling the area of the next most significant diaphragm segment, it could simply be driven so it stroked twice as far. The digital principle of operation and attendant amplifier efficiency benefits would remain.
With the advent of smaller transducer size using manufacture process such as CMOS-MEMS. A more practical approach is to construct an array of speakers, known as Digital Loudspeaker Array (DLA) or Digital Transducer Array (DTA). The least significant bit will be represented by a single transducer, and the amount will double for the next least significant bit. A n-bits speaker arrays will consist of 2n-1 transducers, and the mth bits of said arrays will contain 2m-1 transducers.[2] The entire array basically function as a thermometer-coded DAC that can decode PCM signal of same number of bits as the array into sound wave.[3] Bit grouping or PWM encoding are potential ways to decode 1-bit delta-sigma modulated signal such as DSD.[4][5][6]
Problems[edit]
Although digital speakers can function, there are various problems with this design which make it impractical for any normal uses at present.
Size[edit]
For the number of bits required for high quality sound reproduction, the size of the system becomes impractically large. For example, for a 16-bit system with the same bit depth as the 16-bit audio CD standard, starting with a 0.5 cm² driver for the least significant bit would require a total area for the driver array of 32,000 cm², or over 34 square feet (3.2 m²).
Ultrasonic output[edit]
To work properly, all of the individual diaphragm elements would have to operate cleanly at the clock frequency. The natural frequency response of the various elements will vary with their size. This creates a DAC where the various bits have different bandpass characteristics. Large short-term errors can be expected.
Since this system is converting digital signal to analog, the effect of aliasing is unavoidable, so that the audio output is 'reflected' at equal amplitude in the frequency domain, on the other side of the sampling frequency. One solution would be to overclock the conversion elements, introduce a digital filter and follow them with an acoustical low pass filter.
Even accounting for the vastly lower efficiency of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonics accompanying the desired output.
In electronic digital to analog conversion, this is addressed by the use of low-pass filters to eliminate the spurious upper frequencies produced. Since these frequencies are eliminated in the electrical signal, they are not passed to the speaker and thus ultrasonic airwaves are not generated.
However, electronic filtering is inherently unable to solve this problem with the digital loudspeaker. The speaker elements must operate ultrasonically to avoid introducing (high levels of) audible artifacts, and this means ultrasonic airwaves are inevitable. Electronics can filter electrical signals, but can not remove ultrasonic frequencies already in the air.
Efficiency[edit]
Although amplifier efficiency is good with this system, moving coil speakers operate at relatively low efficiency in the ultrasonic frequency region. Thus the original aim of the method is defeated.
8 Ohm Piezo Speaker
Cost[edit]
The large number of speaker drivers in the array, and the equally large number of amplifier channels to drive them makes for a pricey system.
Ultrasonic Speaker Driver Free
Improvements[edit]
There are ways to tackle the above issues, but none lead to a competitive or even issue free system.
Size[edit]
System size can be easily made practical by using less than 16 bits. With a 0.5 cm² LSB, system sizes are:
- 8 bit: 128 cm² total array area, or 11.3 cm x 11.3 cm (apx 4.5 inches x 4.5 inches)
- 10 bit: 22.6 cm x 22.6 cm array size.
A larger number of bits can be accommodated in a given space by varying the throw of the different elements as well as their area. This can achieve a magnitude or more of area improvement for a given bit depth. One could fit a 13 bit array into a square foot, or a 16 bit array into 4 square feet (0.37 m2).
Ultrasonics[edit]
A passive air coupled diaphragm fitted over the array of digital drivers can act as a mechanical low pass filter. However, a sharp frequency cutoff is impossible, so significant ultrasonics would still be present. Multiple passive diaphragms could improve this, but would never remove all ultrasonics, and only add further to the system's already high cost and complexity.
Intractable problems[edit]
The complexity and thus cost are both high compared to standard moving coil speakers.
The efficiency of speakers operated at ultrasonic frequencies is low, wiping out any efficiency gain in the amplifier.
![Ultrasonic Speaker Driver Ultrasonic Speaker Driver](/uploads/1/2/5/0/125008079/823031782.png)
Practical speakers demand production of fairly high volumes from fairly small cabinets, a combination that is difficult to achieve using ultrasonic speaker drivers.
Full removal of ultrasonic output is impractical.
The large number of required speaker and amplifier elements reduces system reliability significantly
Other more modern approaches to high efficiency, particularly class D amplification, work much better and at much lower cost than digital speakers.
Future Development[edit]
Microelectromechanical systems[edit]
In recent years, there had been research on the topic of construction of digital loudspeaker arrays in Digital Sound Reconstruction system using Microelectromechanical systems (MEMS).[7][8] MEMS microspeaker arrays can be manufactured onto a chip using a CMOS process. A single chip system will have less variations between each subunit compare to a multi-chip system.[7] CMOS-MEMS process reduce the size of each subunit to a few hundred μm in diameter.[9]
Speakers marketed as digital[edit]
Modern speakers marketed as 'digital' are always analog speakers, in most cases driven by an analog amplifier. The widespread use of the term 'digital' with speakers is a marketing ploy intended to claim better suitability with 'digital' source material (e.g., MP3 recordings), or impute 'higher technology' than some other speaker, and perhaps higher price. If pressed, manufacturers may claim the term means the product is 'ready' for input from digital players; this is true of essentially all speaker systems.
There are also a minority of Class D and Class T digital amplifier driven analog speakers, though these are not normally found in separate computer speakers or home stereo systems. These are common in laptops, where their higher cost is justified by battery power savings. The speakers in such equipment are still analog.
See also[edit]
Parametric Speaker Diy
References[edit]
Ultrasonic Transducer Driver Circuit
- ^'Speaker Exchange'. Retrieved 1 May 2012.
- ^Diamond, B.M.; Neumann, J.J.; Gabriel, K.J. Digital sound reconstruction using arrays of CMOS-MEMS microspeakers. IEEE. doi:10.1109/sensor.2003.1215297. ISBN0-7803-7731-1.
- ^Hawksford, Malcolm John (2004-06-01). 'Smart Digital Loudspeaker Arrays (PDF Download Available)'. Journal of the Audio Engineering Society. ISSN1549-4950. Retrieved 2018-01-15.
- ^Tatlas, Alexander; Mourjopoulos, John (2012-01-01). 'Digital Loudspeaker Arrays driven by 1-bit signals (PDF Download Available)'. ResearchGate. Retrieved 2018-01-15.
- ^Tatlas, N.-A.; Kontomichos, Fotios; Mourjopoulos, John (2009-01-01). 'Design and Performance of a Sigma-Delta Digital Loudspeaker Array Prototype (PDF Download Available)'. Journal of the Audio Engineering Society. 57 (1): 38–45. ISSN1549-4950. Retrieved 2018-01-15.
- ^Kontomichos, Fotios; Mourjopoulos, John; Tatlas, Nicolas-Alexander (2007-05-01). 'Alternative Encoding Techniques for Digital Loudspeaker Arrays'. AES. Retrieved 2018-01-15.
- ^ abDiamond, B.M.; Neumann, J.J.; Gabriel, K.J. Digital sound reconstruction using arrays of CMOS-MEMS microspeakers. IEEE. doi:10.1109/memsys.2002.984260. ISBN0-7803-7185-2.
- ^Arevalo, Arpys; Conchouso, D.; Castro, D.; Jaber, N.; Younis, M. I.; Foulds, I. G. (2015). Towards a digital sound reconstruction MEMS device: Characterization of a single PZT based piezoelectric actuator. IEEE. doi:10.1109/nems.2015.7147429. ISBN978-1-4673-6695-3.
- ^Klasco, Mike (2015-08-27). 'MEMS Microspeakers Are Truly Digital Transducers'. audioXpress. Retrieved 2018-01-11.
Best Diy Subwoofer Driver
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