# Sound from ultrasound

Sound from ultrasound is the name given here to situations when modulated ultrasound can make its carried signal audible without needing a receiver set. This happens when the modulated ultrasound passes through anything which behaves nonlinearly and thus acts intentionally or unintentionally as a demodulator.

Scuba divers and naval submarines have been using modulated ultrasound underwater communicators for many years, but to hear the sound signal needed a receiver set to demodulate the received ultrasound signal, much as a radio receiver is needed for RF demodulation.

Researchers since the early 1960s 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 wavelengths much smaller than audible sound and thus can be aimed in a much tighter narrow beam than any traditional audible loudspeaker system.

The first modern device was created in 1998 (105th AES Conv, Preprint 4853, 1998), and is now known by the trademark name "Audio Spotlight", a term first coined in 1983 by the Japanese researchers[1] who abandoned the technology as unfeasible in the mid 1980s.

A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough (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. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small well-defined area. A listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.

There are some criticisms of this approach. Anyone or anything that disrupts the path of the beam will interrupt the progression of the beam, like interrupting the illumination of a spotlight.

## Applications

To aim a sound signal at a particular passer-by without everybody in the area hearing it. In commercial applications it can target sound to a single person without the peripheral sound and related noise that a loudspeaker emits.

### Secure short range communications

Instead of using encrypted electromagnetic radiation, which may be listened to, or jammed regardless, point to point communications could be achieved in a line of sight manner. The only way to jam or eavesdrop on the signal would require a presence in the physical path between the two parties. In cryptographic terms, this is an attempt to produce a secure channel; it has the disadvantage of a line of sight range, limiting application.

### Military and commercial security applications

Military applications have been speculated, such as a "sonic bullet" weapon that aims a highly-directed high-intensity sound wave, causing debilitating pain. However, these devices, such as LRAD, are really just high-powered bullhorns, and contrary to popular misconception, do not use ultrasound at all for sound generation, and instead use traditional loudspeaker elements (tweeters). This type of loudspeaker is unrelated to this article.

## History

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 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.

## Products

There are currently four known devices being marketed that use ultrasound to create an audible "beam" of sound.

• AudioBeam

The German audio company Sennheiser Electronic offers their "AudioBeam" product.[2] There are no known commercial applications of the technology.

• Audio Spotlight

F. Joseph Pompei of MIT developed technology he calls "Audio Spotlight", 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. (Source - ABC news 21 August 2006)

• HyperSonic Sound

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 2002. ATC named and trademarked their device as "HyperSonic Sound" (HSS). In December 2002, Popular Science named HyperSonic Sound the best invention of 2002. Norris received the 2005 Lemelson-MIT Prize for his invention of a "hypersonic sound".[3] Commercial adaptation of the device has occurred at ATC.

• Matsushita Electric Industrial Co., Ltd. (Osaka, JP).

Matsushita apparently offers a sound from ultrasound product named the "MSP-50E"[4] but commercial availability has not been confirmed.

## Literature survey

### Introduction

The first experimental systems were built over 30 years ago, although these first version 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

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[5].

Related topics were researched almost 40 years earlier in the context of underwater acoustics.

The first article[6] consisted of a theoretical formulation of the half pressure angle of the demodulated signal.
The second article[7] 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. It should be noted that 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[8] and underwater[9]. 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[1] 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[1] was the use of 547 ultrasonic transducers to produce a 40 kHz ultrasonic sound source of over 130db at 4m, which would demand significant safety considerations, for reasons outlined in[10][11]. 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

The equations that govern nonlinear acoustics are quite complicated[12][13] and unfortunately they do not have general analytical solutions. They usually require the use of a computer simulation[14]. However, as early as 1965, Berktay performed an analysis[15] 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 60dB 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 equaliser is analogous to an integration function. However these topic areas lie outside the scope of this project.

$p_2(x,t) = K \cdot P_c^2 \cdot \frac{\partial^2}{\partial t^2} E^2(x,t)$

Where

• $p_2(x,t) =\,$ Audible secondary pressure wave
• $K = \,$ misc. physical parameters
• $P_c = \,$ SPL of the ultrasonic carrier wave
• $E(x,t) = \,$ 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[16] with computer simulations by using a precompensation scheme based on Berktay's expression. In 1999 Pompei's article[5] 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

The nonlinear interaction requires two ultrasonic carrier waves to interfere in air to produce sum and difference frequencies. A DSB-AM scheme is straightforward way to generate the required ultrasonic frequencies for a given base band signal. From the basic principles of the Fourier analysis, multiplication in the time domain is analogous to convolution in the frequency domain. Convolution between a baseband signal and a carrier frequency effectively images the baseband signal around both sides of the carrier frequency spectral component, as shown in [Fig]. In addition the modulation depth m has been commonly used as a convenient experimental parameter when assessing the total harmonic distortion in the demodulated signal. Theory suggests that the THD increases proportionally with the square of m[1]. This is because as the side bands gain more power, there is more cross interference between the side bands rather than between the side bands and the carrier frequency component.

[Figure]

Use of Double Side Band Amplitude Modulation (DSB-AM) to conveniently provide ultrasonic interference waves

## Attenuation of ultrasound in air

The Figure provided in[17] 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 1dB per meter at one atmosphere of pressure.

## Safe use of high levels of ultrasound

For the nonlinear effect to occur relatively high intensity ultrasonics are required. The SPL involved was typically greater than 100dB of ultrasound at a nominal distance of 1m from the face of the ultrasonic transducer. Exposure to more intense ultrasound over 140dB near the audible range (20-40kHz) can lead to a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness and fatigue[11], but this is around 100 times the 100dB level cited above, and is generally not a concern. Commercially available Audio Spotlight systems operate in the 60 kHz range.[18]

OSHA specifies a safe ceiling value of ultrasound as 145dB 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).[19] This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety. For frequencies of ultrasound from 25 to 50 kHz, a guideline of 110dB has been recommended by Canada, Japan, the USSR, and the International Radiation Protection Agency, and 115dB by Sweden.[20], 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.

## Use in politics

There are rumors that this technology has been used to send information to candidates during live debates. The sharp sound gradient can be used to send information to a receiver without disturbing the nearby microphone. This was first reported during the infamous Romney Whisper.[21]

## Further resources

USS Patent 6778672 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

1. ^ a b c d Masahide Yoneyama and Jun Ichiroh Fujimoto. 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, 1983.
2. ^ AudioBeam
3. ^ Massachusetts Institute of Technology (2004-04-18). Inventor Wins \$500,000 Lemelson-MIT Prize for Revolutionizing Acoustics. Press release. Retrieved on 2007-11-14.
4. ^ 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 on 2008-11-23.
5. ^ a b F. Joseph Pompei. The use of airborne ultrasonics for generating audible sound beams. Journal of the Audio Engineering Society, 47(9):726{731, 1999.
6. ^ P. J. Westervelt. Parametric acoustic array. Journal of the Acoustical Society of America, 35(4):535{537, 1963.
7. ^ J. L. S. Bellin and R. T. Beyer. Experimental investigation of an end-fire array. Journal of the Acoustical Society of America, 34(8):1051{1054, 1962.
8. ^ Mary Beth Bennett and David T. Blackstock. Parametric array in air. Journal of the Acoustical Society of America, 57(3):562{568, 1974.
9. ^ T. G. Muir and J. G. Willette. Parametric acoustic transmitting arrays. Journal of the Acoustical Society of America, 52(5):1481{1486, 1972.
10. ^ http://www.coolmath.com/decibels1.htm. Everyday Sound Pressure Levels.
11. ^ a b http://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
12. ^ Jacqueline Naze Tjotta and Sigve Tjotta. Nonlinear interaction of two collinear, spherically spreading sound beams. Journal of the Acoustical Society of America, 67(2):484{490, 1980.
13. ^ Jacqueline Naze Tjotta and Sigve Tjotta. Nonlinear equations of acoustics, with application to parametric acoustic arrays. Journal of the Acoustical Society of America, 69(6):1644{1652, 1981.
14. ^ Alexander Kurganov, Sebastian Noelle, and Guergana Petrova. Semidiscrete central-upwind schemes for hyperbolic conservation laws and hamilton-jacobi equations. Society for Industrial and Applied MathematicsJournal on Scientific Computing, 23(3):707{740, 2001.
15. ^ H. O. Berktay. Possible exploitation of nonlinear acoustics in underwater transmitting applications. Journal of Sound and Vibration, 2:435{461, 1965.
16. ^ Thomas D. Kite, John T. Post, and Mark F. Hamilton. Parametric array in air: Distortion reduction by preprocessing. Journal of the Acoustical Society of America, 2:1091{1092, 1998. 54 BIBLIOGRAPHY 55
17. ^ H. E. Bass, L. C. Sutherland, A. J. Zuckerwar, D. T. Blackstock, and D. M. Hester. Atmospheric absorption of sound: Further developments. Journal of the Acoustical Society of America, 97(1):680{683, 1995.
18. ^ "Silent Sound: Exclusive dealer of Audio Spotlight".
19. ^ Noise and Hearing Conservation Technical Manual Chapter: Noise and Health Effects (App I:D)
20. ^ Safety Code 24. Guidelines for the Safe Use of Ultrasound: Part II Industrial and Commercial Applications - Guidelines for Safe Use
21. ^ "The Romney Whisper Mystery".