Electrical audio signal processing systems and devices – Hearing aids – electrical – Frequency transposition
Reexamination Certificate
2000-04-07
2003-10-07
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
Frequency transposition
C381S312000, C381S328000
Reexamination Certificate
active
06631196
ABSTRACT:
BACKGROUND OF THE INVENTION:
1. Field of the Invention:
The present invention relates generally to communication systems, and more particularly, to transducers and transduction methods for reproducing wide audio bandwidth sound using an ultrasonic carrier within a communication system.
2. Background Information:
Communication systems typically operate with transducers that convert audio acoustic signals into electrical signals, and vice versa. The audio acoustic signals are airborne sound pressure waves having frequencies within the bandwidth detectable by the human ear (acoustic signals having frequencies between approximately 20 Hertz (Hz) to 20 kiloHertz (kHz)). Ultrasonic acoustic signals are not output from typical audio circuits because these signals possess frequencies outside the bandwidth detectable by the human ear, and produce inaudible sound pressure waves.
However, communication systems are known wherein ultrasonic signals are used as carrier -signals in the production of audio acoustic signals. These systems typically rely on either: (1) the non-linearities of air to demodulate an audio modulated ultrasonic carrier signal; or (2) rely on bone conduction of ultrasonic signals to create the sensation of audio signals. As such, these systems are ill-suited for or even unable to produce high fidelity sound.
For example, a document entitled “Norris Acoustical Heterodyne™ Technology & HyperSonic™ Sound” (Jul. 26, 1997) by Elwood G. Norris of American Technology Group (California) describes a distributed speaker system wherein the ultrasound transducer superposes an audible signal on an ultrasonic signal of such intensity that airborne audible sound pressure waves, detectable by the human ear, are created. By superposing audible frequencies in the 20 Hz to 20 KHz bandwidth onto an ultrasonic tone, the transducer can be designed to provide uniform audio over a frequency range which constitutes a much smaller percentage of the transducer's center frequency. That is, without the use of an ultrasonic carrier, the total frequency range of the audible bandwidth (i.e., approximately 20 kHz) divided by the lowest frequency in the bandwidth (20 Hz) constitutes a percentage frequency shift from the lowest frequency (20 Hz) to the highest frequency (20 kHz) of 20 kHz/20 Hz, or 100,000%. By superposing this 20 kHz band on an ultrasound carrier in the 200 kHz range, the percentage frequency shift reduces to 20 kHz/200 kHz, or 10%, such that the transducer can be more effectively designed. However, this speaker system requires the use of high intensity output signals because it relies upon the non-linearities of air to demodulate the ultrasonic signals into audible acoustic signals. Thus, efficiencies which are gained in the transducer design are lost in the demodulation.
The Norris document describes transmitting two ultrasound wave trains each having a tone of sufficiently high amplitude that when introduced to the nonlinearity of air in the room produce two “combination” tones corresponding to the sum and difference of the two original ultrasonic tones. For example, if two ultrasonic tones of 200 kHz and 201 kHz were emitted from the ultrasound transducer into air with sufficient energy, a sum tone of 401 kHz and a difference tone of 1 kHz would result, that latter being within the range of human hearing. The distributed speaker system thus relies on the non-linearity of air and the resultant difference tone to produce an audio acoustic signal having pressure waves that can be detected by listeners.
A document entitled “In The Audio Spotlight—A Sonar Technique Allows Loudspeakers To Deliver Focused Sound Beams”,
Scientific American
, October. 1998, pp. 40-41, describes the demodulation of audio tones from ultrasonic waves using the non-linearities of air, and discusses the work of Norris. This document mentions the distortion which occurs at low frequencies of the audible bandwidth when audible tones are produced from ultrasonic waves using the non-linearities of air (i.e., poor bass). This document suggests that using the non-linearities of air to demodulate an ultrasonic carrier to produce sonic energy compromises the ability to achieve high fidelity, wide audio bandwidth sound having a full, bass response. Such a compromised ability would be unacceptable for high fidelity communications.
The lack of audio bass in systems which rely on the non-linearity of air to produce sonic energy occurs because the ultrasound-to-audio-sound transformation is essentially constant volume displacement. That is, the volume of air moved is essentially the same no matter what sonic frequency is being converted. However, the human ear is essentially constant pressure. That is, to hear a constant loudness over a range of audible frequencies requires that those frequencies be presented at the same pressure (also called Sound Pressure Level or SPL). In air, volume displacement and pressure are related, as a function of frequency, as P=V*f, where P is pressure, V is volume displacement and f is the frequency. Thus, to maintain constant pressure (i.e. SPL) as the audible sonic frequency is reduced, the volume displacement must be proportionally increased. Since the Norris technique does not inherently increase the volume displacement at lower audio frequencies, a proportional increase in the ultrasound drive signal must be used. This is called equalization, and it results in a need for very large drive signals to reproduce flat low bass audio frequencies. However, boosting the lower frequencies can not sufficiently compensate the loss without causing distortion when the transducer is driven hard.
Another document entitled “Audio Sound Reproduction Based On Nonlinear Interaction of Acoustic Waves” by Dong Weiguo and Wu Qunli, J. Audio Eng. Soc., Vol. 47, No. 7/8 1999 July/August also describes the nonlinear interaction of two finite-amplitude sound waves of different ultrasonic frequencies in air to produce audible sound waves whose frequencies correspond to the difference of the primary waves. However, this document describes the difficulty in relying on non-linearities of air, as opposed to fluids, to exploit this effect. This difficulty is due to the high absorption of acoustic waves in air and the small non-linearity parameters of air. As experimentally confirmed by Weiguo, the audio frequency SPL output is indeed proportional to frequency; that is, for a constant ultrasound input, the audio output was 34 dB lower at 20 Hz than at 1 kHz. Furthermore, this inherent effect is the same whether the two ultrasonic frequencies are produced from two different ultrasonic transducers or from a single transducer. Thus, like the
Scientific American document
, this document suggests that using the non-linearities of air to demodulate an ultrasonic carrier compromises the bass response, a result which would be unacceptable for high fidelity communications.
Norris, in U.S. Pat. No. 5,889,870, entitled
Acoustic Heterodyne Device and Method
, describes headsets and hearing aids based upon his acoustical heterodyne method. However, both devices rely upon the non-linearities of the air transmission medium. For example, they rely on the non-linearities of air within the ear canal itself to create air borne audible acoustic waves that are subsequently detected by the normal acoustic hearing process of the ear. Furthermore, the Norris patent discloses that this process requires a resonant cavity, and that the ear canal's natural resonance properties provide that necessary element (col. 15, lines 14-27). There is no mention of a system which does not require the nonlinearities of air nor one which works without the “broadly resonant cavity”.
The use of ultrasonic signals is also described in a document entitled “Human Ultrasonic Speech Perception” by Martin L. Lenhardt et al,
Science
1991: 253: 82-85. However, rather than relying on the non-linearities of air to demodulate an ultrasonic carrier, this document is directed to use of bone-conducted ultrasonic signals. The bone conducted ultrasonic sign
Higgins, Jr. Robert J.
Taenzer Jon C.
Beck David G.
Bingham & McCutchen LLP
GN ReSound North America Corporation
Le Huyen
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