Electrical audio signal processing systems and devices – Hearing aids – electrical – Frequency transposition
Reexamination Certificate
2000-07-24
2003-10-07
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
Frequency transposition
C381S312000, C381S326000
Reexamination Certificate
active
06631197
ABSTRACT:
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 bone conduction of an ultrasonic carrier within a communication system.
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 or bone conducted 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)).
It is well known that bone conduction can and does carry audible vibrations directly to the middle and inner ear which, if in the audible range of frequencies, are heard as sound. Hearing aids based on this effect are presently available. 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 audio signal on an ultrasonic signal of such intensity that airborne audible sound pressure waves, detectable by the human ear, are created. By superposing audio frequencies in the 20 Hz to 20 kHz bandwidth onto an ultrasonic tone, the transducer can be designed to provide uniform output 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 non-linearity 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 use of bone conduction of ultrasonic signals is described in a document entitled “Human Ultrasonic Speech Perception” by Martin L. Lenhardt et al,
Science
1991: 253: 82-85. 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 signals are asserted to have potential as an alternative communication channel in the rehabilitation of hearing disorders. However, this document is directed to use of dual (i.e., double) sideband with suppressed carrier modulation or use of dual sideband with single sideband attenuation, neither of which can achieve high fidelity audible sound.
It is unclear from the Lenhardt document exactly how the ultrasonic signals are converted into detectable sensations. However, Lenhardt discloses tests performed using the two sidebands of the dual sideband (DSB) modulated ultrasonic signal. The two sidebands constitute two different ultrasonic frequencies generated using the dual side band suppressed carrier modulation method, and are received via bone conduction by some mechanism around the ear other than the inner ear itself. Two sidebands are spaced from one another by twice the audio frequency used to modulate the ultrasonic carrier. The detectable audio frequencies are doubled and the natural spacing of speech components is not preserved. The double sideband suppressed carrier modulation technique diminishes the intelligibility of speech, and renders the Lenhardt approach unsuitable for high fidelity sound.
The reliance of the Lenhardt approach on the non-linearities of the bone conduction mechanism to produce audible sensations is supported in a document by Staab, et al. entitled “Audible Ultrasound For Profound Losses”,
The Hearing Review
, February 1998, pages 28-36, which cites the Lenhardt et al document and its disclosed use of an amplitude-modulated, suppressed carrier (double sideband modulated) technique, with speech superposed on the carrier. Page 30 of
The Hearing Review
document describes a HiSonic™ hearing aid device developed by Hearing Innovations Inc. of Tucson, Ariz. as an outgrowth of the Lenhardt et al technology.
The Hearing Review
document indicates on page 30 that in a test where a piezoelectric bone conduction driver was applied directly to the mastoid of the skull, no audio signal was measurable using a force transducer on the mastoid or a probe microphone in the ear canal. Thus, it is concluded that the audible sensations detected did not come from any airborne audio signal but must have resulted from some internal non-linearity in the bone conduction path.
The Lenhardt document does not disclose the use of a transducer with an impedance matched to air, and therefore it is incapable of directing inaudible, airborne ultrasonic signals down the ear canal of a user to produce sound that is detectable by the user. Page 36 of
The Hearing Review
document suggests that the bone conducted ultrasound may directly stimulate a nerve, stimulate the cochlea, or stimulate a secondary auditory pathway. However, the use of bone conduction, coupled with the use of a double sideband suppressed carrier, compromises the fidelity of sound achievable with the device.
In the Lenhardt, et. al. U.S. Pat, No's. 4,982,434 and 5,047,994, both entitled Supersonic Bone Conduction Hearing Aid and Method, it is disclosed that the bone conduction method is based on a system of hearing quite disti
Beck David G.
Bingham & McCutchen LLP
GN ReSound North America Corporation
Le Huyen
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