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Title: Electro-acoustic impedance bridges
Claims: I claim: 1. An electro-acoustic impedance bridge apparatus comprising an acoustic coupler whose acoustic impedance is representative of the acoustic impedance of the normal human ear over a predetermined frequency range, a pair of probes, one for connection to the external auditory canal of an ear to be tested and the other attached to the acoustic coupler, a variable frequency sound generator for applying to both probes sound signals of equal frequency within said predetermined frequency range, means in said variable frequency sound generator for sweeping the frequency of said two sound signals through said predetermined frequency range, means for varying the input sound levels applied to the test ear and the acoustic coupler by said variable frequency sound generator as the frequency range is swept through, and comparator means coupled to said probes for comparing the resulting sound levels prevailing in the external auditory canal of the test ear and in the acoustic coupler. 2. An apparatus according to claim 1, having means enabling the frequency sweep to be stopped at any desired frequency within said predetermined frequency range. 3. An apparatus according to claim 1, in which said means for varying the input sound levels applied by the variable frequency sound generator includes means adapted to automatically adjust the level of said two sound signals in dependence upon the frequency of these two signals, such that the sound levels of the sound signals applied to the two probes changes with the frequency of these sound signals in accordance with a predetermined characteristic. 4. An apparatus according to claim 3, in which said predetermined characteristic is such that the sound levels of the sound signals applied to the two probes increases by 6 dB per octave, with increasing frequency. 5. An apparatus according to claim 3, in which the level adjusting means for said two sound signals comprises a variable gain amplifier. 6. An apparatus according to claim 5, in which the variable gain amplifier has a plurality of individually selectable gain steps and, the apparatus further includes a shift register which sequentially selects the gain steps of the variable gain amplifier and which is adapted to shift in accordance with the frequency of the two sound signals. 7. An apparatus according to claim 1, in which said means for varying the input sound levels applied by the variable frequency sound generator includes a feedback control circuit which automatically adjusts the level of the two input sound signals such as to maintain the sound level in the acoustic coupler at a substantially constant level throughout said predetermined frequency range. 8. An apparatus according to claim 7, in which the feedback control circuit includes a variable gain amplifier controlling the level of said two input sound signals, the gain of the latter amplifier being determined by the magnitude of an error signal obtained from the comparison of the sound level in the acoustic coupler with a preset reference level. 9. An apparatus according to claim 5, in which the overall level of the signal emanating from the variable gain amplifier is adjustable to enable the sound level applied to the test ear probe, at all frequencies in said predetermined range, to be kept below that at which the acoustic reflex would be stimulated. 10. An apparatus according to claim 5, in which the variable frequency sound generator includes a voltage controlled oscillator for driving said variable gain amplifier with a signal of variable frequency. 11. An apparatus according to claim 10, in which the variable frequency signal produced by the voltage controlled oscillator is of triangular waveform and a diode function generator is included between the voltage controlled oscillator and the variable gain amplifier for converting said triangular waveform into a corresponding sinusoidal waveform. 12. An apparatus according to claim 10, further comprising a ramp generator for driving the voltage controlled oscillator such that the latter produces an output signal of linearly increasing frequency. 13. An apparatus according to claim 12, further comprising summing means enabling a constant DC level to be added to the output of the ramp generator whereby the frequency sweep of the voltage controlled oscillator starts at a frequency greater than zero. 14. An apparatus according to claim 1, in which the variable frequency sound generator includes a loudspeaker for generating said two sound signals, and respective waveguides connecting the loudspeaker to said two probes. 15. Apparatus according to claim 14, in which the waveguides connecting the loudspeaker to the probes comprise tubes of equal internal diameter. 16. An apparatus according to claim 1, in which the sound level comparison means includes two microphones connected by respective waveguides to passages in the two probes for generating, in use of the apparatus, electrical signals corresponding to the sound levels in the external auditory canal of the test ear and the acoustic coupler, respectively. 17. An apparatus according to claim 16, in which the sound level comparison means further includes first and second signal conditioning channels respectively connected to the two microphones, each channel containing an amplifier and a rectifier for providing a DC signal proportional to the output of the associated microphone. 18. An apparatus according to claim 17, further including a difference amplifier for comparing the two DC signals obtained from the two signal conditioning channels. 19. An apparatus according to claim 17, including a signal divider for comparing the two DC signals obtained from the two signal processing channels. 20. Apparatus according to claim 17, including a phase detector for comparing the phases of the outputs of the two microphones. 21. Apparatus according to claim 17, in which the gain of at least that one of the amplifiers, which is in the signal conditioning channel associated with the test ear, is variable to enable the overall output from this channel to be adjusted. 22. An electro-acoustic impedance bridge comprising an acoustic coupler whose acoustic impedance is representative of the acoustic impedance of the normal human ear over a predetermined audio frequency range, a pair of probed, one for connection to the external auditory canal of an ear to be tested and the other attached to the acoustic coupler, a variable frequency sound generating means for applying to both probes sound signals whose frequency is swept through said predetermined frequency range, means for varying the input sound levels applied to the test ear and the acoustic coupler by said variable frequency sound generating means as the predetermined frequency range is swept through, and means coupled to said probes for comparing the resulting sound levels prevailing in the external auditory canal of the test ear and in the acoustic coupler. Other info: Inventors: Bennett, Michael John Application Number: 707168 Filing Date: 1976-07-21 Publication_date: 1978-03-14 Assignee: Primary Class(es): 600/559 73/585, 73/648 Other Classes: US Patent Ref:
Other Refs: Other References: Domnitz, H., "Headphone Monitor System for Binaural Experiments," J. Ac. Soc. of Am., Aug. 1975. | |