Complex Acoustic Impedance Research Articles

Some Properties of the Acoustic Reflex in Man

This paper describes instrumentation and a technique for measurement of both static acoustic impedance and changes in complex acoustic impedance at the tympanic membrane. White noise was used to activate the acoustic reflex. Five subjects were exposed to this noise at levels between 50 and 110 dB SPL (over all) and with durations between 5 msec and 10 min. Changes in acoustic impedance were recorded at the subject's right ear while the noise was presented to his left. At some noise levels, the acoustic reflex was sustained for at least 10 min. The latency and strength of the reflex, and its adaptation, all were related to the level of the noise. Time required for a maximum change in acoustic impedance was related inversely to noise level. Pulsed noise was more effective than steady noise in eliciting the reflex. The acoustic reflex was observed, on the average, at an over-all SPL of 69 dB for steady noise and 63 dB for pulsed noise. [Work supported by program, project grant B-3856 from the National Institute of Neurological Diseases and Blindness, National Institutes of Health. U. S. Department of Health, Education, and Welfare.]

An Acoustic Impedance Meter for Rapid Field Measurements

A method is developed for rapid, direct measurement of acoustic impedance. It is applicable to large panels and to materials already installed for use, and therefore offers a means of investigating effects of panel vibration and mounting conditions. The pressure and pressure gradient are measured by two microphones near the surface, in a line normal to it. If p1 and p2 are the sound pressures (time phase vectors) at the microphones; d their spacing; λ the wave-length; and θ the phase angle of the impedance; then the complex acoustic impedance ratio is given by zρc≃πdλ|p1+p2p1−p2|ei(θ−π/2). This approximation is reasonably good for λ/d greater than 10 or 15, depending somewhat on the impedance value. The microphone output voltages e1 and e2 are fed into an electronic circuit which gives voltages proportional to (e1+e2) and (e1−e2). These combination voltages are then mixed, and adjusted in relative phase and magnitude to give a null indication. The impedance magnitude and phase angle are determined from the dial settings. Null methods are also used for calibration and alignment, which must be performed with great precision to obtain reasonably accurate results. Preliminary tests with the instrument, using commercial dynamic microphones, appear to yield satisfactory results for frequencies from 50 to 500 c.p.s. and for impedance magnitudes up to 10 ρc.