Method to measure acoustic impedance and reflection coefficient

Abstract

A frequency-domain based system for measuring acoustic impedance and reflection coefficient is described. The calibration procedure uses a least-mean-squares approximation to the Thevenin parameters describing the source and receiver characteristics in which the data measured on closed, cylindrical tubes are matched to a viscothermal tube model. The system is intended for use in acoustical measurement in human ear canals, in which the cross-sectional area of the ear canal at the point of insertion is imprecisely known. This area is acoustically estimated from the impedance data, and the reflection coefficient is calculated in terms of this area and the impedance data. Measurements on a variety of closed tubes show the method is accurate over the frequency range investigated (less than 10.7 kHz). The time-domain reflection function is evaluated by transforming the reflection coefficient from the frequency domain, but the finite bandwidth of the measured data limits the accuracy of time-domain response measurements. The method is well suited for frequency-domain measurements in human ear canals.

Comparison of in-situ calibration methods for quantifying input to the middle ear

“…The Thevenin-equivalent source characteristics of the tube-phone and EM assembly were determined using five brass tubes ͓8.7 mm outside diameter, 8 mm inside diameter ͑i.d.͔͒ of varying lengths ͑83.0, 54.3, 40.0, 25.6, and 18.5 mm͒ with known impedances ͑Allen, 1986; Keefe et al, 1992͒. A 30-mm section of brass tubing was fabricated to function as a coupling device between the source assembly and the five calibration cavities.…”

“…Input to the ME can then be quantified as the pressure incident on the TM, devoid of any standing-wave cancellations. This decomposition of total pressure is achieved by prior determination of the Thevenin-equivalent source impedance and pressure of the transducer ͑Sivian and White, 1933;Rabinowitz, 1981;Allen, 1986;Keefe et al, 1992;Neely and Gorga, 1998;Hudde et al, 1999;Farmer-Fedor and Rabbitt, 2002͒. Knowing the source impedance ͑Z s ͒ and pressure ͑P s ͒, the impedance of any unknown load ͑Z L ͒, whether the ear canal or another cavity, is defined as…”

An Analysis of the Acoustic Input Impedance of the Ear

“…It quantifies the contributions of the ear canal, middle ear, and cochlea, to the total damping, and furnishes values for ear canal radius and length based on sound pressure measurements in the ear canal, in individual ears. The technique to acoustically calibrate a sound source in terms of its Thevenin source pressure and source impedance, developed by Allen (1986) and Keefe et al (1992), provided the means to generate acoustic input impedance data from sound pressure measurements in the ear canal. Future work would include substituting other models of the middle ear for the model used in this study to investigate structure-function relationships of the middle ear further.…”

Reflectance measurement validation using acoustic horns

“…Each stimulus condition was repeated on each of the two sound sources of this system, providing two independent measurements. The measurement system was calibrated prior to data collection to determine the Th evenin-equivalent source impedance and pressure (Allen, 1986;Keefe et al, 1992;Rasetshwane and Neely, 2011). Th evenin-equivalent source parameters are required for the transformation of acoustic pressure to acoustic impedance and reflectance.…”