19th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 ACOUSTIC IMPEDANCES OF EAR CANALS MEASURED BY IMPEDANCE TUBE

During hearing sensitivity tests, the sound field is commonly generated by an earphone placed on a subject ear. One of the factors that can affect the sound transmission in the ear is the acoustic impedance of the ear canal. Its importance is related to the contribution of other elements involved in the transmission such as the earphone impedance. In order to determine the acoustic impedances of human ear canals, the standardized method for measurement of complex impedances used for the measurement of the audiometric earphone impedances is applied. It is based on the transfer function between two microphone locations in an impedance tube. The end of the tube representing the measurement plane is placed at the ear canal entrance. Thus, the impedance seen from the entrance inward is measured on 25 subjects. Most subjects participated in the previous measurement of the ratio between the pressures at the open and blocked ear canal entrance related to the ratio of the earphone and ear canal impedance. The results from present investigation are in good agreement with the existing data and they are going to be further analysed together with the previously obtained results. INTRODUCTION In the test of the hearing sensitivity, the sound pressure level generated in an individual ear by an earphone can be affected by the factors such as the acoustic impedance of the ear canal and the impedance of the earphone [1]. Due to the mentioned influence, the levels generated differ considerably from one subject to another. The variability of the sound pressure level generation has been investigated observing the sound transmission in both human ears and the coupler [1]. The transmission is described by the pressure division representing the ratio of the pressures at the open and blocked ear canal entrance. This pressure ratio is equivalent to the ratio of the ear canal impedance and the sum of this impedance and the earphone impedance. The impedance of several audiometric earphones is already determined [2]. Since the human ear canal can is often assumed as a linear passive one-dimensional system, a complete description of such a system can be obtained by its acoustic impedance [3]. Unfortunately, measurement of the ear canal impedance is sensitive to the measurement conditions (measurement position) [3-5]. Some of the problems can be circumvented by focusing on the energy reflectance (the ratio of reflected and incident energy) or pressure reflection coefficient pressure reflectance (the ratio of reflected to incident pressure) rather than on impedance. When energy losses due to the wave propagation in the ear canal are small and the ear canal area function is slowly varying, which is usually the case, the energy (pressure) reflectances measured at different positions in the ear canal are nearly equal [3, 4]. As a continuation of the previous investigations [1, 2], the pressure reflectance and impedance of the human ear canal were measured by the standardized method for measurement of complex impedance using the four-microphone impedance tube [2]. The transfer function method was applied for the measurements, and determination of the impedance was based on the sound pressure variation along the tube [6, 7]. The measurements were carried out on 25 subjects. The results for the pressure reflectance and the ear canal impedance are analysed. MEASUREMENT OF EAR CANAL IMPEDANCE Measurement method The details of the developed four-microphone measuring tube, Fig. 1, are given in [2]. The tube consists of an inner brass tube of thickness of about 1 mm inserted in an external nylon tube in which the housings for the microphones (Sennheiser KE-4-211) are drilled. The pairs of microphones 4-1, 3-1 and 2-1 are intended to be used in the following frequency ranges, respectively 200-750 Hz, 750-3500 Hz, and 3500-16000 Hz. The distance of the first microphone from the tube end (lx) is somewhat larger than the one used in the measurement of the earphone impedances [2] since the tube is somewhat extended by the additional plug fitted to the tube diameter to enable better and easier coupling of the tube to the human ear canal. The stationary sound wave is generated by the loudspeaker (Audax TW010F) placed at one end of the tube, while the other end represents the measurement plane. Besides, the absorbing material is placed near the loudspeaker to reduce the effects of reflections and tube modes [2]. Figure 1.Four-microphone tube coupled to the human ear canal at the canal entrance The details of the transfer function method used for determination of the pressure reflectance and ear canal impedance are given in [2, 6]. The pressure reflectance R is determined as i i x i i x l jk i i l jk kl j l jk i i l jk kl j e H H e e e p p p e p e R 1 1