The estimation of tube wall compliance using acoustic input impedance.

An acoustic-electric analog and transmission line theory have been used to examine acoustic wave propagation in a tube with a compliant wall. The input impedance (i.e., input pressure-flow) has been simulated using a distributed element model. A relative minimum and maximum, denoted by fr and f2, respectively, that are independent of tube length have been identified theoretically and confirmed experimentally from input impedance measurements on a compliant tube. A method has been devised which uses measured values of fr and f2 to deduce the tube wall properties from the theoretical model. This method has been validated on a tube with known wall properties determined using standard methods. In practice, the input impedance is measured through a short section of rigid connecting pipe. In this case fr remains constant while f2 is reduced. This reduction can be accounted for by the volume compliance of the gas within the lumen of the rigid pipe. The theory could have useful applications such as estimating the wall properties of the airways from noninvasive measurements made through the mouth.

Consonant burst enhancement: a possible means to improve intelligibility for the hard of hearing.

The possibility of using a circuit to amplify selectively the burst of a stop consonant is investigated. It is shown that such a circuit used in the speech channel of an amplifying system can improve the discrimination between stop consonants. Such a system could be of value in assisting the hard of hearing.

A mechanism for stimulated acoustic emissions in the cochlea.

Stimulated acoustic emissions in the cochlea are explained in terms of its hydraulic properties. The mathematical model predicts that these 'echoes' are caused by reflections which result from a discontinuity in the resistive and reactive components of the impedance which occurs at resonance. This discontinuity is a direct result of the wavelength-dependent nature of the fluid inertance and occurs without the consideration of non-linearities. Calculations of the time delay of tone bursts to and from the places of reflection by determination of the group velocity agree with observations of the response latencies for the frequencies concerned.

The effect of frequency tolerance on audiometer accuracy.

Investigations into the methods used to calibrate audiometers reveal that the 6 000 Hz frequency is particularly liable to yield inconsistent results when calibrated in the usual way. It is shown that the TDH39 telephone receiver which is usually calibrated on a 9A coupler in accordance with the International Standards Organization recommendation R389 will depend to a considerable extent on the precise frequency used and may differ by as much as 7 db when the frequency is varied but still retained within the specified tolerance limits. A new telephone receiver, the TDH50, is much less sensitive to frequency variation. It is shown that this receiver should be calibrated with the same threshold figures as the TDH39.