The effects on transient evoked otoacoustic emissions following changes in external auditory canal acoustic impedance.

The form of the transient evoked otoacoustic emission (TEOAE) generated by an acoustic impulse and recorded in the external auditory canal is dependent on many factors. Of these, the acoustic impedance of the external canal is possibly the most important. The effect of a change of acoustic impedance on the TEOAEs, induced by filling the external canal with helium or sulphur hexafluoride gas, is measured in a group of normal hearing subjects. The fundamental frequency of the TEOAEs is found to depend upon the density of the gas in the external canal. Compared to that of air (density 1.29 g/l) the frequency increased with helium (density 0.178 g/l) and decreased with sulphur hexafluoride (density 6.6 g/l). The amplitude of the TEOAEs was also found to depend upon the density of the gas in the canal, decreasing with helium and increasing with sulphur hexafluoride. These results suggest that the TEOAEs observed in the group of subjects tested arose within the external auditory canal. This does not imply that cochlear emissions are not generated in response to transient acoustic impulses, rather it implies that this study failed to show the presence of a cochlear-mediated emission.

Detection of mechanical changes to prosthetic heart valves by spectral analysis of valve closing sounds.

This study investigates the possibility of using spectral analysis of the closing sounds of a prosthetic heart valve to indicate minor changes to its mechanical state. Sounds are recorded in vitro in a large effectively anechoic tank of water, and the elastically mounted valve is actuated pneumatically within the water. This provides a noise-free environment for recordings. Analysis of the valve sounds is by spectral averaging and digital filtering, with each individual valve used to create its own filter. Results show that it is possible using this technique to detect reliably changes in the mechanical state of a valve, such as the addition of a small mass to the structure of the valve.

Acoustic analysis of the closing sounds of implanted prosthetic heart valves.

Previous in vitro studies [K. J. Reynolds, Ph.D. thesis (1994); K. J. Reynolds and R. O. Stephen, J. Acoust. Soc. Am. 97, 60-68 (1995)] have shown that it is possible to detect minor mechanical changes to a prosthetic heart valve by acoustic analysis of the valve's closing sounds. This current study investigates the sounds produced by prosthetic valves implanted in patients. To reduce signal distortion at the thoracic surface, recordings were made with the patient submerged in water. Results show that stable reproducible averaged spectra can be obtained from implanted valves provided recording conditions are kept constant. The respiration cycle does not affect the recorded spectrum.

Generating precise mechanical stimuli and recording chordotonal organ discharge patterns using a microcomputer.

A computer-controlled system for the investigation of the response properties of the tibio-femoral chordotonal organ in the locust is described. The computer is used to generate small amplitude sinusoidal movements of the tibia via a small servo-controlled motor. The resulting response recorded via a suction electrode is simultaneously detected, processed and stored on disk. Full constructional details for all hardware required are given. The software, developed for a BBC microcomputer, in addition to controlling all the hardware, has graphics and analysis routines enabling the operator to display and manipulate the stored data.

Signal transmission in noisy environments: auditory masking in the tympanic nerve of the bushcricket Metaballus litus (Orthoptera: Tettigoniinae).

Physiological responses of the auditory leg nerve were recorded in the tettigoniid Metaballus litus to suprathreshold tone pulses of 12.45 kHz, which is close to the carrier frequency of the male's call. This stimulus tone frequency was determined by characterizing the polar response of the foreleg. Physiological threshold of the receptors was calculated from intensity input/output curves, and the experimental stimulus was set at 40 dB above this threshold value. There was low variance in threshold values between preparations. Continuous octave filtered white noise centered on the stimulus frequency was presented at the same time as the tone pulse at increasing intensities. The summed action potentials (SAPs) of the whole leg nerve were averaged over 256 stimulus presentations and the magnitude of the response was calibrated to dB values. The range of noise levels was set between that inducing no decrease in the SAP response to the tone pulse stimulus, up to a masking intensity where the response to the tone pulse was only just observable. Decrement in SAP magnitude was linear, and complete masking occurred when the noise level was 20-25 dB above the initial level of zero masking. This final level was comparable in magnitude to the sound-pressure level of the tone pulse and within the natural range of the insect's auditory behavior. Following the cessation of the noise signal, the SAPs were monitored over intervals of 2 min until the SAP asymptoted to the preexperimental condition. The reduction in SAP magnitude during noise presentation was attributed to a loss in synchrony from the individual tympanic receptors.

Directionality and auditory slit function: a theory of hearing in bushcrickets.

Female bushcrickets locate their mates by sound. Auditory acuity is mediated by the complex anatomy of the tympanal slits, associated trachea, and spiracle. By manipulating the acoustic properties of this system and measuring the neural output, a bilobed, highly directional mechanism associated with the slits is observed, confirming Autrum's original hypothesis.