A.M. Martin

Development of an electroacoustic analogue model of the middle ear and acoustic reflex

Abstract Experimental measurements of changes in the acoustic admittance of the eardrum caused by stapedius muscle contractions in human subjects are used to develop and electroacoustic analogue model of the middle ear. In this model the stapedius muscle is included as an explicit functional unit. The acoustical characteristics of the external ear canal are also included. The model is extensively evaluated by comparing its properties with the known characteristics of real ears of humans and other animals. Subsequently, the model is used to predict the effects of the acoustic reflex on middle ear sound transmission, which cannot easily be measured in humans. The model predicts attenuation of potentially hazardous high level sounds at frequencies below 1 kHz of up to about 10 dB, but very little effect at higher frequencies unless the reflex-eliciting stimulus is of sufficient intensity to cause partial disarticulation of the incudo-stapedial joint by stapedius muscle contraction. Overall attenuation for typical industrial noises is unlikely to be greater than approximately 3 dB(A) and will probably be even less in practice, resulting in little effective protection from the harmful effects of high intensity noise. It is considered that the model will be of benefit in the analysis of middle ear function, including the interpretation of audiological measurements of eardrum impedance and acoustic reflex response. This should lead to more versatile diagnosis of peripheral auditory dysfunction than has been possible hitherto.

An investigation of the effects of hearing protectors on vocal communication in noise

Previous research has indicated that the wearing of hearing protectors in noise above about 85 dB does not reduce speech intelligibility compared to the open ear condition. However, in spite of this it is still reported by many industrial workers that verbal communication is impeded by such devices being worn in the working environment. Two experiments were carried out to investigate the effects of both earplugs and earmuffs on speech intelligibility for the listener and on the voice level of the talker in the presence of broadband masking noise. It was found that hearing protectors do not degrade speech intelligibility for the listener and may even effect a slight improvement. However, if worn by the talker, his speech is reduced in level and quality to such an extent that intelligibility for the listener is also reduced. When hearing protectors are worn by both talker and listener, the composite effect is an overall reduction in speech intelligibility compared to the open ear condition.

Adaptation of the acoustic reflex to combinations of sustained steady-state and repeated pulse stimuli

Abstract Responses of the acoustic reflex to steady-state and pulsed noise stimuli with overall durations of 5 minutes were measured in human subjects. The responses were found to decrease considerably during stimulation by steady-state noise. This decrease in response did not occur for pulsed noise stimuli with large dynamic ranges. For combinations of steady-state and pulsed noise with a dynamic range of up to 10 dB, the response approximated to that for the steady-state noise alone. It was also observed that the reflex responses did not rise rapidly to a maximum at the onset of stimulation, but increased gradually over a period of between 10 and 30 seconds. This gradual rise has not been described previously and is termed augmentation here. The lack of a decrease in response for the pulsed stimuli indicates that the decrease measured for the other stimuli is due to adaptation of the reflex rather than fatigue of the stapedius muscle. The components of the observed responses are modelled in terms of exponential functions and the possible mechanisms of adaptation and augmentation are discussed. It is concluded that the most likely sites of reflex adaptation are within the afferent or central portions of the reflex arc, rather than in the efferent portion. Augmentation may occur within the stapedius muscle.

The role of acoustical characteristics in the perception of warning sounds and the effects of wearing hearing protection

Abstract The relative importance of the acoustical contrasts of a warning sound with other irrelevant sounds and with the ambient noise has been investigated in the context of the perception of warning sounds and the effects of hearing protection being worn by normally hearing subjects. The results indicate that both contrast factors had an influence on the attention demand of the auditory warnings, but that the contrast with the irrelevant sounds was more potent for the specific stimuli investigated. Thus, they emphasize the importance of including a typical recognition requirement when assessing the attention demand of warning sounds. A small additional reduction in the attention demand of both sounds was evident when the subjects were wearing hearing protectors. This effect could occur in the industrial setting, but further research is required to establish its extent and implications.

The variability of acoustic reflex threshold measurements

Abstract Repeated measurements of the acoustic reflex threshold for various stimuli were obtained in normal subjects following a procedure similar to that commonly used in clinical audiological work, and also using a recording method with a well-defined criterion for determining the threshold value. The variability of measurements was estimated for each technique using a statistical model approach. The standard deviations of repeated measurements for a given stimulus were 2·8 and 2·5 dB, respectively, for the two methods. When the apparatus was refitted to the subject between measurements, the variability was increased. The small gain in repeatability resulting from the recording method is unlikely to be of practical significance in the clinical situation but may be of importance in research work where greater precision is desirable.

Impulse noise damage risk criteria

New possibilities for unification of impulse noise damage risk criteria with the “immision” principle of equivalent A-weighted sound energy for continuous noise exposure are presented and discussed.

The acoustic reflex threshold for impulses

Acoustic reflex thresholds, measured as changes in the acoustic admittance of the eardrum, were obtained from normal human subjects for various stimuli, presented as pulses with either rectangular or exponentially-decaying sound pressure envelopes. The influence of the frequency of the probe tone of the admittance measurement system on the reflex threshold was also investigated. The reflex threshold for exponentially-decaying pulses was found to be about 7 dB higher in terms of peak sound level than for rectangular pulses with the same total duration. This difference may be explained in terms of the dynamics of the reflex response. Reflex thresholds measured with a 660 Hz probe tone were found to be approximately 3 dB lower than with a 220 Hz probe tone. A significant correlation was found to exist between the magnitude of response of the reflex and the resting eardrum admittance. The effect of this on the measurement of the reflex threshold is discussed.

The response of the acoustic reflex as a function of the intensity and temporal characteristics of pulsed stimuli

Abstract The response of the acoustic reflex in normal human subjects was measured as a change in the complex acoustic admittance of the eardrum for pulsed stimuli at various intensities and with various durations. Both rectangular and exponentially decaying pulse shapes were used. Thus, detailed quantitative information was obtained regarding the relative effects of stimulus intensity, duration and pulse shape on the magnitude of the reflex response. The magnitude of the acoustic reflex response was found to increase monotonically with increasing stimulus intensity and duration, as might be anticipated from previous studies of the reflex threshold alone. The response was found to be greater for rectangular pulses than for exponentially decaying pulses with the same peak intensity and total duration. The dependence of the acoustic reflex response magnitude on both stimulus intensity and duration was compared with the temporal summation of loudness which has been observed in many past psychoacoustical experiments. The two phenomena were found to be quantitatively different, suggesting that, contrary to previous assumptions, they are not the same process. Temporal summation of the acoustic reflex response was also investigated by analysis of the time-course of the response, using a modelling approach. Comparison of this analysis with previous electromyographic data indicated that the temporal summation of the reflex is probably mainly a function of the transduction of neural excitation into mechanical force within the stapedius muscle itself, rather than summation of neural excitation within the afferent auditory pathway. The absolute measurements of the conductance and susceptance of the eardrum at 220 and 660 Hz allowed a comprehensive description to be made of the changes in eardrum admittance caused by the various levels of reflex response. This information had not been available previously. At both frequencies, the effect of the reflex could be represented approximately by an increase in eardrum reactance, with resistance remaining constant. This simplified description should benefit the diagnostic interpretation of acoustic reflex responses.

Studies on quiet threshold shift in the absence of noise

Previous work has indicated that after removal from moderate ambient noise for one hour, subjects with normal hearing can show substantial improvements in hearing acuity. Such effects could have important consequences in the measurement of the threshold of hearing over periods of time, as for example in the evaluation of hearing protectors. An experiment was carried out to measure the extent of such a shift and to obtain some indication of its frequency characteristics. Possible methods of re-arousing the threshold were also investigated. The results are discussed in terms of recovery from temporary threshold shift, an audiometric learning effect and possible changes in the level of physiological noise.

The influence of the acoustic reflex on the loudness of pulsed pure tones

The influence of stapedius muscle contraction elicited by contralateral acoustic stimulation on the loudness of pulsed pure tones at frequencies between 0·25 and 2 kHz was measured by means of a monaural loudness balance technique. Considerable care was taken in the experimental technique to minimize unwanted psychological interference by the reflex-eliciting stimulus on loudness judgments. The results indicated a minimal ( These results are compared with the divergent results from past studies and the existing hypothesis that only high intensity sounds are attenuated by contraction of the stapedius muscle. The conclusion is tentatively drawn that changes in the level of stimulation reaching the cochlea due to stapedius muscle contraction may be compensated for in the judgment of loudness at a higher level of the auditory system.