A distribution based definition of impulse noise.

Determining when a worksite exposure should be considered impulsive is a problem which has complicated both research into mechanisms of auditory pathology and protection of the exposed worker. An ideal solution requires that a definition of impulsive noise be developed which is independent of specific characteristics such as duration and amplitude. As an alternative to duration independence, a fixed time window over which a statistic is calculated may also serve as a basis for classification. Additionally, selecting this window on the basis of TTS production in the ear provides a biological basis for the definition of impulsiveness. Classification of impulsiveness based on the sample kurtosis meets the requirements of a generally applicable impulse definition.

Aural distortion product fl + fh: growth rate and phase characteristics at different frequencies.

This paper addresses the question of whether the nonlinear amplitude response of the ear depends only on stimulus level or on both level and frequency. The level of the quadratic distortion product fh + fl was estimated using the tone-on-tone masking paradigm under a 2AFC transformed UP-DOWN procedure. The higher frequency tone (fh) was maintained at 1.2 times the lower (fl = 250, 500, 1000, or 2000 HZ). Changes in the maximum and minimum threshold as maskee phase changes, [the monaural phase effect (MPE) amplitude], changes in the phase of the aural distortion product, and changes in the growth of the combination tone amplitude were examined as functions of primary frequency and level. The results indicate that MPE amplitude is simply a function of S/N ratio between the combination tone and the masking at that frequency. Distortion product phase varies over 360 degrees with respect to the primary tones and is unrelated to primary frequency or intensity. Within listeners the range of distortion product phase is more narrow (70-90 deg) at a single frequency. The absolute level of the summation tone does not correlate well with primary frequency and the growth rate of the combination tone varies with frequency and observers over a range of 1.1-2.3 dB/dB. The results suggest frequency independent nonlinearity when the ratio of primary frequencies is constant.

Thermal hazards in mastoid surgery: an evaluation of instruments and irrigation techniques.

Study of the mechanical properties of three commonly used mastoid drills disclosed major differences in their power and potential for heat production. Based on available information regarding heat tolerance of neural tissue, all drills were found capable of producing hazardous temperature elevations. After testing several irrigation techniques, continuous and copious suction irrigation was found to be most effective in preventing bone heating.

Thermal hazard during mastoid surgery.

Torque speed characteristics of two mastoid drills were measured. Potential temperature elevations in temporal bones were calculated from these measurements. Measured temperature elevations in temporal bones drilled without irrigation agreed well with predictions. Irrigation was demonstrated to be of critical importance in minimizing thermal hazard from mastoid drills.

Intermodulation product pair 2f1 +/- fh: masking and growth.

To examine the question of whether the distortion product 2f1 - fh is unique--whether a special nonlinear mechanism is responsible for its production--estimates of 2f1 - fh and of 2f1 + fh amplitudes were obtained in three listeners. Threshold shifts measured with a tone-on-tone masking paradigm for each distortion product suggest that each have similar properties and are masked by other tones as if they had a physical realization in the cochlea. Their masking is not a function of stimulus fine-time structure. The 1 dB/dB growth of 2f1 - fh was not changed by rendering it inaudible as previously suggested. Possible explanations of the 1 dB/dB growth of 2f1 - fh compared with the accelerated growth of 2f1 + fh are examined.