Audiometric and histological differences between the effects of continuous and impulsive noise exposures.

An experiment was designed to determine if, for equal SPL and power spectrum, the effects on hearing of high-kurtosis noise exposures and a Gaussian noise exposure are different and the extent to which any differences measured in terms of audiometric and histological variables are frequency specific. Three groups of chinchillas with 10 animals/group were exposed for 5 days at 90 dB SPL to one of three types of noise, each with the same power spectrum. The impulsiveness, defined by the kurtosis, and the region of the spectrum from which the impulsive components of the noise were created differed for two of the noises, while the third was a continuous Gaussian noise. The results show that the most impulsive noise produced up to 20 dB greater permanent threshold shift at the high frequencies than did the Gaussian noise exposure. However, these audiometric results were difficult to reconcile with the pattern of sensory cell losses that showed statistically significant larger losses of outer hair cells for the impulsive exposure in the 0.25-kHz region. When the impacts in a high-kurtosis noise were created from the energy in the 1- through 6-kHz region of the spectrum, the audiometric profile of hearing loss was similar to that produced by the Gaussian noise; however, inner hair cell losses were significantly greater in the 4-kHz octave band region of the cochlea.

The energy spectrum of an impulse: its relation to hearing loss.

Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noise paradigms have been related to the energy spectra of the impulses. The impulses were generated by three different shock tubes that produced impulse noise spectra whose A-weighted energies showed peaks at 0.25, 1, and 2 kHz. The results show that there is an increasing susceptibility to NIPTS as the audiometric test frequency increases from 0.5 to 16 kHz. This increase in susceptibility to NIPTS is further accentuated by approximately 5 to 10 dB for impulses whose spectra peak at 2 kHz.

A generalized approach to random noise synthesis: theory and computer simulation.

A generalized approach to the synthesis of Gaussian and non-Gaussian random noises as well as purely impulsive waveforms having a preselected amplitude spectrum has been developed. The basic idea behind the synthesis is to construct the amplitude-time waveform from the frequency domain, i.e., from the amplitude and phase spectra. By maintaining a predetermined (reference) amplitude spectrum and performing certain specific manipulations of the phase spectrum within any selected band of frequencies and then applying the inverse discrete Fourier transform (IDFT), peaks in the non-Gaussian random waveform can be constructed from the selected band of frequencies that have been phase manipulated. Entire families of signals can thus be produced having the same energy spectrum, but statistical characteristics that vary along the continuum from Gaussian (skewness = 0 and kurtosis = 3) through non-Gaussian (variable skewness, kurtosis, and crest factor) to purely impulsive (shock/transient) signals. The theoretical background and the results of a series of numerical simulations will be presented which demonstrate the functional relation between various phase spectrum manipulations and the descriptors of the synthesized random noise. The results show that the approach is viable and that the synthesized random waveforms can be easily tailored to simulate a variety of real-world acoustic/vibration signals, e.g., high kurtosis (impulsive) industrial noises, helicopter noises, missile vibrational signals, etc.