^{1,a)}, Roger P. Hamernik

^{1}and Robert I. Davis

^{1}

### Abstract

A series of Gaussian and non-Gaussian equal energy noise exposures were designed with the objective of establishing the extent to which the kurtosis statistic could be used to grade the severity of noise trauma produced by the exposures. Here, 225 chinchillas distributed in 29 groups, with 6 to 8 animals per group, were exposed at 97 dB SPL. The equal energy exposures were presented either continuously for 5 d or on an interrupted schedule for 19 d. The non-Gaussian noises all differed in the level of the kurtosis statistic or in the temporal structure of the noise, where the latter was defined by different peak, interval, and duration histograms of the impact noise transients embedded in the noise signal. Noise-induced trauma was estimated from auditory evoked potential hearing thresholds and surface preparation histology that quantified sensory cell loss. Results indicated that the equal energy hypothesis is a valid unifying principle for estimating the consequences of an exposure if and only if the equivalent energy exposures had the same kurtosis. Furthermore, for the same level of kurtosis the detailed temporal structure of an exposure does not have a strong effect on trauma.

This work was supported by grant 1-R01-OH02317 from the National Institute for Occupational Safety and Health. The able technical assistance of Adam Bouchard, Anton Norwood, and George A. Turrentine is appreciated.

I. INTRODUCTION

II. MATERIALS AND METHODS

A. Auditory evoked potential

B. Histology

C. Noise exposures

D. Noise measurement and analyses

E. Experimental protocol

F. Statistical analysis

III. RESULTS

A. Preexposure thresholds

B. The acoustic stimulus

C. For a fixed kurtosis and energy spectrum, is noise-induced trauma independent of the detailed temporal structure of the noise, i.e., independent of the combinations of the I, P, and D histograms?

D. What is the effect of the kurtosis on noise-induced trauma?

E. For the same noise energy and exposure parameters what is the effect on noise-induced trauma of interrupting the exposure?

IV. DISCUSSION

V. CONCLUSIONS

##### G10K11/00

## Figures

The impact interval histograms (I1 and I2), peak amplitude histograms (P1 and P2) and duration histograms (D1 and D2) computed over a 5.5 min digitized segment of the non-Gaussian noise.

The impact interval histograms (I1 and I2), peak amplitude histograms (P1 and P2) and duration histograms (D1 and D2) computed over a 5.5 min digitized segment of the non-Gaussian noise.

Mean preexposure evoked potential thresholds for all (n = 225) chinchillas compared to laboratory norms (shaded area) based on a sample of 1572 chinchillas.

Mean preexposure evoked potential thresholds for all (n = 225) chinchillas compared to laboratory norms (shaded area) based on a sample of 1572 chinchillas.

(a) The relative spectral level of the 97 dB SPL Gaussian noise. (b) The relative spectral level of the 97 dB SPL nonG, β(t) = 50, noise. The insert shows a representative 40 s sample of the nonG waveform. Both spectra were computed over a 5.5 min sample of the digitized waveform. (c) The relative spectral level of a representative impact noise transient along with the temporal waveform. All the synthesized impacts had similar spectra. The impacts were designed using an exponential decay function. The duration of the impact was taken as the time from the initial peak to the 20 dB down point.

(a) The relative spectral level of the 97 dB SPL Gaussian noise. (b) The relative spectral level of the 97 dB SPL nonG, β(t) = 50, noise. The insert shows a representative 40 s sample of the nonG waveform. Both spectra were computed over a 5.5 min sample of the digitized waveform. (c) The relative spectral level of a representative impact noise transient along with the temporal waveform. All the synthesized impacts had similar spectra. The impacts were designed using an exponential decay function. The duration of the impact was taken as the time from the initial peak to the 20 dB down point.

(a) The octave band Leq of the four β(t) classes of noise used in the exposures. The dotted line represents the A weighted Leq. The data points were computed over a 5.5 min sample of the digitized waveform. (b) The frequency specific kurtosis, β(f), computed on consecutive octave bands of the filtered noise signal for each of the four classes of noise. Each data point represents a value of β(f) computed on consecutive 40 s bins of the filtered 5.5 min waveform and then averaged.

(a) The octave band Leq of the four β(t) classes of noise used in the exposures. The dotted line represents the A weighted Leq. The data points were computed over a 5.5 min sample of the digitized waveform. (b) The frequency specific kurtosis, β(f), computed on consecutive octave bands of the filtered noise signal for each of the four classes of noise. Each data point represents a value of β(f) computed on consecutive 40 s bins of the filtered 5.5 min waveform and then averaged.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 25 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P), and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 25 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P), and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC) and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 50 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P) and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC) and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 50 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P) and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 100 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P), and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

Group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the eight groups of animals exposed to the interrupted β(t) = 100 nonG noise at 97 dB SPL for 19 d. Symbols refer to the different combinations of the impact noise interval (I), peak (P), and duration (D) histograms used to create the nonG noise (Table I ). The exposure for each group differed only in the I, P, and D histograms used to create the nonG noise.

The averaged (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) of all groups exposed to the β(t) = 25 (n = 62), 50 (n = 63), or 100 (n = 63) nonG noise at 97 dB SPL for 19 d regardless of the temporal structure of the noise exposure. The mean data (n = 6) from the group exposed to the equivalent energy 19 d Gaussian [β(t) = 3] noise is shown in each panel for comparison.

The averaged (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) of all groups exposed to the β(t) = 25 (n = 62), 50 (n = 63), or 100 (n = 63) nonG noise at 97 dB SPL for 19 d regardless of the temporal structure of the noise exposure. The mean data (n = 6) from the group exposed to the equivalent energy 19 d Gaussian [β(t) = 3] noise is shown in each panel for comparison.

The group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for groups exposed to the β(t) = 25 (n = 8), 50 (n = 8), or 100 (n = 8) nonG noise at 97 dB SPL for 19 d. All three groups were exposed to the nonG noise having the same I1P1D1 temporal structure. The mean data (n = 6) from the group exposed to the equivalent energy 19 d Gaussian [β(t) = 3] noise is shown in each panel for comparison.

The group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for groups exposed to the β(t) = 25 (n = 8), 50 (n = 8), or 100 (n = 8) nonG noise at 97 dB SPL for 19 d. All three groups were exposed to the nonG noise having the same I1P1D1 temporal structure. The mean data (n = 6) from the group exposed to the equivalent energy 19 d Gaussian [β(t) = 3] noise is shown in each panel for comparison.

The mean percent (a) outer hair cells (OHC) and (b) inner hair cells (IHC) lost within the 1.0, 2.0, 4.0, and 8.0 kHz octave band region of the basilar membrane for all animals exposed at a fixed β(t) to the 97 dB SPL noise for 19 interrupted days as a function of the frequency specific kurtosis β(f) computed on that octave band [see Fig. 4(b) ].

The mean percent (a) outer hair cells (OHC) and (b) inner hair cells (IHC) lost within the 1.0, 2.0, 4.0, and 8.0 kHz octave band region of the basilar membrane for all animals exposed at a fixed β(t) to the 97 dB SPL noise for 19 interrupted days as a function of the frequency specific kurtosis β(f) computed on that octave band [see Fig. 4(b) ].

The group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the groups exposed to the β(t) = 3, 25, 50, and 100 noise at 97 dB SPL for 5 continuous days. All three nonG groups were exposed to the noise having the same I1P1D1 temporal structure.

The group mean (a) permanent threshold shift (PTS), (b) percent inner hair cell loss (%IHC), and (c) percent outer hair cell loss (%OHC) for the groups exposed to the β(t) = 3, 25, 50, and 100 noise at 97 dB SPL for 5 continuous days. All three nonG groups were exposed to the noise having the same I1P1D1 temporal structure.

The group mean asymptotic threshold shift (ATS) for the groups exposed to the β(t) = 3, 25, 50, or 100 nonG noise at 97 dB SPL for 5 continuous days. All three nonG groups were exposed to the noise having the same I1P1D1 temporal structure.

The group mean asymptotic threshold shift (ATS) for the groups exposed to the β(t) = 3, 25, 50, or 100 nonG noise at 97 dB SPL for 5 continuous days. All three nonG groups were exposed to the noise having the same I1P1D1 temporal structure.

The mean total number of (a) outer hair cells (OHC) and (b) inner hair cells (IHC) lost for all animals exposed to the 97 dB SPL noise for 19 interrupted days and for 5 continuous days is shown as a function of the kurtosis β(t).

The mean total number of (a) outer hair cells (OHC) and (b) inner hair cells (IHC) lost for all animals exposed to the 97 dB SPL noise for 19 interrupted days and for 5 continuous days is shown as a function of the kurtosis β(t).

The mean threshold recovery (Tr) for the groups exposed to the β(t) = 25, 50, or 100 the interrupted nonG noise at 97 dB SPL for 19 days regardless of the temporal structure of the noise exposure. The mean Tr data (n = 6) from the group exposed to the equivalent energy Gaussian [β(t) = 3] noise is also shown for comparison. Note: Tr is the difference between the threshold measured following the first day (T1) and the mean of the thresholds measured following the last three days (T17–19) of the interrupted exposure [Tr = (T1) – (T17–19)].

The mean threshold recovery (Tr) for the groups exposed to the β(t) = 25, 50, or 100 the interrupted nonG noise at 97 dB SPL for 19 days regardless of the temporal structure of the noise exposure. The mean Tr data (n = 6) from the group exposed to the equivalent energy Gaussian [β(t) = 3] noise is also shown for comparison. Note: Tr is the difference between the threshold measured following the first day (T1) and the mean of the thresholds measured following the last three days (T17–19) of the interrupted exposure [Tr = (T1) – (T17–19)].

Scatter plots of the individual animal (n = 194) PTS and OHC loss from all the 19 day exposures.

Scatter plots of the individual animal (n = 194) PTS and OHC loss from all the 19 day exposures.

## Tables

Outline of the 29 groups of animals exposed to the various broadband, 97 dB SPL noise exposures (Σn = 225).

Outline of the 29 groups of animals exposed to the various broadband, 97 dB SPL noise exposures (Σn = 225).

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