A laboratory method for the study of acoustic trauma

Sound exposure level as a metric for analyzing and managing underwater soundscapes

The auditory frequency weighted daily sound exposure level (SEL) is used in many jurisdictions to assess possible injury to the hearing of marine life. Therefore, using daily SEL to describe soundscapes would provide baseline information about the environment using the same tools used to measure injury. Here, the daily SEL from 12 recordings with durations of 18-97 days are analyzed to: (1) identify natural soundscapes versus environments affected by human activity, (2) demonstrate how SEL accumulates from different types of sources, (3) show the effects of recorder duty cycling on daily SEL, (4) make recommendations on collecting data for daily SEL analysis, and (5) discuss the use of the daily SEL as an indicator of cumulative effects. The autocorrelation of the one-minute sound exposure is used to help identify soundscapes not affected by human activity. Human sound sources reduce the autocorrelation and add low-frequency energy to the soundscapes. To measure the daily SEL for all marine mammal auditory frequency weighting groups, data should be sampled at 64 kHz or higher, for at least 1 min out of every 30 min. The daily autocorrelation of the one-minute SEL provides a confidence interval for the daily SEL computed with duty-cycled data.

Noise-induced hair-cell loss and total exposure energy: analysis of a large data set

The relation between total noise-exposure energy, recovery time, or rest during the exposure and amount of hair-cell loss was examined in 416 chinchillas. The exposures were octave bands of noise (OBN) with a center frequency of either 4 kHz at 47-108 dB sound pressure level (SPL) for 0.5 h to 36 d, or 0.5 kHz at 65-128 dB SPL for 3.5 h to 432 d. Recovery times varied from 0 to 365 d. With both OBNs, some animals were exposed on interrupted schedules. Hair-cell loss as a function of age in nonexposed animals (N = 117) was used to correct for sensory-cell loss due to aging. For both OBNs, the ears (N = 607) were separated into three subsets to characterize the primary hair-cell loss from noise and the secondary post-exposure loss and to determine if rest during the exposure decreased loss. Cluster and regression analyses were performed on data from the basal and apical halves of the cochlea to determine the specific rates for these three factors. It was found that: (1) when the OBN was above a critical level, there was no relation between total energy and hair-cell loss; (2) below a critical level, there were highly significant log-linear relations between total energy and hair-cell loss, but not at rates predicted by the equal-energy hypothesis; (3) rest periods during either OBN exposure reduced hair-cell loss; more so for the 4 kHz OBN than the 0.5 kHz OBN; (4) except for the highest exposure levels, the majority of outer hair cell loss from the 4 kHz OBN occurred after the exposure had terminated, while that from the 0.5 kHz OBN occurred during the exposure; and (5) a majority of the inner hair cell loss from both OBNs occurred post-exposure.

Cochlear damage caused by continuous and intermittent noise exposure

We compared the extent of permanent threshold shifts (PTS) and cochlear hair cell damage caused by continuous noise exposure with those caused by intermittent noise exposure. Twenty male pigmented guinea pigs that had been exposed to a one-octave band of noise at 4 kHz for 5 h were placed in four groups: exposure to 115 dB SPL continuous noise (group 1, n=5), 115 dB SPL intermittent noise (group 2, n=5), 125 dB SPL continuous noise (group 3, n=5), and 125 dB SPL intermittent noise (group 4, n=5). PTS at 2, 4, 8, and 16 kHz were assessed by means of auditory brainstem responses measured before noise exposure and 10 days after. The guinea pigs were killed 15 days after noise exposure, and the number of hair cells missing counted in surface preparations of the organs of Corti stained with rhodamine phalloidin. Groups 1 and 3 had significantly greater PTS (P<0.05) at all frequencies than intermittent groups 2 and 4. Group 3 had the greatest PTS at all the frequencies. Intermittent 125 dB noise total energy was greater than that of continuous 115 dB noise, but the latter elicited more PTS than the former. The extent of hair cell damage was comparable to the physiological findings. This indicates that continuous noise causes greater damage to the cochlea than intermittent noise of the same intensity and that, at the intensities tested, damage to the cochlea is not proportional to the total noise energy.

Action potential threshold elevation in the guinea-pig as a function of impact noise exposure energy

Eighteen groups of guinea pigs were exposed to a simulated impact noise for periods of 1.5-24 h. The peak level was kept constant at 131.5 dB and the repetition rate was varied to give seven different equivalent levels (Leq) between 96 and 117 dB. The auditory thresholds were assessed by electrocochleography after 1 month and compared with those of a control group. Significant damage occurred even at the lowest exposure energy used. When the total exposure energy was expressed on a decibel scale, the threshold elevation (1-10 kHz) increased 1.07 dB for each decibel increase in the exposure energy, regardless of the combination of Leq and exposure time. The results of the study thus support the equal-energy hypothesis under these conditions.

Holes in the reticular lamina after noise exposure: implication for continuing damage in the organ of Corti

Three different histological techniques (scanning electron microscopy, phase contrast microscopy and light microscopy) were used to examine the organ of Corti after a damaging noise exposure. In the region of maximal injury, all outer hair cells were missing 1-2 h after the exposure had ended. Degenerated hair cells are eventually replaced by phalangeal scars. However, in these short-term recovery ears, a series of holes which were the exact size and shape of the missing outer hair cell apices were found in the reticular lamina. These holes may provide a communication route between the endolymphatic space and the fluid spaces of the organ of Corti for a period of time following a damaging noise exposure. The noise-related degeneration of supporting cells, nerve fibers and even some sensory cells may be secondary to contamination of the fluid spaces with potassium-rich endolymph.

Influence of argon laser stapedotomy on cochlear potentials. I: Alteration of cochlear microphonics (CM)

In guinea pigs the cochlea microphonics (CM) were recorded during and after argon laser stapedotomy. Short termed drastic depression of CM was observed. This effect involved the entire frequency range of the cochlea. The recovery was mostly complete and often started already during laser irradiation or at the end of the laser exposure. A marked sensitivity loss of the cochlea only occurred with large stapedotomy holes followed by massive perilymph leakage. A momentary heat related instability of the entire stapes is suggested to cause the CM depression. In addition we observed a thermoacoustic effect on the CM. This laser evoked response seems to cause no harm to the cochlea.

Cochlear vascular and electrophysiological effects in the guinea pig to 4 kHz pure tones of different durations and intensities

Round window cochlear microphonic recordings from 33 guinea pigs were obtained prior to and following exposure to a 4 kHz pure tone at levels ranging from 124 dB to 140 dB for 5 to 80 minutes. When electro-physiological evaluation was complete, the animals were killed and tje cochlear tissues prepared using a soft-surface preparation technique. Experimental and control animals were mixed and randomly assessed without prior knowledge of the groups to which the specimens belonged. Histopathological observations were quantified and computer analysed. Statistical analyses suggest that noise exposure results in an overall decrease in blood flow to the cochlea with slight intracochlear increases in flow, perhaps compensatory in nature, to locations corresponding to the 4 kHz exposed region. Results implied that the initial localized hair cell damage known to occur as a result of overexposure to a discrete pure tone is not caused by a decreased blood flow. Relationships between electro-physiological and vascular changes and noise 'dose' are discussed.