Most Convenient Noise Exposure Duration for Experimentally Induced Acoustic Trauma Model

There is a large variation in the duration, intensity, and frequency of the given noise among different experimental acoustic trauma models. The aim of this study is to contribute to the development of standardization in acoustic trauma models, especially in terms of noise exposure duration.

Thirty rats were randomly divided into three groups according to noise duration; 110 dB SPL narrow-band noise at 4 kHz was applied to only the right ears for 30 min (group I), 1 h (group II), and 2 h (group III). The left ears were kept as control. Hearing function of both ears of all rats was evaluated with distortion product otoacoustic emission and auditory brainstem response tests electrophysiologically. After the hearing tests, 60 cochleae were stained using immunohistochemistry and terminal deoxynucleotidyl transferase dUTP nick-end labeling methods.

Hearing thresholds in the traumatized right ears in all groups were increased ( P < 0.001). The difference obtained in group III was higher than in the other groups ( P =  0.002). An increase in the hearing thresholds also was observed in the left ears, especially on the 7th post-traumatic day, only for group III, with a trend toward significance ( P =  0.083). In the immunohistological staining, a very strong reaction was observed, especially in the inner and outer hair cells of group III.

Application of 110 dB SPL narrow-band noise for 2 h via insert earphones produced the most reproducible permanent acoustic trauma, as confirmed by both audiological and histopathological findings. In addition, a temporary threshold shift was observed in the untraumatized ear, highlighting the importance of considering contralateral effects when designing future experimental studies.