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We
recently built a new acoustic testing facility at Long Marine Lab, a sound
attenuating, hemi-anechoic chamber. We received funding from the Department
of Defense University Research Instrumentation Program to construct the
facility, which includes a testing chamber for the animal (and sometimes
human) subjects and a control room for experimenters and equipment.
While raising the funds and making plans to build the test chamber took over two years, the construction process took only a few weeks. The chamber, custom built by Eckel Acoustics, was completed in December 2000, and training the animals to work comfortably inside the small space began immediately.
The superior ambient noise control provided by the chamber immediately led us to some important discoveries about pinniped hearing. We found that the aerial hearing curves (audiograms) obtained in the chamber were much lower (better) than previously measured for any pinniped species. Rather than having somewhat poor aerial hearing sensitivity (as had previously been believed from studies conducted using headphones), we measured thresholds up to 30 dB more sensitive, suggesting that all previous aerial hearing assessments for pinnipeds have been noise limited. The harbor seal and California sea lion actually demonstrated hearing thresholds lower than 0dB for their best frequencies, indicating that their aerial hearing rivals that of the most sensitive terrestrial mammals.Currently, we are looking at noise impacts on hearing in the chamber to determine if hearing is affected in similar ways by noise presented in air and under water. Thus far, our masking and TTS studies suggest that, although absolute hearing sensitivity is different in air and water, the relative effects of noise exposure are the same.
