Auditory thresholds are determined by exposing an animal to a pure tone at a distinct frequency and amplitude.

Doing this repeatedly at different frequencies forms the points on the audiogram, also known as the hearing curve.

Then, noise of a certain frequency bandwidth is added, which

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increases the hearing threshold, called the “masked threshold”.

This continues until you reach a point where a wider bandwidth of noise no longer increases the hearing threshold, called the “critical bandwidth”.

The critical bandwidth correlates with the bandwidth of the animal’s auditory filter, which provides information about the shape and function of the cochlea.

The term “critical ratio” in psychoacoustics denotes the detectability of a tone embedded in white noise; this is the ratio of acoustic power between the tone and the noise that is presented experimentally as described above.

Southall et al 2007 provided the first comprehensive information about hearing in marine mammals.

Recent studies have provided audiograms for several new species of pinnipeds (e.g., harbor seals, Kastelein et al 2009, Reichmuth et al 2013; spotted seals, Sills et al 2014; ringed seals, Sills et al 2015; California sea lions, Reichmuth et al 2012), but none of these studies have tested hearing at frequencies below 100 Hz.

That is because it is very difficult to conduct these tests in tanks due to issues with standing waves for sounds with large wavelengths, and the fact that many underwater speakers cannot produce low-frequency sounds.

Work in mammals thus far has shown that the critical bandwidth is about 1/3 of an octave, but since very little testing has been done below 100 Hz, we lack information about their auditory filters at these lowest frequencies.

This work will provide the first data on hearing below 100 Hz in two species of pinnipeds:
the California sea lion; and the bearded seal.

Current investigations (funded by the Sound and Marine Life Joint Industry Program) on bearded seals are already testing at 63 Hz and 100 Hz, but not below 63 Hz.

Previous tests on California sea lions have not gone below 100 Hz.

Testing in these ranges is important because many sound sources (e.g., airguns, marine vibroseis, pile-driving) have significant energy in low frequencies.

This study will enhance our understanding of hearing sensitivity and critical bandwidths at these low frequencies, which may provide insights on low-frequency hearing in other marine mammals as well.

This work is important to BOEM because many active acoustic sources – used across all three programs - introduce acoustic energy at these low frequencies.