Kastelein, R.A., Wensveen, P.J., Hoek, L., Verboom, W.C., & Terhune, J.M. (2009). Underwater detection of tonal signals between 0.125 and 100 kHz by harbor seals (Phoca vitulina). Journal of the Acoustical Society of America, 125(2), 1222-1229.

Underwater hearing sensitivities of two harbor seals were calculated behaviorally in tanks specially designed for acoustic research. The study was conducted with two captive-born, female seals between the ages of 14 and 18 months. The study tank was built with coconut fiber embedded in the walls and a layer of sand on the floor to reduce sound reverberation. For longer wavelength test stimuli (between 0.125 and 0.25 kHz), pure tones were used, and between 0.5 and 100 kHz, narrow-band, frequency-modulated tonal signals were used. Signal duration was 900 ms. Seals were trained on a go/no-go procedure in which they would respond when they heard a stimulus and remain still when they did not hear one. Trials began when the seal was stationed underwater at a listening station. Thresholds determined for the two seals were similar. Although thresholds did show the typical mammalian U-shape, the bottom of the U was very flat and wide, the high frequency cutoff was steep, and the low frequency thresholds decreased gradually. Best sensitivities for the seals occurred between 0.5 and 40 kHz.

It is not an easy task to compare these results with previous audiograms from other harbor seal subjects. Investigators have used different stimuli, testing methods, calibration techniques, and threshold calculations. Above 4 kHz, sensitivities from this study generally agree with other studies. However, below 4 kHz this study showed thresholds up to 20 dB lower than other studies, which could be due to several factors, including the care taken to avoid background noise in this study.

Our lectures on properties of sound, sound propagation and sound reception are all relevant to this paper. The concept of acoustic impedance becomes important when discussing animal communication underwater. The acoustic impedance for water is about 5000 times higher than for air. Sound travels about 4.5 times faster in water than in air, and therefore acoustic communication is often an efficient form of communication for marine animals (including the seals discussed here).

Harbor seals produce low frequency, broadband vocalizations with most energy below 2 kHz, and this paper demonstrates great sensitivity at these low frequencies. Because medium absorption increases with the frequency of the sound, these low frequency vocalizations could allow the seals to communicate over long ranges. However, in water, the energy distribution of noise is weighted towards low frequencies because low frequencies attenuate less rapidly than high frequencies. So these seals could be competing with many other noise sources, including surf, wind, rain and currents. According to our textbook, marine mammals optimally should produce higher frequency signals to maximize the distance signals can travel, but surprisingly, these seals are very sensitive to and produce signals at lower frequencies. The lecture on sound propagation also discusses reverberation. Reverberation in water can be a major problem in distorting temporal waveforms. The investigators in this study designed their tank carefully with coconut fibers embedded in the walls to avoid reverberations.

This paper also applies to the lecture on sound reception. Because these seals communicate at low frequencies, they must have a very sensitive low frequency hearing to distinguish between conspecific calls and all of the low frequency natural and anthropogenic noise sources that are abundant in the water.