Sounds We Can Hear: Science of Sound in the Sea

Humans don't hear all sounds. For example, we can't hear the sound a dog whistle makes, but dogs can hear that sound. Marine animals can often hear sounds that humans can't hear. The sounds we hear best are sounds like human conversation. Marine animals' hearing is similar. They hear the sounds they use every day the best. What sounds we, or a marine animal, can hear depends on the frequency of the sound and the intensity of the sound.

Humans can hear sounds at frequencies from about 20Hz to 20,000Hz. Hearing loss may reduce the range of frequencies a person can hear. The human ear does not perceive all frequencies equally. We hear sounds best at around 3,000 to 4,000Hz, where human speech is centered.

The sound intensity that scientists measure is not the same as the loudness. Loudness describes how people perceive sound. (See How do you characterize sounds?) The softest sounds that people can hear at a frequency of 1000 Hertz have a measured sound intensity of 0 decibels relative to the intensity of a sound wave with a pressure of 20 microPascals (dB re 20 µP). (The Pascal is the international unit for pressure, named after Blaise Pascal, a famous 17th century mathematician and physicist.)

The graph below is an Equal Loudness Curve. The y-axis (vertical) is relative intensity in decibels referenced to 20µP. The x-axis (horizontal) is the frequency of a sound on a logarithmic scale. The contour lines are lines of equal perceived loudness for sounds at different frequencies. For example, a sound at a frequency of 30Hz and a measured relative intensity of 80dB re 20µP -- the purple dot -- has the same perceived loudness as a sound at a frequency at 1000Hz and a measured relative intensity of about 30dB re 20µP --the red dot. If people could hear equally well at all frequencies, the contour lines would be flat because the same measured sound intensity would be perceived to be equally loud regardless of the sound frequency. In fact, people do not hear as well at low frequencies. Therefore, the relative sound intensity has to be much greater for a low frequency sound to be perceived to be as loud as a sound at a frequency that we hear well, such as 1000 Hertz.

Equal Loudness Curve. Courtesy Audio Engineering Society and Brian Moore. Modified from Figure 13 of Moore, et al., 1997.

The loudness at which humans can just barely hear a sound is known as the Threshold of Hearing. The bottom red line in the graph is the Threshold of Hearing. We would not hear sounds that are below the threshold level at each frequency. For example, at a frequency of 50Hz, people cannot hear a sound at 30 decibels (dB) re 20µPa -- the blue dot. However, at a frequency of 200Hz and 30 decibels (dB) re 20µPa -- the green dot -- a typical person would have no problems hearing the sound.

At high sound levels there is the Threshold of Pain. Very loud sounds (115-130dB re 20µPa) can actually cause physical pain. Sufficiently loud sounds will also damage a person's hearing.

Marine mammals have a similar response to sound. They hear best at the frequencies that they use. Other marine organisms that use sound also have variable sensitivity to sounds of different frequencies. Marine mammals have thresholds of hearing that differ for each species.

The figure below shows estimated hearing thresholds for several groups of marine mammals and the typical ambient noise (background noise) level in the ocean at different frequencies. It is important to realize that sound intensity given in underwater dB is not directly comparable to sound intensity given in dB in air, in part because different reference levels are used.

Estimates of the hearing thresholds for some groups of marine mammals along with typical ambient noise levels. The y-axis (vertical) for the hearing thresholds is relative intensity in underwater dB. The y-axis for the ambient noise curve is spectral level in 1 Hertz frequency bands with units of dB re 1 µPa²/Hz. The x-axis (horizontal) is the frequency of a sound on a logarithmic scale. (Figure is adapted from Office of Naval Research, 2001. See notes regarding the figure).

All species of baleen whales (Mysticetes), such as blue, fin, and humpback whales, produce low frequency sounds. There are no direct measurements of hearing thresholds for baleen whales. Anatomical evidence and vocalizations strongly suggest that they are adapted to hear low frequencies. Direct studies have been performed on seven toothed whale species including dolphins, beluga whales and harbor porpoises. All species of toothed whales (Odontocetes) hear best in the high-frequency range (10,000 to 50,000⁺ Hz). Pinnipeds (seal and sea lions) have increasing sensitivity from low to high frequencies. Several species of pinnipeds, including harbor seals and elephant seals, have been directly studied.