E14 Sound Production and Propagation

Topic

Sound production and propagation in fish involve the acoustics of underwater sources, the vibrational mechanics of the fish's biological structures, and the acoustics of the waveguides formed by the ocean itself. Fish produce sound through three main mechanisms: vibration of the swim bladder caused by the contraction of specialized sonic muscles—the most common and powerful mechanism, capable of producing pulses of up to 200 decibels at a distance of one meter; the rubbing together of hard surfaces, such as the stridulation of teeth or spines; and hydrodynamic turbulence generated during the fish's own movement.

When acting as a sound source, the swim bladder behaves like a spherical Helmholtz resonator—that is, a gas-filled cavity with an opening to the surrounding fluid—whose resonant frequency depends on the gas volume and the speed of sound within it.

The force exerted by the sonic muscle on the swim bladder wall generates pressure oscillations that propagate into the surrounding water as compression waves. The fraction of the muscle's mechanical energy that is converted into radiated sound, rather than being lost, depends on the relationship between the size of the resonator and the wavelength of the sound produced: the larger the resonator relative to the wavelength, the more efficiently it radiates sound into the water.

Once the sound propagates through the ocean, it follows the same linear wave equation describing wave propagation in any fluid, with a speed of sound that varies according to water temperature, salinity, and pressure, typically hovering around 1,500 meters per second. In shallow waters, that same sound ceases to propagate freely in all directions and instead travels as a guided wave, trapped between the seabed and the water's surface.

As the sound travels through the water, it undergoes absorption—minimal at low frequencies but significant at high frequencies. This difference is precisely what limits long-range communication among fish to low frequencies (below 1,000 Hz), as only these frequencies can travel a sufficient distance without being lost along the way.

In acoustically active fish, vocalization patterns are typically linked to reproductive and territorial behaviors, and the frequency of these calls is inversely related to the fish's body size: larger fish—with larger swim bladders acting as the Helmholtz resonator described earlier in this model—produce lower-frequency calls than smaller fish.

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