What components of sound are used for hearing?

All vertebrates have inner ears with similar components, but aquatic vertebrates have modified some of these to hear well underwater. Scientists are trying to understand exactly how ears of each aquatic group work.

Sound waves are characterized by compression and expansion of the medium as sound energy moves through it. This represents the pressure component of sound. At the same time, there is also back and forth motion of the particles making up the medium (particle motion). Animals can detect these different aspects of sound. Scientists are trying to unravel exactly which of these aspects of sound different species can detect.

Detection of Particle Motion

Particle motion is described by displacement, velocity, and acceleration. All fishes detect particle motion, while some also detect pressure. (For more on fish hearing see: How do fish hear sounds?)

Most fish tissues have a density that is approximately the same as water and move with the impinging sound wave.

The otoliths (literally, “ear stones”) in the inner ears of fishes are significantly denser than other fish tissues. Because of their higher density, otoliths move with a different amplitude and phase than the rest of the body. It is this relative motion between the otoliths and the fish’s body that bends stereociliary bundles of the hair cells and allows fish to detect sounds.

Some invertebrates, including cephalopods (octopus and squid) and decapod crustaceans (lobster, shrimp, and crab) sense the particle motion component of sound. [1] [2] [3] It has been suggested that this is done using an organ called the statocyst, that primarily detects gravity and motion of the animal.

Detection of Sound Pressure

CT scan of a squirrelfish swim bladder and otolith.
3D Volume Rendering Technique (VRT) reconstruction from CT scans of the swimbladder (teal) and otoliths (red) of an Indo-Pacific squirrelfish. Image courtesy of Darlene Ketten, WHOI/Harvard Medical School. (related publication: Webb et al. 2010)
Some animals are also able to detect the pressure component of sound. Most bony fishes have a swim bladder, a gas-filled structure located in the abdominal cavity. Because gas is much more compressible than water, the swim bladder will expand and contract with the pressure changes that occur within a sound field. Therefore, some fishes sense sound pressure as well as particle motion. Fishes that detect pressure often have a wider hearing range and better sensitivity than fishes that do not use pressure for sound detection.[4] (For more on fish hearing see: How do fish hear sounds?)

It is known that different species of fish vary in their relative use of pressure and particle motion to detect sound. Some fishes that have a swim bladder mechanically coupled to the inner ear are primarily pressure detectors, whereas fishes without a swim bladder only detect particle motion. These are two extremes of a continuum between only detecting pressure, such as goldfish and catfish, and only detecting particle motion, such as flatfish. Most fishes, such as Atlantic cod and salmon, fit somewhere on that continuum.[4]

Some research[5] suggests that pinnipeds and cetaceans are sensitive only to sound pressure in water. It is difficult to be certain if this is true, because all land mammals that have been tested use both particle motion and pressure, and it is difficult to do similar experiments with aquatic mammals. [6] [7] [8]

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  1. Packard, A., Karlsen, H.E., and Sand, O. 1990, "Low frequency hearing in cephalopods." Journal of Comparative Physiology A 166:501-505. 
  2. Budelmann, B.U. 1992, "Hearing in non-arthropod invertebrates." In: Webster DB, Fay RR, Popper AN (eds) The evolutionary biology of hearing. Springer-Verlag, New York, pp 141–155. 
  3. Budelmann, B. U. and Williamson, R. 1994, "Directional sensitivity of hair cell afferents in the octopus statocyst." The Journal of Experimental Biology. 187:245-259. 
  4. Popper, A. N. and Fay, R. R. 2011, "Rethinking sound detection by fishes." Hearing Research. 273 (1–2): 25–36. 
  5. Finneran, J.J., Carder, D.A., and Ridgway, S.H. 2002, "Low-frequency acoustic pressure, velocity, and intensity thresholds in a bottlenose dolphin (Tursiops truncatus) and white whale (Delphinapterus leucas)" Journal of the Acoustical Society of America 111(1 Pt.1):447-456. 
  6. Lynch, T.J. III, Nedzelnitsky, V., and Peake, W.T. 1982, "Input impedance of the cochlea in cat." Journal of the Acoustical Society of America. 72 (1), 108-130. 
  7. Peake, W.T., Rosowskia, J. J., and Lynch, T.J. III. 1992, "Middle-ear transmission: Acoustic versus ossicular coupling in cat and human." Hearing Research. 57 (2): 245–268. 
  8. Brundina, L. and Russell, I. 1994, "Tuned phasic and tonic motile responses of isolated outer hair cells to direct mechanical stimulation of the cell body." Hearing Research. 73 (1), 35–45. 
  • Fay, R. R. 1984, "The goldfish ear codes the axis of acoustic particle motion in three dimensions." Science 225:951-954. 
  • Kastak, D. and Schusterman, R.J. 1998, "Low-frequency amphibious hearing in pinnipeds: Methods, measurements, noise, and ecology." Journal of the Acoustical Society of America 103(4): 2216-2228. 
  • Wartzok, D. and Ketten, D.R. 1999, "Marine Mammal Sensory Systems." Pages 117-175 in Reynolds, J.E. III and Rommel, S.A. (eds.). Biology of Marine Mammals. Smithsonian Institution Press, Washington D.C. 
  • Webb, J.F., Fay, R.R., and Popper, A.N. (eds). 2008, "Fish Bioacoustics." Science+Business Media, LLC, New York. 
  • Webb, JF, Herman, JL, Woods, CF and Ketten, DR. 2010, "The ears of butterflyfishes: “Hearing generalists” on noisy coral reefs?" J. Fish Biol. 77: 1434-1451.