Ocean acoustics is the study of sound and its behavior in the marine environment. Sound waves are created when underwater objects vibrate, producing pressure waves that compress and decompress water molecules as they propagate. These waves radiate outward in all directions, similar to ripples on a pond’s surface. Hydrophones, or underwater microphones, detect these compressions and decompressions as changes in pressure.
The fundamental characteristics of a sound wave are frequency, wavelength, and amplitude.
Frequency refers to the number of pressure waves passing a fixed point per unit of time, measured in Hertz (Hz). Humans perceive higher frequencies as higher-pitched sounds and lower frequencies as lower-pitched sounds. The human ear can typically detect sounds between 20 and 20,000 Hz. Sounds below 20 Hz are infrasonic, while those above 20,000 Hz are ultrasonic. For reference, middle C on a piano has a frequency of 246 Hz.
Wavelength is the distance between two consecutive peaks of a sound wave. It is inversely related to frequency, meaning that lower frequencies have longer wavelengths.
Amplitude describes the intensity or “loudness” of a sound wave, often measured in decibels (dB). Small amplitude variations correspond to weak or quiet sounds, while large amplitude variations produce strong or loud sounds.
The figure illustrates waves having same amplitude but different frequencies
The decibel scale is a logarithmic scale used to quantify sound amplitude. A constant increase in sound amplitude results in successively smaller perceived increases in loudness. A decibel expresses a ratio between a measured pressure and a reference pressure, rather than a direct unit of measurement. The decibel scale is based on power, which is amplitude squared. Furthermore, the reference pressure differs between air and water. Consequently, a 150 dB sound in water is not equivalent to a 150 dB sound in air. Therefore, specifying the medium, air or sea, is crucial when describing sound waves.
| Amplitude of Example Sounds | In Air (dB re 20µPa @ 1m) | In Water (dB re 1µPa @ 1m) |
|---|---|---|
| Threshold of hearing | 0 dB | — |
| Whisper at 1 meter | 20 dB | — |
| Normal conversation | 60 dB | — |
| Painful to human ear | 130 dB | — |
| Jet engine | 140 dB | — |
| Blue whale | — | 165 dB |
| Earthquake | — | 210 dB |
| Supertanker | 128 dB (example conversion) | 190 dB |
Note on Acoustic Noise Level Units: Hydrophones measure sound pressure in micropascals (µPa). Because human ears perceive sound differences logarithmically, a relative logarithmic scale (dB) was adopted. Sound levels are referenced to a standard pressure at a standard distance. The reference level in air (20µPa @ 1m) corresponds to human hearing sensitivity. A different reference level is used underwater (1µPa @ 1m). Consequently, noise levels in air do NOT equal underwater levels. To compare them, subtract 26 dB from the underwater noise level. For instance, a supertanker radiating noise at 190 dB (re 1µPa @ 1m) has an equivalent noise level in air of about 128 dB (re 20µPa @ 1m). Note that these numbers are approximate, and amplitude varies with frequency.
Sound Speed: Faster in Water
Wave speed describes how quickly vibrations travel through a medium. Sound travels faster in water (approximately 1500 meters/second) than in air (approximately 340 meters/second) due to the different mechanical properties of water and air. Water’s greater density and incompressibility enable faster sound transmission. Temperature also influences sound speed; sound travels faster in warmer water. Wavelength and frequency are related: lower frequencies have longer wavelengths. The wavelength of a sound equals the speed of sound in either air or water divided by the wave’s frequency. For example, a 20 Hz sound wave has a wavelength of 75 meters in water (1500/20 = 75) but only 17 meters in air (340/20 = 17).
The image displays hearing a scale of various frequencies
As depth increases, temperature decreases, causing a corresponding decrease in sound speed. The minimum sound speed is reached at the bottom of the thermocline, which is also the axis of the sound channel. Below the thermocline, temperature remains constant, but increasing pressure causes sound speed to increase again. Sound waves bend, or refract, toward areas of minimum sound speed, leading to sound waves traveling within the sound channel bending up and down over long distances.
The Deep Sound Channel (SOFAR Channel)
Sound in the ocean can be trapped within the deep sound channel, also known as the SOFAR (SOund Fixing And Ranging) channel, enabling it to travel vast distances with minimal signal loss. The SOFAR channel was discovered when researchers found that the acoustic energy from small explosive charges deployed in the deep ocean could travel long distances. Hydrophone arrays could then be used to locate the source of the charge, aiding in the rescue of downed pilots far out at sea. Low-frequency sound, in particular, can travel thousands of kilometers with very little attenuation.
Ocean acoustics offers scientists valuable tools for quantitatively describing sound in the sea. By analyzing the frequency, amplitude, location, and seasonality of underwater sounds, we can gain significant insights into the marine environment and its inhabitants. Hydroacoustic monitoring allows scientists to measure global warming, detect earthquakes and magma movement during volcanic eruptions, and record the low-frequency calls of large whales across the globe. As our oceans become increasingly noisy, the field of ocean acoustics will continue to grow in importance. By understanding how sound travels through water, we can better assess and mitigate the impacts of noise pollution on marine life.
