Ocean acoustics is the study of sound and its behavior in the sea. When underwater objects vibrate, they create sound-pressure waves that alternately compress and decompress the water molecules as the sound wave travels through the sea. Sound waves radiate in all directions away from the source like ripples on the surface of a pond. The compressions and decompressions associated with sound waves are detected as changes in pressure by the structures in our ears and most man-made sound receptors such as a hydrophone, or underwater microphone.
The basic components of a sound wave are frequency, wavelength and amplitude.All waves act in a exact similar fashion either transverse or longitudinal waves form.
Frequency is the number of pressure waves that pass by a reference point per unit time and is measured in Hertz (Hz) or cycles per second. To the human ear, an increase in frequency is perceived as a higher pitched sound, while a decrease in frequency is perceived as a lower pitched sound. Humans generally hear sound waves whose frequencies are between 20 and 20,000 Hz. Below 20 Hz, sounds are referred to as infrasonic, and above 20,000 Hz as ultrasonic. The frequency of middle “C” on a piano is 246 Hz.
Wavelength is the distance between two peaks of a sound wave. It is related to frequency because the lower the frequency of the wave, the longer the wavelength.
Amplitude describes the height of the sound pressure wave or the “loudness” of a sound and is often measured using the decibel (dB) scale. Small variations in amplitude (“short” pressure waves) produce weak or quiet sounds, while large variations (“tall” pressure waves) produce strong or loud sounds.
The decibel scale is a logarithmic scale used to measure the amplitude of a sound. If the amplitude of a sound is increased in a series of equal steps, the loudness of the sound will increase in steps which are perceived as successively smaller. A decibel doesn’t really represent a unit of measure like a yard or meter, but instead a pressure value in decibels expresses a ratio between the measured pressure and a reference pressure. On the decibel scale, everything refers to power, which is amplitude squared. And just to confuse things, the reference pressure in air differs from that in water. Therefore a 150 dB sound in water is not the same as a 150 dB sound in air. So when you are describing sound waves and how they behave it is very important to know whether you are describing sound in the sea or in air.
Early acousticians working with sound in air, realized that human ears perceive differences in sound on a logarithmic scale, so the convention of using a relative logarithmic scale (dB) was adopted. In order to be useful, the sound levels need to be referenced to some standard pressure at a standard distance. The reference level used in air (20µPa @ 1m) was selected to match human hearing sensitivity.
A different reference level is used for underwater sound (1µPa @ 1m). Because of these differences in reference standards, noise levels cited in air do NOT equal underwater levels. To compare noise levels in water to noise levels in air, one must subtract 26 dB from the noise level referenced in water. For example, a supertanker radiating noise at 190 dB (re 1µPa @ 1m) has an equivalent noise level in air of about 128 dB (re 20µPa @ 1m). These numbers are approximate, and amplitude often varies with frequency.
Faster than the Speed of Sound…The speed of a wave is the rate at which vibrations move through the medium. Sound moves at a faster speed in water (1500 meters/sec) than in air (about 340 meters/sec) because the mechanical properties of water differ from air. Temperature also affects the speed of sound (e.g. sound travels faster in warm water than in cold water) and is very influential in some parts of the ocean. Remember that wavelength and frequency are related because the lower the frequency the longer the wavelength. More specifically, the wavelength of a sound equals the speed of sound in either air or water divided by the frequency of the wave. Therefore, a 20 Hz sound wave is 75 m long in the water (1500/20 = 75) whereas a 20 Hz sound wave in air is only 17 m long (340/20 = 17) in air.
As we descend below the surface of the sea, the speed of sound decreases with decreasing temperature. At the bottom of the thermocline, the speed of sound reaches its minimum; this is also the axis of the sound channel. Below the thermocline the temperature remains constant, but pressure increases which causes the speed of sound to increase again.
Sound waves bend, or refract, towards the area of minimum sound speed. Therefore, a sound wave traveling in the sound channel bends up and down and up and down and can travel thousands of meters. This “channeling” of sound occurs because of the properties of sound and the temperature and pressure differences at different depths in the ocean. The ocean is divided into horizontal layers in which the speed of sound is greatly influenced by temperature in the upper layers and by pressure in the deeper layers. As temperature decreases, the speed of sound decreases, and as pressure (depth) increases, the speed of sound increases. Sound waves bend, or refract, towards the area of minimum sound speed.
Therefore, a sound wave traveling through a thermocline (a region of rapid change in temperature with depth) tends to bend downward as the speed of sound decreases with decreasing water temperature, but then is refracted back upward as the speed of sound increases with increasing depth and pressure. This up-down-up-down bending of low-frequency sound waves allows the sound to travel many thousands of meters without the signal losing significant energy.
The depth of this varies in different oceans depending on the salinity, the temperature, and depth of the water. At low and middle latitudes, the SOFAR channel axis lies between 600-1200 m below the sea surface.
It is deepest in the subtropics and comes to the surface in high latitudes, where the sound propagates in the surface layer. Scientists often take advantage of the properties of the SOFAR channel. We have learned that by placing hydrophones at just the right depth (that is, at the axis of the sound channel) we are able to record sounds such as whale calls, earthquakes and man-made noise that occur many kilometers from the hydrophone. As a matter of fact, sometimes we can hear low-frequency sounds across entire Ocean basins.
Mating fish take the blame for night-time hum.
Tom Whipple Science Correspondent Vibrating water pipes have been considered and a cement factory investigated, but so far no answer has been forthcom ing regarding the origin of the night- time “Hampshire hum”.
Marine biology, however, has a new theory — that the mysterious drone heard across Southampton may be the sound of fish in search of love. Midshipman fish, which croak to attract mates, have been blamed for a similar hum in Seattle, and Sarah Neen- an, a PhD student at the University of Southampton, argues that their Eng- lish cousins may be doing the same.
“Many fish have been observed to produce vocalisations during reproduc- tion,” said Ms Neenan, who specialises in fish acoustics. “These courtship events tend to occur through the night or at dawn. It is definitely in the realms of possibility that fish are causing the mysterious hum.”
She suggested sand gobies as a potential culprit. “If the noisy species are abundant enough they could keep people awake. Fish sounds are limited by the size of their vocal apparatus and swim bladders, but with enough fish present it could be possible to annoy local residents.” Dr Paul Kemp from the university has not heard the hum but has done a lot of work in fish vocalisations. “If you live in the turbid waters of an estuary it makes sense to be vocal,” he said.
I remember diving off Pigeon Island “Jack Cousteau diving reserve” and clearly hearing the errie sound of distant Humpback Whale singing his haunting song.
Jack Cousteau Marine Reserve https://youtu.be/t0DHEldqfIchttps://youtu.be/POITH02VVrwhttps://youtu.be/Pktln-WWAho