Sound is a longitudinal wave because molecules disturbed by the source of waves oscillate parallel to the direction of the disturbance, rather than the perpendicular oscillation observed with transverse waves. Energy released by the source of waves results in compression (area of higher pressure) and rarefaction (area of lower pressure) of solids, liquids or gases. Due to these properties, sound will not travel through a vacuum. On a graph, sound appears as a wavy horizontal line in which troughs and crests represent rarefactions and compressions over time.
|A visible pattern of sound waves. This new technique of studying sound demonstrates the focusing effect of an acoustical lens on sound waves issuing from the horn at extreme left. Wave pattern is produced by a scanning technique. – Bell Telephone Laboratories photograph, from the book The First Book of Sound: A Basic Guide to the Science of Acoustics by David C. Knight, Franklin Watts, Inc. New York (1960). p. 80.
Amplitude is the measure of how powerful sound waves are in terms of pressure. Since the displacement of particles of matter is being measured, amplitude appears graphically as the height of a crest or the depth of a trough. A crest, or compression, indicates an area of higher pressure and higher displacement of particles, and a trough indicates an area of lower pressure and lower displacement of particles, or a rarefaction. Pressure is measured in pascals and amplitude is measured in decibels (dBSPL, dBspl or dB(SPL)). The frequency of sound waves influences how loudness is perceived. Consequently, scientists have come up with an adjusted measurement referred to as dBA or A-weighting. Humans can hear sounds between 0 dBSPL (the threshold of hearing) and 85 dBSPL (the threshold of pain), although sounds at 85 dBSPL begin to damage the ears.
|Northeast Pacific blue whale call
Northeastern Pacific blue whale calls are perhaps the best known blue whale call to date.They generally consist of two parts, A and B. The A call is a series of pulses (on the order of 1.5 pulses/s) which often exhibits side-banding and the B call is a long FM moan. Click here (338 kb, 10x) to hear the northeastern blue whale call.
Frequency of Sound
The frequency of sound is the number of vibrations or cycles per second. Hertz (Hz) symbolizes frequency. Sounds audible to humans have a frequency between 20 and 20,000 Hz. As an individual gets older, the range of frequencies and decibels audible usually shrinks or changes. Interestingly, amplitude is not the only factor in how loud sounds seem; sounds near a frequency of 3,500 Hz usually sound louder to humans than those with the same amplitude but a much higher or lower frequency. Above the range of audible sounds to humans is ultrasound and below it is infrasound.
|Rapid vibrations of air molecules create a high-pitched sound (treble); a slower rate of vibration creates a low-pitched sound (bass) (Beggs, Josh and Thede, Dylan, 2001).
Particle velocity is the speed at which the particles in the medium oscillate and is different from the speed of sound. The propagation of the actual sound wave can be faster or slower depending on the temperature, pressure or type of medium the waves are traveling through. Since air is a nearly perfect gas at standard temperature and pressure (STP), air pressure is not usually a factor with the velocity of sound. In dry air, sound travels approximately 1 m per 3 milliseconds.
Perception of Sound
Sounds are usually perceived through the ears, but some loud or low frequency sounds can be sensed by contact with other parts of the body. Animals use sounds to communicate, for music and also to acquire spatial information about their environment. Echolocation, the sending of sounds and interpretation of echoes produced, is used by whales and dolphins to navigate, for orientation and to hunt for food. Bats and humans use sonar to search for food and to acquire spatial information.
Sound Under Water
Surprisingly, sound travels at a speed five times greater under water than in air. However, hearing under water for humans is not as easy as it is in air. Sound volume actually depends mostly on perception by audial organs and the amplitude of sound waves. Sounds can be perceived through air conductivity, using the audial bones of the inner ear or through bone conductivity, using the vibration of bones in the skull. One reason humans can’t hear under water as well as in air is because bone conductivity is prevalent under water and bone conductivity is 40% less effective than air conductivity. Bone conductivity is prevalent because acoustic resistance of water is close to that of human tissue and less energy is lost in the transition of sound waves into skeletal bones under water than in the air. Additionally, the eardrum cannot vibrate under water because the outer audial opening is filled with water. Tonality of the sound also influences the distance that the sound wave travels. Sounds with greater tonality can travel farther distances. Sounds emitted under water usually cannot be heard above water.
There is a layer of water deep in the ocean known as the Sonar Fixing and Ranging Channel (SOFAR) where the speed of sound is very slow. In the SOFAR channel, low frequency waves may travel thousands of miles before weakening. Minimum sound depth is shallower in temperate waters and reaches the surface at approximately 60°N or 60°S. It is thought that the SOFAR channel is controlled by a deep thermocline and a seasonal thermocline, which form a lower and upper barrier that channels sound waves. Humpback whales and baleen whales are thought to be responsible for the strange and mysterious low-frequency sounds in this area. Even more amazing, scientists believe, is that humpbacks actually dive down to this particular channel to communicate or “sing” with other humpback whales many kilometers away.
» Wikipedia: Sound
» Dive Training Magazine: The Underwater Science of Sound