Acoustics << uh KOO stihks >> is the branch of physics dealing with sound. Acoustics includes the study of how sound is produced, transmitted, detected, measured, and used. It is also concerned with the effect of sound on living things. Sound includes all mechanical vibrations that travel as waves through solid, liquid, or gas.
Sound waves are often described in terms of their frequency, the number of vibrations in a given time. Frequency is often measured in units called hertz. One hertz equals one vibration per second. The higher the frequency of a sound wave, the higher the sound’s pitch. The acoustic spectrum (range of all possible frequencies) can be broken up into three regions: (1) audible sound; (2) infrasound; and (3) ultrasound. The audible part of the spectrum refers to sounds human beings can hear. Infrasound includes sounds too low in pitch to be heard by people (below about 20 hertz). Ultrasound contains sounds too high to be heard (above 20,000 hertz).
Human ears are sophisticated sound receivers. They can distinguish a sound’s pitch, loudness, and direction. Sound waves make part of the ear vibrate. The vibration is converted to a nerve signal, which is sent to the brain.
Acousticians (scientists who specialize in acoustics) use microphones to measure sound in air. Hydrophones measure sound underwater. Accelerometers and geophones detect vibrations. They are often used in engineering design and geological studies. Laser beams can be reflected off an object to detect vibration without touching it. All of these devices produce electrical signals that can be studied with acoustic test equipment, such as oscilloscopes and frequency analyzers.
Because sound is a wave, it behaves much like light. When sound strikes an object, it can be diffracted (spread out), dissipated (absorbed), reflected, or refracted (bent). Polarization describes the direction of the vibrations, in relation to the wave’s direction of travel. Reverberation is heard when a sound is received after reflection from many surfaces. The Doppler effect is a shift in the pitch of sound as its source moves toward or away from a receiver. It can be heard when a train or a vehicle with a siren passes by. Interference is the reinforcing or canceling effect between two or more sound waves traveling through the same space. Over time, interference between two tones similar in frequency can produce variations in volume called beats. Resonance is the vibration of an object at its natural frequency upon being struck by a sound wave of the same frequency.
There are many branches of acoustics. Architectural acoustics deals with making rooms and buildings quiet and providing good conditions for listening to speech and music. Environmental acoustics involves the control of noise pollution (unwanted sound). Underwater acoustics studies sound waves in water. It involves detecting objects underwater using sonar, a device similar to radar that relies on sound waves rather than radio waves. Physiological acoustics is the study of how people hear sounds. Psychological acoustics studies the way people interpret sounds. Speech communication studies the way people produce, transmit, and hear speech. Musical acoustics deals with the way instruments and voices produce sounds. Medical acoustics involves imaging and treating disease with sound. Bioacoustics studies the sounds emitted by animals and the effect of sound on animals.
See also Sound and its list of Related articles.
