Acoustics is the science of sound. It can be divided into various branches, but the boundaries between the various branches are often ill-defined. Knowledge of the physical principles of acoustics has practical application in planning recreational and other developments to reduce the adverse effects of noise, but, because of its importance as a modern environmental health problem, noise and its control are treated as a separate branch of acoustics.
Branches of Acoustics
Physical acoustics encompasses propagation and absorption of sound at all frequencies in air and other gases, liquids, semi-solids and solids. It deals with airborne, audible sound, infrasound and ultrasound. Physical acoustics includes both linear processes such as the propagation of sound from traffic, and non-linear processes such as the shock waves that are generated by planes flying faster than the speed of sound.
Sound waves in gases generate alternating small increases and decreases in pressure about the equilibrium value that propagate through the gas; the molecules oscillate back and forth along the direction of propagation. This kind of propagation is called a longitudinal wave. In air, these pressure variations stimulate the ear, which sends signals that are processed by the brain as sound. The velocity of sound in air increases with increasing temperature; at normal room temperatures around 22°C, the velocity of sound is about 345 metres per second. The velocity of sound is generally greater in liquids and solids than it is in gases. The sound speed in water, for example, is about 1500 metres per second at ordinary temperatures. In materials such as concrete and steel, it is higher still.
In solids, several different types of acoustic waves, such as shear waves (which cause changes in shape but not in volume) and surface waves, occur. Longitudinal waves can also propagate in solids. In plates, rods and beams, bending waves are important. Seismic waves (whether on Earth or the moon) have surface and shear components and are a legitimate part of acoustics. Although in everyday language the word "sound" is mostly used to refer to sound in air, propagating vibration in solids is an acoustical phenomenon. Sound travelling in the solid materials of a building is called structureborne sound.
Canadian Acoustics Research
In Canada, acoustics research takes place in government laboratories (eg, National Research Council), the universities (eg, Sherbrooke, Toronto) and industry. Canadian research into the transmission of sound in the atmosphere has been extensive; areas studied include the effect of temperature gradient, turbulence, obstruction, ground reflections and ground impedance. This research has applications in urban and regional planning, for locating buildings and for sound barriers near expressways, airports, railways, etc.
More esoteric work, such as measuring the velocity and absorption of high ultrasonic sound in various liquids to deduce the detailed architecture of their constituent molecules and the energy needed to change their structure, has application in various fields of engineering. Such work is related to the use of sonar and radar techniques to determine the structure of media (water and air) and the position of objects in them, to seismic techniques used in petroleum exploration and to the use of ultrasound in medicine.
The effect of the complex shape of the outer ear on transmission of sound has been studied by segments of the record player, radio and television receiver industry concerned with the development of high-fidelity earphones. Industrial work has also been done on the effect of room shape and furnishings on music reproduced through loudspeakers. Means have been developed for assessing loudspeakers subjectively in normal rooms and correlating the results with standard, objective anechoic-chamber (ie, echo-free) measurements.
Engineering Acoustics deals with the development of devices to generate (eg, loudspeakers), record (eg, microphones) and analyze (eg, frequency analyzers) sound of all kinds. The field of sound production, recording and reproduction, with all its attendant electronics and measuring instruments, is an important part of engineering acoustics.
Architectural Acoustics is concerned with sound in buildings. One aspect of this field is the control of sound within rooms to maximize the acceptability of music or intelligibility of speech. This branch of architectural acoustics deals with sound in lecture or Concert Halls, meeting rooms and classrooms. Sound production, by one or more live players, vocalists or lecturers, whether assisted by loudspeakers or not, is greatly affected by the character of the room in which it takes place. The correct choice and placement of sound-absorbing and sound-reflecting materials in a room and the level of noise in it are critical to obtaining an acceptable space. Properly placed reflectors can direct the sound to where it will do most good. Properly placed sound-absorbing materials will prevent reflections that might cause unpleasant echoes or cause sounds to interfere and cancel each other out. In rooms that are too reverberant, it is difficult to understand speech. In rooms that are not reverberant enough, music will sound dead and lifeless.
The reverberant characteristics of a room are described by its reverberation times which vary with frequency; reverberation time is usually longer at frequencies around 100 Hz than it is around 5000 Hz. In concert halls, the average reverberation time is around 2 seconds. In homes, it is around 0.5 seconds. In a highly reverberant space with hard surfaces, such as a swimming pool, the average reverberation time can be several seconds unless material is added to absorb sound.
Another concern of architectural acoustics is sometimes called building acoustics and deals with the reduction of noise transfer between rooms in buildings.
Musical acoustics considers the workings of traditional, experimental and electronic musical instruments. The interaction of musicians, instruments, listeners and performance spaces means that many branches of acoustics influence work in this field. Why, for example, does a Stradivarius violin sound the way it does and why do people like that sound?
Canada is the birthplace of the world's first music synthesizer, constructed by Hugh LE Caine in 1945. Le Caine called his instrument the Electronic Sackbut. It combined tape recordings of real sounds with electronically generated ones.
Psychological acoustics studies the brain's signal-processing function, which takes nerve impulses from the ear and interprets them. One practical application of this field is the study of the elements important to achieving a stereophonic effect. Another is the determination of those factors that make one sound unpleasant or annoying and another the reverse. In offices, the sound from ventilation systems can provide a neutral, fairly low level of background noise that helps to mask intruding sounds from adjacent workstations. Other sounds with approximately the same loudness are annoying, such as an irritating hiss, whistle, rumble or roar. There is no direct correlation between loudness and annoyance.
Physiological acoustics deals with models and theories of the operation of the ear and its anatomy. Cochlear mechanics, the physiology of hair cells, the auditory central nervous system and the effects of noise and trauma on the auditory system are among some of the areas studied.
Bioacoustics studies all aspects of acoustic behaviour in animals and biological media in general. This field includes such topics as sound production by animals, biosonar, sound reception by animals, effects of noise on animals and medical diagnostics using acoustics, especially ultrasonics.