ACOUSTICS (Greek akustikos — acoustical) — the doctrine about a sound; the section of physics studying properties, emergence, distribution and reception of elastic waves in gaseous, fluid or firm media.
And. — one of the most ancient fields of physics — arose in connection with the need to offer an explanation for the phenomena of hearing and the speech. So, still Empedocles (490 — 430 BC) explained distribution and perception of sounds by the movements of the special (thin) substance, the coming from sounding body and getting into an ear. Aristotle (384 — 322 BC) already understood that the sounding body causes compression of N of depression of air, and managed to explain process of emergence of an echo. It accurately differentiated height, force and a timbre of a sound and connected them with distinctions in the speed and amount of moving air and with the device of the voice device. Pythagoras (6 century BC) the first formulated laws of fluctuation of strings.
A stage in development And. there were Galilei and Mersenn's works (17 century) which established quantitative laws of fluctuation of strings and the first determined the speed of a sound in air. Gassendi (17 century) established that the speed of a sound does not depend on its height. Brothers Weber (1825) and Savart (1820) showed that distribution of a sound in liquids and elastic bodies is made under the same laws, as in air. In 1863 there was Helmholtz's book «The doctrine about sound feelings», and in 1877 — 1878 — Rayleigh's work «The theory of a sound».
Helmholtz explained the physical nature of sounds, proceeding from the method of the analysis of sounds (Helmholtz's resonators) developed by it, explained perception of a sound with laws of physics.
New stage of development And. began in connection with development of the electronic equipment, creation of electronic amplifiers, finding of new ways of generation of sounds up to very high frequencies (millions of fluctuations a second). Especially intensively And. began to develop in connection with a problem of radio - and telecastings.
Modern And. it is possible to subdivide into the general, or theoretical, physiological, medical, musical, architectural, technical and atmospheric; mark out also electroacoustics and hydroacoustics.
The general, iliteoretichesky, acoustics studies (theoretically and experimentally) processes of emergence and distribution sound (see), and also methods of acoustic measurements.
The fluctuating body (a source of fluctuations) creates in the environment of a zone of alternate increase and reduction of pressure, extending in different directions in the form of elastic vibrations (waves) with the speeds determined by properties of the environment in a cut they extend. E.g., rate of propagation of elastic waves in air at t ° 0 ° makes 331 m/s, in water — 1440 — 1500 m/s, in a bone tissue — 3380 m/s. Elastic vibrations are characterized by the frequency of fluctuation (f), wavelength (λ), intensity of fluctuation (I). Frequency of fluctuations is defined in hertz (Hz); 1 Hz is equal to one fluctuation a second. If the frequency of elastic vibrations is in limits of 16 — 20000 Hz, then they are perceived by an acoustic organ of the person in the form of a sound, height to-rogo is defined by the frequency of fluctuations; at the same time to bigger frequency there correspond more high-pitched sounds.
Sound intensity is defined by intensity of a sound or amount of the sound energy proceeding through 1 cm 2 for 1 sec. Intensity of fluctuation is maximum at a source of fluctuations, decreases with distance.
Fluctuations are lower than 16 and higher than 20000 Hz (with deviations in this or that party) by an ear of the person in the form of sounds are not perceived and carry the name infrasounds (see) and ultrasounds (see). At the same time the person through bones of a skull is capable to perceive ultrasounds with a frequency about 100000 — 150000 Hz. Infrasonic fluctuations can be perceived by an organism vibrotaktilno (see. Vibration ). Borders of perception of sound waves significantly differ in animals from the specified figures (e.g., Guinea pigs, hamsters and some other animals perceive sounds with a frequency up to 100000 Hz).
Physiological acoustics studies physics and biophysics of acoustic organs and the speech, and also an effect of action of elastic vibrations as the last are capable to render on biological objects (including and on an organism in general) mechanical, thermal and physical. - chemical influences. At the same time intensity of sound energy and frequency are important. So, e.g., at intensity of a sound of an order 10-4óÔ/cm 2 there comes the sensation of pain. The intensive sounds lying even below a threshold of sensation of pain harmfully affect on health and working capacity. Long action of strong noise can bring to relative deafness (see), sometimes to to deafness (see) or to specific damage of an acoustic organ as a result of influence of sounds of excessive force (see. Acoustic injury ). At the same time sensitivity of an ear of the person to sounds of various height is not identical. The greatest sensitivity the ear has 1000 — 3000 Hz to tones.
Elastic vibrations of various frequency ranges cause specific influences, however for all frequency ranges there is the general in the nature of their action: 1) at small intensivnost sound impact on biological substrate is practically absent; 2) at average intensivnost impact of elastic vibrations causes mechanical, thermal and physical. - chemical changes; 3) at big intensivnost in biological substrate there are irreversible changes leading sometimes to death of an organism (see. Sound , biological effect of action of sounds of big intensity).
Medical acoustics, using receptions and methods physiological And., investigates and finds possibilities of use of elastic vibrations in applied medicine (diagnosis, therapy, surgery).
Special attention is paid to studying of the elastic vibrations arising in a human body during the work of his internals and circulatory system (e.g., mechanical action of the heart, lungs, pulse waves etc.). These researches conducted in the conditions of norm and pathology form a basis of creation of acoustic devices and devices, and also some methods of a research (e.g., auscultation, pneumography, a phonocardiography). For diagnosis of diseases of an acoustic organ, and also a research of the acoustic analyzer the external sound generator is used (see. Audiometriya , audiometer).
One of sections of use of sound vibrations in medicine are devices for prosthetics of the voice device and correction of hearing of the patient (see. Hearing aids ).
Ultrasound is especially widely applied. It is used for therapy, providing high performance of medical action, is more and more widely applied in the diagnostic purposes, supplementing a X-ray analysis. Ultrasound found application in surgery that is caused by ease of receiving powerful ultrasounds, in need of a type of posterior pyramids with a possibility of their focusing is similar to optical beams. It is used at treatment of some diseases of a brain when it is necessary to nekrotizirovat locally fabric (intensity of each of sent to the set point of beams of ultrasound is insufficient to cause any pathological change, but in focus their total intensity is sufficient to nekrotizirovat fabric).
Ultrasounds have the expressed bactericidal properties that found application, e.g., at sterilization of milk, canned food etc. Ultrasound is applied also during the cleaning of tools (on the basis of the phenomenon of cavitation), in particular surgical, and first of all hollow injection cannulas (in more detail about use of ultrasound in medicine — see. Ultrasound ).
One of aspects of practical application of results of researches in the area A. the dignity is. rationing noise (see). Noise level and its spectral analysis measure by audio-noise meters and analyzers of a range of sounds. On the basis of the special works considering harmful action of noise on a human body marginal norms of noise are developed for various conditions. Similar works are carried out also in the field of sanitary rationing vibrations (see).
Main types of use of acoustics in medical practice see above.
Architectural acoustics studies sound processes in the enclosed space from the point of view of ensuring good audibility of the speech and music in all points where there can be listeners, etc.
Atmospheric acoustics is engaged in hl. obr. studying of patterns of distribution of a sound in the free atmosphere.
Technical acoustics considers generally practical possibility of appendix A. to technology of transfer of separate sounds, the speech and music that is connected by hl. obr. with problems of transformation of sound energy to electric; therefore technical And. quite often call electroacoustics. Technical And. along with the general, or theoretical, deals with issues of creation of the measuring, accepting and transferring equipment.
Special section technical And. the hydroacoustics studying distribution of sound waves and beams in fluid medium, and first of all in water makes.
Bibliography: Beranek L. Acoustic measurements, the lane with English, M., 1952; Krasilnikov V. A. Sound and ultrasonic waves in air, water and solids, M., 1960; Lamb G. The dynamic theory of a sound, the lane with English, M., 1960; Paul R. V. Mechanics, acoustics and the doctrine about warmth, the lane with it., M., 1971; Strett D. V. (D. V. rayleigh), the Theory of a sound, the lane with English, t. 1 — 2, M., 1955; Skuchik E. Bases of acoustics, the lane with it., t. 1 — 2, M., 19 58 — 1959; Morse P. M. a. Ingard K. U. Theoretical acoustics, N. Y. and. lake, 1968.
L. A. Vodolazsky, A. A. Chevnenko.