Sound

Sound, oscillatory movements that propagate in the form of waves through a material medium; such movements, upon reaching the ear, create irritation in it, which is the cause of auditory sensation (see also Acoustics). For a sound to arise in a medium, there must exist a body performing periodic movements that would serve as a source of waves. If all points of the material medium in which oscillations are excited are interconnected, then the oscillations are transmitted from point to point, and each of them comes into independent oscillations; in this case, the oscillations of each point of the medium lag in phase compared to the oscillations of the source, the more so the farther this point is from the source. The oscillatory movements propagating in the medium are called waves. The frequency of oscillations determines the pitch of the tone, which is the lower the smaller this frequency. Sounds perceived by the ear are usually complex sounds, composed of the most intense oscillation of a certain frequency, which determines the fundamental tone, and a series of oscillations with frequencies that are multiples of the frequency of the fundamental tone. Tones with frequencies that are multiples of the fundamental are called overtones; the timbre of the sound depends on the presence of certain overtones and their relative intensity. A complex of sounds that vary in intensity, pitch, and timbre is noise. - Speed of sound. The distance at which sound waves propagate in 1 sec. is the speed of sound (c). Obviously, c=λ/T. (Values of the speed of sound-see Acoustics.) Since the cause of the transmission of oscillations in the medium are the elastic forces between its particles, the propagation of sound is possible only in an elastic medium. The speed of sound is determined by the ratio of the elasticity of the medium to its density. In solids possessing both elasticity of form and elasticity of volume, longitudinal, transverse, and torsional waves are possible; their speeds are somewhat different. In liquids and gases, which possess only volume elasticity, only longitudinal oscillations are possible, as a result of which periodic condensations and rarefactions occur when waves pass through a liquid or gaseous medium. The speed of sound propagating with the wind is greater, and against the wind is less than normal. Excessively large speeds are observed in the propagation of very strong sound impulses (explosive waves); 56 S

SOUND

in tubes, pulse velocities of up to 14,000 m/sec were observed. However, excessively large velocities are observed only near the source itself (epicenter of explosion, gun barrel)—with distance, the velocity drops to a normal value. The intensity of sound is the amount of energy carried by propagating oscillations through 1 cm² of area in 1 sec.; therefore, the intensity is measured in erg/cm²/sec. From the objectively measured intensity of sound, one must distinguish the intensity of sound perceived subjectively, called loudness, which shows what the relationship is between a given objective intensity of sound and the intensity of sound at the threshold of hearing. The intensity of sound depends on the amplitude of oscillations. Absolute measurement of the intensity of sound can be performed by means of a Rayleigh disk—a light disk suspended by a thin thread so that its plane makes a 45° angle with the direction of the waves: the stronger the sound, the greater the angle of rotation of the disk, which tends to become perpendicular to the direction of the sound. Measurements of the intensity of sound can be made according to the magnitude of pressure exerted by sound waves on an obstacle. Among other methods, the microphone condenser is often used; in it, a very thin metal membrane serves as one of the plates of the condenser with an insulating air layer of several tenths of a mm. A potential difference of several hundred volts is applied to the plates of the condenser; when the light membrane begins to oscillate under the influence of sound waves, a variable electromotive force is created in the circuit in which the condenser is included, due to periodic changes in the capacity of the condenser; by measuring this electromotive force after amplifying it by a cathode amplifier, one can determine the intensity of sound. For subjective measurement of noises, Barkhausen constructed an apparatus in which the intensity of a buzzer (an electromagnet with a buzzing spring-armature) can be measurably weakened. To measure the intensity of sound, the following is established: such a loudness of the buzzer that seems equal to the sound heard by the other ear. The form of the sound wave and absorption of sound. Depending on what the source of sound represents (a point, an oscillating line, or a plane), the waves spreading from it are spherical, cylindrical, or plane. In spherical waves, the intensity of sound decreases according to the law J = 1/r², in cylindrical waves—according to the law J = 1/r, in plane waves it remains constant. However, this happens only in the absence of absorption of sound energy by the medium. In reality, due to internal friction and thermal conductivity of the medium, sound energy is absorbed the more strongly the shorter the wavelength of sound. The absorption of sound waves in water is much less than in air. This provides a basis for replacing air signaling in navigation with underwater signaling. This also shows that there are limiting, shortest waves that can propagate without being absorbed even at very close distances. The shortest waves observed in the air have a wavelength of about 0.2 mm at a frequency of 1,500,000 (Pierce; 1925). Such short waves cannot, of course, be perceived by the ear and are called ultrasonic waves. To create them, use is made of the piezoelectric properties of quartz. A piezo-quartz plate serves as the insulating layer of a condenser, to the plates of which rapidly alternating voltage is applied. Under the influence of an alternating electric field, piezo-quartz periodically compresses and expands; its mechanical vibrations are especially strong when the frequency of the alternating electric field is the same as the natural frequency of the quartz plate, determined by its geometric dimensions (electromechanical resonance). By selecting the dimensions of the quartz and the frequency of the current, one can obtain very intense sound waves with sufficient power. Short waves have the advantage that they can be directed in a certain direction—in a beam, therefore they are used, on the initiative of Langevin, in underwater signaling. Wood (R. Wood; 1927) discovered interesting biological effects of these waves: a hand subjected to the action of powerful short waves (in water) feels pain, paramecia under their action lose the ability to move and die; red blood cells in a physiol. solution quickly disintegrate; small fish and frogs die in one to two minutes; mice are less sensitive to the action of ultrasonic waves and only lose the ability to move; under the action of short waves, very stable emulsions are created, etc. When in air (or another medium) oscillations are simultaneously created by several sources, the movement of each individual particle is the result of the combination of several oscillations, and the entire picture of wave propagation changes accordingly. The results of the combination of waves (interference) can be infinitely varied. Of particular interest are cases of the combination of oscillations of the same period and equal amplitude, propagating in the same direction: when the phases are equal, oscillations occur with double amplitude, when the phases are opposite, they mutually cancel each other. As a result of the combination of waves of the same periods and amplitudes, moving towards each other (as happens when combining progressively moving and reflected waves), standing waves arise; in them, the medium is divided into sections with the largest amplitudes (antinodes), separated from each other by places where oscillations do not occur at all (nodes). The distance between adjacent nodes, or antinodes, is equal to λ/2. Standing waves are used to measure the velocity of sound. When two oscillations with close periods are combined, periodic increases and decreases of amplitude occur—beats. The number of beats per second equals the difference in frequencies f₁ and f₂ of the combining sounds. Beats are easily detected by the ear, and by them, having one source of a definite number of oscillations, one can determine the frequency of sound with great accuracy. The transition of sound from one medium to another. When sound waves pass into another medium with a different propagation velocity, reflection and refraction of sound occur at the boundary according to the laws of optical phenomena. Therefore, sound propagating in the free atmosphere experiences different deviations from a straight line, depending on whether the temperature increases or decreases with height. When sound passes from a medium where its velocity is less (e.g., air) into a medium where its velocity is greater (e.g., water), the phenomenon of total internal reflection, analogous to optical, may occur. Sound is transmitted from one medium to another the better (i.e., the less amount of reflected energy) the closer the "acoustic rigidities" of the media (the acoustic rigidity of a medium is the product of the density of the medium and the speed of sound in it). For the purpose of sound insulation, multi-layered screens made of layers with significantly different acoustic rigidities are used. When propagating in a closed room (auditoriums, theaters, etc.), sound can experience multiple reflections from walls and ceiling, as a result of which sound that has arisen in the room lasts for some time until it weakens to the value of the intensity of sound at the threshold of hearing (reverberation of sound—see Auditorium). With such "accumulation" of sound, speech and music become indistinct. However, in halls devoid of reverberation, musical sounds turn out to be ugly—dry: for music and speech there is an optimum reverberation: for large halls—about 1.6 sec., for small rooms—less. Sources of sound. The sources of sound used can be classified according to their dimensions—having one dimension (string, rod, long tube), two dimensions (membrane, diaphragm) and three dimensions (cubic resonator)—and also according to the method of excitation, which can be mechanical (impact, friction, blowing), magnetic (telephone), electric (electrostatic loudspeakers), thermal (thermophone). The purest tones are delivered by tuning forks, which are therefore used as standards of sound of a definite pitch. In the oscillations of a tuning fork, a series of nodal points are formed, as a result of which overtones are produced, weak compared to the fundamental tone and not harmonic relative to it: the frequencies of the fundamental tone and overtones of the tuning fork are related as 1:6, 2:18, 3:35:58 (Auerbach). A source of sound, especially convenient for measuring the sensitivity of the ear, is the thermophone, consisting of a very thin metal sheet, through which constant and alternating currents pass simultaneously. The periodic heatings created by the alternating current excite expansions and compressions of air, perceived as sound. Knowing the volume of the thermophone chamber, the temperature of the sheet, and the frequency of the current, one can measure the intensity of sound in absolute terms.—As receivers of sound, membranes are usually used, which can be either fixed at the edges or moving entirely (like a piston). In a receiver of sound, the most intense oscillations occur when the frequency of incoming oscillations is the same as the frequency of natural oscillations of the membrane or other receiving device (resonance); however, when it is required that the receiver equally capture oscillations of all frequencies (as is necessary, for example, in a microphone), resonance phenomena should be expressed as little as possible.

Generally, sound receivers can react either to the displacement of oscillating particles (displacement receivers) or to changes in pressure (pressure receivers). In order for the receiver itself not to distort the sound field with its reverse (radiating) action, its dimensions should be as small as possible. Combination tones. The simultaneous existence of several strong sounds (with large amplitudes) is accompanied by the formation of additional combination tones. Helmholtz showed that combination tones are also created during asymmetric oscillations of a body. This explains the appearance of subjective overtones, caused by the asymmetric structure of the eardrum. Every strong tone (even with pure sinusoidal oscillations) is also accompanied by combination tones: therefore a strong tone cannot be pure. This circumstance is important for the loud reproduction of sound (in loudspeakers). Combination tones play an important role in the perception of sound: if the fundamental tone of speech (or another sound) is artificially excluded, it reappears as the sum tone of the overtones that make up the sound of the voice. Therefore, the exclusion of the fundamental tone (and overtones close to it) little distorts the timbre of the voice and little affects the intelligibility of speech, whereas the exclusion of the upper overtones, despite their small energy, strongly impairs intelligibility. When several tones sound simultaneously (a chord), beats are created between the fundamental tones, their overtones, and the accompanying combination tones. As Helmholtz found, many and frequent beats are the cause of dissonance in a chord; the fewer the number of tones that produce beats, the more consonant the chord appears.-3 a п и c ь 3. For the purpose of sound analysis, resort is made to the recording of sound waves, which can be done in various ways. This is usually achieved by recording the oscillations of a membrane set in motion by changes in pressure in the medium. By subjecting the curves obtained in this way to analysis, it is possible to determine the presence and relative intensity of all overtones that make up the sound. The results of analysis are usually expressed in the form of so-called acoustic spectra, where the intensities are characterized by the heights of the vertical segments, and their positions by the frequencies of the overtones that make up the sound.