Measurements of Sound Intensity; Using Air Resonators.— The amplitudes at various points in a vibrating air column or cavity may be examined qualitatively by means of small mano metric capsules (fitted with tube to a sensitive flame) or by sensitive membranes carrying fine particles or small mirrors. Quantitative results have been obtained by means of a Rayleigh disc which, when inclined at to the axis of a neck of a double resonator, forms a very sensitive disc tending to set itself at right angles to the vibrating air current in the neck of the resonator (see p. 14.) Alternatively the hot-wire "micro phone" (see p. 31) may be used.
Kundt's Dust Figures.—To exhibit the nodes and antinodes in a resonating air column contained in a glass tube, Kundt devised the method of spreading fine powder (lycopodium seed or fine dry cork dust) along the inside of the tube. The dust is immediately thrown into a pattern indicating the positions of nodes and loops along the tube. The experiment is capable of a number of variations yielding valuable data relative to the veloc ity of sound in gases and solid rods. The wave length X5 in the gas contained in the tube is at once obtained from twice the nodal wave length X in the rod which excites the vibrations in the air column being, for the fundamental longitudinal vibration, equal to twice the length of the rod. These values give at once the relative velocities in the gas and the rod (Seep. 21.) Sirens.—The sources of sound to which we have hitherto referred are dependent on the principle of resonance for their efficient action. We shall now deal briefly with an entirely dif ferent type of sound generator. In the earliest form of siren a revolving disc perforated with a ring of equally spaced holes interrupted a jet of air from the nozzle of a tube mounted oppo site the ring of holes. The fundamental frequency of the note produced by the successive puffs of air emerging beyond the disc is clearly equal to the product of the number of holes and the revolutions per second of the disc. This primitive form is known as Seebeck's Siren In later forms of siren (C. de la Tour's form) the holes were drilled obliquely to the surface of the disc so that the blast of air also produced the necessary rotation of the disc. In this form the intensity of the note increased with sound amplitude being roughly proportional to the pressure of the air supply. In recent
laboratory forms the siren is motor driven, means being provided for indicating the speed of the revolving disc and consequently the frequency of the sound. A motor driven siren forms a very convenient means of determining, by a direct method, the pitch of a note. With a circular nozzle and equal circular holes the siren note is generally impure—containing a plentiful supply of harmonics in addition to the fundamental. Milne and Fowler (Roy. Soc., 98, 414, 1921) describe a special form of siren which gives comparatively pure tones. The holes in the rotating disc are specially shaped so that the uncovered area of the nozzle varies sinusoidally.
Dove and Helmholtz produced sirens having several rings of holes provided with corresponding nozzles, so that one or more tones could be produced simultaneously. Sirens driven by corn pressed air or steam are in common use as sources of sound. Similar devices modified to suit the special conditions have been used for signalling under water—the air supply of the ordinary siren being replaced in this case by a jet of water which causes rotation of the disc and gives the necessary vibratory impulses to the surrounding water.
The Diaphone, a powerful long range signalling device for use in air, is essentially a siren of the same group as those of Seebeck and La Tour since it differs from them only in the fact that the opening and closing of the ports is effected by a recip rocating instead of a rotary motion—this reciprocation being produced directly by means of the compressed air supply.
The Centrifugal Siren.—This device consists of a radially vaned cylindrical rotor revolving in a stationary casing in which ports are cut. The vanes are connected in pairs at their outer edges by cylindrical segments, so that rotation causes opening and closing of the fixed ports. Air is drawn through an axial aperture where the vanes are cut away. The sound is produced by the expulsion of air through the ports by the centrifugal action of the rotor which may be driven by an electric motor or internal combustion engine. Sirens of this type constitute very powerful and efficient sources of sound for long distance trans mission in air, e.g., for use on light-vessels or shore stations as a means of signalling to ships at night or in foggy weather.