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Sounds in Air

1911 Encyclopedia Britannica

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"{| border=1

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Aeroplane engine

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Frequency (vibrations per

second)

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S. E. 5. .. .. .

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130

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R. E.8. .. .

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90

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F. E. 2B

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70

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Avro.. ... .

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90

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Gotha .

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80

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SOUNDS IN AIR 3 Detection. - The human ear itself is a remarkably sensitive detector of the air vibrations which constitute sound. It is still much superior in this respect to any mechanical device which has yet been produced for recording the vibrations visually. Thus the perception of feeble sounds of necessity depends upon the limitations of audibility, either indirect listening, or with the ear aided by the intervention of an electrical device such as a microphone. The audibility of a feeble sound can be very largely augmented by making use of the principle of resonance, provided that the sound itself approximates to a pure tone. This can be secured, for example, by the use of a Helmholtz resonator applied to the ear in the case of direct listening, and in addition, by tuning the diaphragm receiver when microphonic listening is adopted. It has happened fortuitously that one of the chief sounds in air which it is important to be able to detect, viz. those emitted by aircraft, do contain predominant notes which enable the application of resonance, as above indicated, to increase largely the range of audibility. Typical predominant frequencies (apparently due to engine exhaust) are given in the following table, which relates to the engine running at the usual speed: - 1 Owing to the abnormality of the conditions, it is impossible to follow the usual practices in writing the present article. Experiments on sound, with military ends in view, have been carried out in nearly all the belligerent countries. Comparatively few of the results have found their way into the recognized scientific journals, largely by reason of the secrecy which is still frequently enforced by the various Governments, under whose control most of the work was done. In the circumstances it is not safe to attempt to assign credit to particular investigators, nor is it possible to give adequate references. The present article has been drawn up, therefore, upon broad general lines which, since they fulfil censorship conditions, form necessarily a by no means complete survey; and names have been avoided as far as possible.

2 Lord Rayleigh's work is contained in his Collected Papers (No. 6, 1920). His contributions were numerous between 1911 and 1919, when he died.

3 The information contained in this section is largely drawn from a manual entitled Development of Sounds, kindly placed at the writer's disposal by the British Munitions Inventions Dept.

The following frequencies have been detected in the sounds from the Maybach engines of a Zeppelin airship: Slow speed-27, 54, 108, 1 35, 243. High speed-57, 114, 171, 228.

The operation of the Doppler effect, arising from the relative motion between the aircraft and the observer, prevents the possibility of the identification of the machine by means of the observed frequency, this being liable to change by as much as 20%, according to the speed and direction of flight. An interesting observation which has been constantly made is that the notes of low pitch continue to be heard at ranges where those of high pitch have ceased to be audible. This is in accordance with the theoretical expectation that damping increases with frequency.

The determination of the direction whence a sound arrives is theoretically possible by a variety of methods dealt with below, several of which have been tried in aircraft localization.

(a) Binaural Listening. 4

Lord Rayleigh's experiments (Collected Papers, vol. 5, p. 347) have shown that low-pitched sounds are determined in direction by the observation of the phase difference between the vibrations arriving at the two ears. This principle has been applied in direction-finding, and the effect has been exaggerated by increasing the distance between the two points of reception. The sound is received by two equal trumpets or horns rigidly connected together and capable of rotation about an axis perpendicular to the line joining them. Separate and exactly equal tubes lead from the trumpets to the two ears, respectively, and the apparatus is rotated until the sound under observation appears to come from directly in front. The line joining the sound receivers is then perpendicular to the incident sound stream. An alternative method which dispenses with the necessity of rotating the apparatus is to use a compensator or phase-measurer, which consists of tubes, adjustable in length, inserted between the sound receivers and the appropriate ears, so as to provide a path difference equal to that between the distant source of sound and the two receivers. Adjustment of the tube lengths is made until the impression received is that the sound is neither to the right nor to the left, and the determination of direction is then a matter of simple geometry. In practice the compensator is graduated to give direct angular readings. The practice of binaural listening has verified theoretical conclusions in several important respects. It has been found that it is easier to perceive the direction of a mixed sound, or noise, than a pure note. Apparently it is necessary that the wave train should contain more or less isolated special characteristics whereby the phase difference can be readily appreciated. In the regular sine wave corresponding to a pure tone each vibration is exactly like those which immediately precede and follow it, and the ears are unable to identify corresponding displacements. It is apparently also necessary for successful binaural listening that the two portions of the incident wave which enter the two receivers should be free from subsequent distortion; in particular, that the sound receivers should be as nearly as possible non-resonant for the vibrations in question. Any amplification of the sound which depends upon resonance, therefore, such as the use of Helmholtz resonators already referred to, is incompatible with efficient direction-finding by observations of phase difference.

The construction and arrangement of the receivers used has varied very much in practice. As a typical system, that commonly used in the British army may be quoted, namely, circular cones, 2 to 4 ft. long and of semi-angle 20°, as receivers, placed about 7 ft. apart - a distance which proved to be sufficient for attaining nearly the maximum practical accuracy of setting.

The method is subject to many errors, chiefly those arising from the motion of the sound source, refraction due to temperature inequalities in the air, and the effect of winds. The necessary corrections are tabulated for use in practice.

(b) Sound Mirrors

Some success has been attained in directionfinding by means of concave sound reflectors. The chief limitations have arisen from the question of size, and, consequently, of 4 This method of perception of direction has been largely used also in a connexion which scarcely justifies treatment in a separate section. The geophone is an instrument for direction-finding of sounds proceeding through the earth, and its particular use during the war was for localizing the sounds of picks, etc. used in tunnelling and land mining. It consists of two hollow boxes connected by equal tubes to a stethoscope arranged so that the sounds proceed from the two boxes to separate ears. The boxes are laid upon the ground a few feet apart, and moved about until the sounds of the pick appear to come from straight ahead. It is then known that the sound source is on a line perpendicular to that joining the two geophone receivers, since the sounds arrive through the earth in synchronism. By combining several pairs of geophones separated by considerable distances, the actual position of the pick can be estimated, for it lies at the intersection of the several perpendiculars above specified.

portability. In optics the size of mirrors commonly in use is very great in comparison with the wave-lengths of the light; in the corresponding problem in acoustics it is almost impossible to make them so; and yet this is a necessary condition for the geometrical laws of reflection to apply with accuracy. In the largest sound mirrors - perhaps 20 ft. in diameter - the size is at most only a few wave-lengths for the aircraft sounds under investigation, with the result that the image of a distant sound obtained at the focus proves to be an area much larger than that corresponding to optical calculations. There is therefore no advantage secured by making the mirror paraboloidal instead of spherical, and considerable roughness of the surface is not detrimental. The mirrors were usually made of concrete, and listening was effected either by means of a small horn receiver placed in the focal plane and connected by a tube to the ears, or by means of a microphone placed in a similar position. If, as was more usual, the mirror was fixed, the direction of the sound source could be found by determining the position of maximum intensity in the focal plane. It ma y be noted that in this method of directionfinding amplification is obtained on account of the area of the mirror, and that further augmentation is attainable by using resonators, to which the same objections do not apply as in binaural listening. The accuracy of the determinations vary very much with frequency, being much greater for notes of high pitch than for low, as would be anticipated from considerations of wave-length.

(c) Interference and Diffraction Methods

There have been many attempts to apply the principle of interference as a substitute for binaural listening, i.e., by ultimately mixing the sounds entering the two receivers, instead of leading them to different ears, and adjusting the compensator until the total sound heard is as loud as possible. Theoretically this will occur when there has been provided in the compensator a difference of path equal to the path difference outside the receivers. The method has not proved very successful, for a variety of reasons, some of which are obscure. We shall not elaborate them here.

On the other hand, remarkable results have been obtained by the application to sound waves of a phenomenon well known in the diffraction of light. A small distant source of light gives in the middle of the shadow of a small circular obstacle a luminous region, called the " white spot," arising from the diffraction of light round the edges of the obstacle. The same phenomenon is observable in sound under suitable conditions. Thus a large horizontal disc, at least 20 ft. in diameter, and made of material which either reflects or absorbs sound, will give below itself a sound shadow of a sound source, such as an aeroplane, above it. Near the centre of the shadow, in a position depending on that of the source, there is a region where the sound heard is comparatively loud - in many cases much louder than it would be if the disc were absent. The relation between the direction of incidence of the sound and the position of maximum intensity has been calculated, and the method provides, perhaps, the most reliable means of perceiving the direction of air-borne sounds.

(d) Sound Ranging

This special military aspect of the localization of sound sources, viz. those arising from gun-fire and shell bursts, is dealt with in the article Range-Finders.

2. DETECTION AND PERCEPTION OF DIRECTION OF