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Aviation History
1917
1917 - 1080.PDF
OCTOBER 18, 1917. SOUND WAVES. By S. T. G. ANDREWS, B.Sc. (Eng.), Lond THE following article, which sets forth with admirable clearness the fundamental principles a good basis for a discussion of'the reasons for the change: in pitch of the sound of j^ According to fundamental laws the note should, as pointed out in the,a ride rise as fall as the machine travels away. ^^^/^ff'^^ZS^^ h Wih h pt tl quite certain thai the reverse islh the noise of an aeroplane fndtl lM« C«5«. With the present article as a basis it should P/J^J^^ Zm a single source must follow the fundamental laws,appears to differ. We say appears, as the noise ^f'/^t/foZcZesioTdents to state most emphatically that the sound falls Personally we*are inclined to think that the reasons that have led some covresponuen * u . rnm/nq/>ll nr tmn ma4n no(es one with the "approach and rises unth the_ retreat are that ^jeneralnoU^^^^ ThTUgtr'Ze P^dheard machine.above the lower. A slhecoming from the engine and the other from the passage through the air oj ine iprobably carries farther than the deeper one ; therefore at a distance the higher noie is near, ™* 27uuToweve7Tke to Year machine approaches the lower note becomes more audible and finally predominates the higher. We should, however, like other correspondents' views.—ED. As the question as to whether there is or is not an increase The loudness of th/ * "7^^°° StrCetS, - , *• .• __. 4i4.^j „,. „,, O»™T^T, O annrnar.hPK an is usually unnoticed by the average Londoner.As the question as to hof pitch in the note emitted as an aeroplane approaches an observer appears to be of somewhat general interest, it has occurred to the writer that a few notes on the propagation of sound waves will be generally appreciated by readers of i th averag The simplest types of waves are transverse and longi- tudinal. - Let the arrow in Fig. 1 represent the direction in whichthe waves are travelling. If we divide up the medium h tlli b f is" In'ofd'er to prevent misconceptions arising from a loose or through which the waves~are travelling by ^sJ^^improper use of terms, the exact significance of the terms of equidistant lines I 2, 3, 4, 5. 6- 7,• &c., then the motion ol usePd will be defined in all cases. The ear is the organ by each of these successive layers would be up and down per- means of which we appreciate the sensations of sounds, pendicular to the arrow in the case of ^7^^^^ This organ is a very complex structure, and as authorities and to and ^ytV^\^^^^^^^°\^^^torgan is a very compldiffer as to the exact manner in which the various parts act, no attempt at a description will be made here. We , T-„ • ., * ~ - M^ja_ know, howeve?, that we can readily distinguish with our of representing air waves will probably enable the reader ears the sound sensations of pitch, quality and loudness. to realise much more clearly and effectively the r Pitch is the difference existing between different notes, as dition^reahsed m the various g%g^a%^1 JK^O* 1 1 1 1 1 1 1 1 1 1 / 23*56789 10 p g waves. It can be shown that sound waves in air are of the latter, that is the longitudinal, type. The usual graphic method of representing air waves will probably enable the reader to realise much more clearly and effectively the exact con- i f i FJG I. r Length yTTfTfrs /. >K< FIG. 2 *. J K L ?My* *,J ... ...1. L »s£ O F * Wave Length A, » /* X FIG. 3. FIG. for example, between a bass note and an alto note. The former is said to be lower in pitch than the latter. All musical sounds have a definite pitch, but some sounds, such as street noises, gun reports, &c, have no definite pitch. In the first case there is a vibrating source which sends out regular waves, and by experiment we can show that the pitch of the note heard by the ear depends upon the number of waves reaching the ear per second. The number of waves which reach the ear per second is termed the frequency of the sound, so that the same pitch will always correspond to the same frequency, whatever may be the source of the sound. There is, however, a considerable difference between the note produced by, say, a piano and the same note as produced by the human voice. They are both of the same pitch and frequency, but they differ considerably in quality. We shall see later how sound waves can give rise to notes of the same pitch and frequency, but of variable quality. Loudness of sound sensations depends upon the amplitude of the waves, and is largely influenced by accompanying conditions. For example, the air-raid warning syrens recently tried in Central London Would have been thought to be very loud indeed by those in the immediate vicinity, but their sound sensation Was entirely lost amidst the traffic noises always prevalent in London during the daytime. of the particles in the direction of motion of the sound waves before disturbance. Then, at some instant when the wave is passing, the positions of these particles will be displaced into some such positions as A, b, c, d, E, /, g, h, &c, where distances like B6, Cc, Dd, /F, gG, AH, &c, represent the displacement of the particles from the positions originally occupied. At B, C, D, &c, ordinates are erected perpendi- cular to OX and proportional to Bfc, Cc, Dd, &c, respectively. These ordinates are drawn above OX for forward displace- ments such as Bb, and below OX for backward displacements such as/F. By joining the ends of these ordinates we obtain a curve as shown in Fig. 2. It must be clearly remembered that the ordinates of this diagram represent forward or back- ward displacements in the direction of OX, and that they are merely drawn at right angles to OX for convenience. Bearipg this in mind, longitudinal sound waves in air can be graphically represented as shown in Fig. 2. It will be observed from Fig. 2 that at some point such as I the wave is about to repeat itself. The distance Al is called the wave length and is denoted by A. If the velocity of the waves forward is V and n is number of waves which pass a given point per second, then a length V of the displacement diagram would contain n waves. Hence we have the relationship that V — n\. To an observer stationed at I, n represents the frequency 1080
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