ACOUSTICS. 



ACOUSTICS. III. 



KI.KKA. 1!<N 





B 



Pig. 13. 



RK80NANCK REFLECTION OF BOUND ECHO 



OF BOUND. 



WE must now inquire as to the rate at which sound travels 

 through tho nir, and wo shall then be able to calculate tho 

 f tho wave* produced by any given note. This inquiry 

 la rather a difficult one, an there are many disturbing causes, 

 such as the temperature of the air, the amount of watery 

 vapour present in it, and tho pressure as indicated by a baro- 

 meter. A calm night is usually selected for the experiment, 

 a* the air ia then much more quiet. Two etations of observa- 

 tion uro chosen, several miles apart, 

 but so situated that each can bo 

 seen from tho other. Cannons or 

 guns are then discharged at regular 

 intervals of about ten minutes, and, 

 since the passage of light is practi- 

 cally instantaneous, the moment of 

 firing is thus seen, and the distant 

 observers note very accurately, by 

 means of a chronometer, the interval 

 between seeing the flash and hearing 

 the report. The true distance be- 

 tween tho stations is then measured, 

 and, dividing this by the number of 

 seconds, the velocity of the sound ia 

 ascertained. In an experiment of 

 this nature tried in France many 

 years ago, tho distance between tho 

 observers was 20,354 yards, and, 

 as the mean of several observations, 

 the time occupied by the sound in 

 travelling this distance was found 

 to be 54-6 seconds. This gives a 

 velocity of 1,118 feet a second, when tho air is at 60, that 

 being the temperature during the experiment. As the tem- 

 perature increases, tho speed increases likewise at about the 

 rate of a foot a second for every degree. Generally, then, we 

 may state the velocity of sound in the air at 32 to be 1,090 

 feet a second, and to increase one foot for every degree that 

 the temperature is raised. In other gases the velocity of sound 

 is somewhat different : wo can, however, easily determine it, 

 since it is found to vary inversely as the square roots of their 

 densities. Hydrogen, for example, is sixteen times less dense 

 than oxygen, and sound travels through it at four times the 

 speed. An increase of density thus serves to diminish the velo- 

 city, and this is 

 why sound travels 

 more slowly in air 

 at a low tempera- 

 ture. 



In water, the 

 sound-waves are 

 propagated at a 

 rate of about 4, 700 

 feet a second, and 

 many solids con- 

 vey them much 

 more rapidly; 

 along an iron rod, 

 they travel nearly 

 17,000 feet in the 

 same time. A 

 good illustration of the different rates at which gases and 

 solids conduct sound may be observed by standing near a 

 long iron railing, and getting a friend at a distance to strike 

 it a violent blow. Two distinct sounds will be perceived, the 

 first caused by the vibrations conducted along the railing, 

 while the other has travelled through the air, and hence arrives 

 considerably after the first. In blasting operations, two con- 

 cussions are often heard, from a similar cause, the one being 

 conveyed by the solid rock, and the other transmitted through 

 the air. 



In substances which exhibit a fibrous or crystalline structure, 

 the sound travels in different directions at different speeds. 

 Along wood, for instance, it is conveyed in the direction of the 

 fibres nearly fotir times as rapidly as across them. 



149 X.K. 



Having now ascertained the Telocity at which sound travel*, 

 we can easily determine the length of the sonorous ware*. It 

 U, however, important for us first of all to obtain a dear idea of 

 their nature. In water, each wave consists of an elevation and 

 corresponding depression, and the length is measured from 

 crest to crest. In sound-waves, we have in place of these on 

 area of condensation and one of rarefaction, and the length u 

 measured from one centre of compression to the next. 



Now sound, as we have seen, travels 1,120 feet a second in 

 air at the temperature of 60, and a C tuning-fork that is, one 

 sounding the note an octave above middle C produces 612 

 vibrations in the same time. Dividing 1,120 by this, we find 

 the length ot the waves produced by 

 that note to be about 2 feet 2 inches. 

 An octave lower, the waves ore about 

 double the length, or about 4 feet 

 4 inches. 



This calculation may easily be 

 verified by the student in a rather 

 remarkable way, and in doing so he 

 will obtain a good illustration of the 

 manner in which a sound may be 

 increased by resonance. 



Take a tall glass jar, A (Fig. 12), 

 and having struck the tuning-fork, 

 B, hold it over the mouth of the jar, 

 as shown. The sound will probably 

 bo unaffected. Now gently pour in 

 water from a jug, making as little 

 splash as possible ; when it attains t 

 certain height, the sound will bi 

 found to burst suddenly forth with 

 greatly increased power. On pouring 

 more water in, the sound sinks again 

 to its former intensity. Ascertain, 



by repeating the experiment, the exact point at which the 

 maximum intensity is attained, and then measure its depth 

 from the top of the jar. If we are using a C fork, we shall find 

 this depth to be 6.J inches, or just one-fourth the length of the 

 wave. The return wave, therefore, is exactly synchronous with 

 tho return vibrations of the fork, and thus tie sound is greatly 

 increased and swells out with augmented intensity. When the 

 water is at a different level the vibrations interfere with one 

 another, and clash to a certain extent. 



The manner in which the power of any sound is increased by 

 resonance is well shown by an apparatus devised by Savart, and 

 shown in Fig. 13. A large open-mouthed bell, A, is set in 



vibration by draw- 

 ing a violin-bow 

 across its edge. 

 Close to it is a 

 hollow cylinder, 

 B, the length of 

 which can be ad- 

 justed by means 

 of a sliding tube. 

 This cylinder is 

 mounted on a 

 universal joint, so 

 that it can be 

 turned in any di- 

 rection, and its 

 distance from A 

 can be adjusted 

 by means of the slide, c, on which it ia carried. The intensity 

 of the sound will now be found to be greatly affected by the 

 position of B. When the vibrations have almost ceased, so that 

 the bell is nearly inaudible, the sound will at once swell out, on 

 properly placing the cylinder. The air contained in B is made 

 to vibrate in unison with the bell, and hence the greatly in- 

 creased power of the sound. 



It is stated that in ancient times large metal vessels were 

 placed in theatres upon the stage, in order to increase, by their 

 resonance, the power of the actors' voices. In the present day 

 care is taken, in the construction of large buildings, to give 

 them such a form as to render the speaker's voice audible with 

 the least effort to himself. 



In many respects waves of sound are closely analogous to 



