PROPERTIES OF SOUND 



tone produced by a sound source can be considerably increased by the use 

 of sound augmentors. These are called resonators, sounding boards or 

 trumpets according to the form they take or the sound source (musical 

 instrument) to which they are applied. If from any string instrument 

 (e.g. violin) the box be removed, the tones generated are found to be 

 greatly reduced in intensity. The function of these sound amplifiers appears 

 to be to transmit the vibrations of the source (e.g. the stretched wire) to the 

 greatest possible volume of air, i.e. to turn into sound as much of the kinetic 

 energy of the vibrating wire as possible. In the case of the trumpet or horn, 

 there is in addition the effect of increasing the volume of sound in some chosen 

 direction at the expense of that in others. This effect is well illustrated in 

 the gramophone. In most musical instruments the amplifier must be 

 capable of responding to a large range of tones indifferently, and the more 

 perfectly it can do this the better is the instrument. Such perfection is diffi- 

 cult to obtain, and more usually it is found that in spite of all care one note 

 is accentuated more than others, e.g. ' the wolf note ' of the violin. Sound 

 amplifiers for the reed stops of the organ are on the contrary made as sharply 

 selective as possible, in order that of the many tones emitted by the vibrating 

 reed, the chosen one shall alone be augmented. This form of amplifier is 

 called a resonator, although the term is strictly speaking applicable to other 

 classes of sound amplifier, e.g. the sounding board. This power of 

 augmenting one chosen tone has great value, because in a musical chord it is 

 possible at once to detect the presence of any particular tone, by ascertaining 

 whether its resonator responds when the chord is sounded. For such analysis 

 the resonators of Helmholtz are generally employed. These are hollow 

 vessels open at one end and having a tube at the other to which the ear may 

 be applied. A series of graduated sizes are used, each of which has a definite 

 period of vibration (pitch). 



TIMBRE OR QUALITY. When the same note is sounded on different 

 instruments, i.e. tuning-fork, violin, piano, trumpet, human voice, every 

 person, whether he has an educated musical ear or not, can say at once what 

 kind of instrument is being used. This fact shows that the sound wave pro- 

 duced by these instruments must differ, altogether apart from any differences 

 in amplitude or in number of vibrations per second, and if the sound waves 

 produoed by these instruments be recorded an actual difference is found in 

 the shape of the curve. 



If a stretched wire be plucked 'so as to set it into transverse vibrations it 

 will give out a certain note, dependent on its length, its thickness, and the 

 tension to which it is subjected. If its length be halved it will give out a 

 note which is of double the number of vibrations per second. If only one- 

 third of the wire be set into vibrations the sound wave produced will have 

 three times the number of vibrations of that of the whole string. When the 

 string is free to vibrate as a whole the segments of it tend to vibrate even 

 while the whole string is vibrating. If therefore we take the note given out 

 by the whole string, the ' fundamental tone,' as corresponding to 132 vibra- 

 tions per second, there will also be a series of notes superadded to the funda- 



