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THE NATURE OF SOUND 303
too much or too little response to certain of the original frequencies but does not introduce any new or spurious frequencies.
Most of the sounds of voices and instruments have a wave form which is much more complicated than the one used here for illustration.
It will be noticed in Fig. 10 that components 1 and 2 do not start in the same "phase." That is, they do not cross the horizontal axis at the same time. If we were to shift the three components so that they all cross the axis together at the left hand side of the figure, and then add them together, the result would be a curve which would look different, but actually would represent the same sound so far as the ear is concerned.
In other words, the ear appears to be affected by the various component frequencies which make up the entire sound, without regard to the phases of these components.
A projecting oscillograph can be used to demonstrate the character of the wave form of various voice and instrument tones. Also some specially prepared phonograph records have been made by the Bell Technical Laboratories and the Victor Company to show the effects of non-linear response and the effects on music and speech of eliminating certain frequency bands.
COMBINATION TONES
When two tones, of nearly the same frequency are sounded together an effect called "beats" is produced. This is a waxing and waning or throbbing effect caused by the two tones alternately helping and destroying each other. The number of beats per second is equal to the difference of the frequencies of the two tones. Thus if one tone were 200 double vibrations per second and the other were 202, then there would be two beats heard per second.
When the number of beats per second reaches a higher value, the effect may be that of a new tone whose frequency is that of the beats. This can be demonstrated by means of a whistle having two barrels. Each alone may give a high pitched tone, but together the effect will be a lew pitch which depends on the difference in frequency of the two high tones. The same effect is used by organ builders to obtain low tones without the use of long pipes.
"Summation" tones are also heard under certain circum