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CHAPTEE XII

ACOUSTICAL MEASUREMENTS

In the preceding chapters we have described a few such
methods of measurement as fell naturally into the line of our
argument. In the present chapter we shall gather together a
number of other methods which have been used by investigators.
The aim will be not so much to provide a complete list, as to
describe representatives of the various types of measurement.
We shall deal first with the measurement of the pitch.
230. Measurement of pitch by Cagniard de la
Tour's Siren. In Chapter I it was shewn that the frequency




Fig. 93



230] ACOUSTICAL MEASUREMENTS 197

of a note can be found by means of Savart's Toothed Wheel
or the Disc Siren. These instruments are capable of giving
only a rough approximation to the frequency. A better
instrument of the same class, where the number of vibrations
per second is counted directly, is Oagniard de la Tour's Siren
shewn in Fig. 93.
This siren consists of two circular discs nearly in contact,
of which the lower forms the fixed top of a wind chest, and
the upper can rotate freely on a spindle. Each of the discs
is pierced with a circle of holes, the holes in one disc corre-
sponding in number and position with those in the other.
The wind chest is supplied with air from bellows with
a pressure regulator. Suppose now the upper disc is rotating.
