diaphragm becomes a sounding body

"At each of these points a line is drawn at right angles to

MD, and on this line the corresponding value of the

distance of the oscillating body from M is marked ;
thus, for instance, at length





o ^ = MA ^ a x cos o

1 „ i^ =■ Mb^ a X cos iO°

2 „ 2^ = Mc = a X cos 6o^

3 „ o = o = a X ^^j 90°

4 „ 4^ = Me=^ a X cos 120°

And generally 7 = a cos 0, a being the distance MA^
and fl the angle which the radius to any point on the
circumference of tne reference circle ADg makes with
MA.

The curve drawn through the points A, 6, c, 3, e, f.g^
etc., is the curve of vibration of a body oscillating after
the manner of a pendulum^ and following the law of
simple harmonic motion. This is more frequently ex-
pressed thus \ y ^s:. a sin 0, ^ being the complement of
0, or the angle which MD makes with the axis as it
revolves about Mva the positive direction. It forms what
is called a sinusoidal curve or curve of sines. The interval
between two crests of waves ^^ is called the wave length.
The distance MA through which the particle vibrates is
called the amplitude of the vibration. The whole dura-
tion Z, which is represented by the time o to 1 2, is called
the period of the vibration. The amplitude and period
of a vibration are quite independent of each other.
The oscillating particle will make the same number of
vibrations per second whether the amplitude of its
excursion be great or small. This equality of period
for all amplitudes is called isochronism.
Such a simple harmonic curve can be actually pro-
duced by providing a tuning-fork of a rather large size
with a pen or stylus, which during its oscillation leaves
traces on a strip of paper which is drawn from under
the tracing stylus with uniform velocity, in a direction
at right angles to the line of vibration.
The vibrations of a sounding body give rise in the
surrounding atmosphere to a wave-motion which is
transmitted by the successive molecules of air executing
similar vibrations as the vibrating body itself. When
a tuning fork vibrates in air it gives to the air a series of
pushes, each of which produces a momentary increase of
pressure and density in front of the advancing prongs,
while a momentary decrease of density and pressure is
produced behind them. As the prongs advance, first in
one direction and then in the opposite, a series of
compressions and extensions are produced in alternate
succession. But each compressed portion tends to relieve
itself by expanding into the neighbouring air, which is
thus in its turn compressed, and the extended portions
in like manner communicate extension. Hence a
series of compressions and extensions are propagated
through the surrounding air, and these constitute an
undulation whose period is the same as that of the
vibrations of the tuning fork. If the sonorous vibrations
take place in open space their amplitude must get
smaller and smaller, and the sound will die away. If
the air waves impinge on a diaphragm of any kind, the
diaphragm will take up the sonorous vibrations and
oscillate with the same period and the same form. The
diaphragm becomes a sounding body."

- Sir William Henry Preece