Doppler’s Principle:—­As pitch depends upon the frequency at which sound waves strike the ear, an object may emit sound waves at a constant frequency, yet may produce different pitches in ears differently situated.  Such a case is not usual, but an example of it will serve a useful purpose in fixing certain facts as to pitch.  Conceive two railroad trains to pass each other, running in opposite directions, the engine bells of both trains ringing.  Passengers on each train will hear the bell of the other, first as a rising pitch, then as a falling one.  Passengers on each train will hear the bell of their own train at a constant pitch.

The difference in the observations in such a case is due to relative positions between the ear and the source of the sound.  As to the bell of their own train, the passengers are a fixed distance from it, whether the train moves or stands; as to the bell of the other train, the passengers first rapidly approach it, then pass it, then recede from it.  The distances at which it is heard vary as the secants of a circle, the radius in this case being a length which is the closest approach of the ear to the bell.

If the bell have a constant intrinsic fundamental pitch of 200 waves per second (a wave-length of about 5.5 feet), it first will be heard at a pitch of about 200 waves per second.  But this pitch rises rapidly, as if the bell were changing its own pitch, which bells do not do.  The rising pitch is heard because the ear is rushing down the wave-train, every instant nearer to the source.  At a speed of 45 miles an hour, the pitch rises rapidly, about 12 vibrations per second.  If the rate of approach between the ear and the bell were constant, the pitch of the bell would be heard at 212 waves per second.  But suddenly the ear passes the bell, hears the pitch stop rising and begin to fall; and the tone drops 12 waves per second as it had risen.  Such a circumflex is an excellent example of the bearing of wavelengths and frequencies upon pitch.

Vibration of Diaphragms:—­Sound waves in air have the power to move other diaphragms than that of the ear.  Sound waves constantly vibrate such diaphragms as panes of windows and the walls of houses.  The recording diaphragm of a phonograph is a window pane bearing a stylus adapted to engrave a groove in a record blank.  In the cylinder form of record, the groove varies in depth with the vibrations of the diaphragm.  In the disk type of phonograph, the groove varies sidewise from its normal true spiral.

If the disk record be dusted with talcum powder, wiped, and examined with a magnifying glass, the waving spiral line may be seen.  Its variations are the result of the blows struck upon the diaphragm by a train of sound waves.

In reproducing a phonograph record, increasing the speed of the record rotation causes the pitch to rise, because the blows upon the air are increased in frequency and the wave-lengths shortened.  A transitory decrease in speed in recording will cause a transitory rise in pitch when that record is reproduced at uniform speed.