The Eaton Democrat. (Eaton, Ohio) 1875-1903, 1881-12-29
A SOUND PHENOMENON.
why E1Eine Bells and WhistIes Appear
Sometimes to Vary I'm Fitch.

ID LkEME1 au0
sometimes to Vary I'm Pitch.
From the Providence Journal.J
"When two trains, going in an OPP
posite direction at q rapid rate, meet
each other, with the engine bells ring
ing, OF whistles blowing, a passenger OF
the n train notices a marked variation
in the pitch of the bell or whistle of the
other train. When the sound first
strikes the ear, the pitch is at its highest.
Gradually it goes down IS the train
dashes past, and the lowest pitch is
reached when the last notes fall OF the
ear. Why is this?"
This question was ~propounded to an
engineer by an individual thirsting for
information.
"The question is simple enough"
was the reply. "To start with, it is an
axiom which needs no proving that the
pitch of a sound depends on its number
of vibrations. Thus, while forty vibra-
tons a second produce the lowest sound,
40, 900 a second produce the highest.
Pitch rises with an increase Of the num-
Der of vibrations.
"a certain number Of vibrations are
emitted by the bell or whistle during
the time the train is running a certain
distance-say a quarter Of a mile. Sup
pose each train runs this distance in half
a minute. Then, as the n train ap-
breaches that in which the listening
passenger is seated, all the vibrations
emitted during the half-minute will
strike the ear in less than half a minute
(supposing, of course, that they can be
heard over SO great a d~istance.) The
reason for this is that the first will not
be heard until more than two seconds
| after it has been emitted, as the sound
will have to travel half a mile, while the
last will be heard the instant it is emit
ted, because the engine will then be
within a few feet of the ear. Thus,
| thirty seconds vibrations will be heard
in twenty-eight seconds. When the
trains are receding from each other, the
vibrations emitted during the half-minute
will take rather more than thirty-two
seconds to reach the ear. az that emitted
when the train is half a mile off will
have to travel to the ear"
"Can you illustrate this to make it
| plainer?" the engineer was asked.
Certainly / can. Suppose a man
with a rubber hose stands ten yards
from a tub. The capacity of his hose
enables him to squirt a pint of water per
second into that tub. But if, during,
say, five seconds, he walks up to the
tub, all the while allowing his hose to
pour water into it, there will be nor
than five pints of water as the result of
that five seconds work. There will be
five pints, plus the quantity contained in
the stream which would have fallen to
the ground if he had stood still and at
the end of the five seconds turned the
cock, shutting off the stream. By the
approach of the whistle or bell of an en-
CINE a greater number of vibrations
meet the ear in a given time, just as a
greater quantity of water reaches the
tub from the hose by the approach Of
the nozzle. And, accepting as correct
the axiom hat the greater number of
vibrations the higher the pitch, it will
be seen that when the trains approach
the ear gets more than its due share OF
vibrations per second, and when they
recede it gets less than its share.