Acoustics is the science relating to sound and hearing. Sound is heard
when any shock or impulse is given to the air, or to any other body
which is in contact directly or indirectly with the ear.
Difference Between Sound and Noise.
Noises are made by the crack of whips, the beating of hammers, the
creak of a file or saw, or the hubbub of a multitude. But when a bell
is struck, the bow of a violin drawn across the strings, or the wetted
finger turned round a musical glass, we have what are properly called
sounds.
Sounds, How Propagated.
Sounds are propagated on all bodies much after the manner that waves
are in water, with a velocity of 1,142 feet in a second. Sounds in
liquids and in solids are more rapid than in air. Two stones rubbed
together may be heard in water at half a mile; solid bodies convey
sounds to great distances, and pipes may be made to convey the voice
over every part of the house.
To Show How Sound Travels Through a Solid.
Take a long piece of wood, such as the handle of a hair broom, and
placing a watch at one end, apply your ear to the other, and the
tickings will be distinctly heard.
To Show That Sound Depends on Vibration.
Touch a bell when it is sounding, and the noise ceases; the same may
be done to a musical string with the same results. Hold a musical
pitch-fork to the lips, when it is made to sound, and a quivering
motion will be felt from its vibrations. These experiments show that
sound is produced by the quick motions and vibrations of different
bodies.
Musical Figures Resulting From Sound.
Cover the mouth of a wine-glass, having a foot-stalk, with a thin sheet
of membrane, over which scatter a layer of fine sand. The vibrations
excited in the air by the sound of a musical instrument, held within a
few inches of the membrane, will cause the sand on its surface to form
regular lines and figures with astonishing celerity, which vary with
the sound produced.
To Make an Æolian Harp.
This instrument consists of a long, narrow box of very thin deal, about
six inches deep, with a circle in the middle of the upper side of an
inch and a half in diameter, in which are to be drilled small holes. On
this side seven, ten or more strings of very fine catgut are stretched
over bridges at each end, like the bridges of a fiddle, and screwed up
or relaxed with screw-pins. The strings must all be tuned to one and
the same note, and the instrument should be placed in a window partly
open, in which the width is exactly equal to the length of the harp,
with the sash just raised to give the air admission. When the air blows
upon these strings with different degrees of force, it will excite
different tones of sound. Sometimes the blast brings out all the tones
in full concert, and sometimes it sinks them to the softest murmurs.
A colossal imitation of the instrument just described was invented at
Milan in 1786 by the Abbate Gattoni. He stretched seven strong iron
wires, tuned to the notes of the gamut, from the top of a tower sixty
feet high, to the house of a Signor Moscate, who was interested in
the success of the experiment; and this apparatus, called the “giant’s
harp,” in blowing weather yielded lengthened peals of harmonious music.
In a storm this music was heard at a greater distance.