If you can picture to yourself these waves spreading out in all
directions, you will easily see why sound grows fainter at the
distance. Just close round my hands when I clap them, there is
a small quantity of air, and so the shock I give it is very
violent, but as the sound-waves spread on all sides they have
more and more air to move, and so the air-atoms are shaken less
violently and strike with less force on your ear.
If we can prevent the sound-wave from spreading, then the sound
is not weakened. The Frenchman Biot found that a low whisper
could be heard distinctly for a distance of half a mile through a
tube, because the waves could not spread beyond the small column
of air. But unless you speak into a small space of some kind,
you cannot prevent the waves going out from you in all
directions.
Try and imagine that you see these waves spreading all round me
now and hitting on your ears as they pass, then on the ears of
those behind you, and on and on in widening globes till they
reach the wall. What will happen when they get there? If the
wall were thin, as a wooden partition is, they would shake it,
and it again would shake the air on the other side, and so anyone
in the next room would have the sound of my voice brought to
their ear.
But something more will happen. In any case the sound-waves
hitting against the wall will bound back from it just as a ball
bounds back when thrown against anything, and so another set of
sound-waves reflected from the wall will come back across the
room. If these waves come to your ear so quickly that they mix
with direct waves, they help to make the sound louder in this
room than you would in the open air, for the "Ha" from my mouth
and a second "Ha" from the wall come to your ear so
instantaneously that they make one sound. This is why you can
often hear better at the far end of a church when you stand
against a screen or a wall, then when you are half-way up the
building nearer to the speaker, because near the wall the
reflected waves strike strongly on your ear and make the sound
louder.
Sometimes, when the sound comes from a great explosion, these
reflected waves are so strong that they are able to break glass.
In the explosion of gunpowder in St. John's Wood, many houses in
the back streets had their windows broken; for the sound-waves
bounded off at angles from the walls and struck back upon them.
Now suppose the wall were so far behind you that the reflected
sound-waves only hit upon your ear after those coming straight
from me had died away; then you would hear the sound twice, "Ha"
from me and "Ha" from the wall, and here you have an echo, "Ha,
ha." In order for this to happen in ordinary air, you must be
standing at least 56 feet away from the point from which the
waves are reflected, for then the second blow will come one-tenth
of a second after the first one, and that is long enough for you
to feel them separately.* Miss C. A. Martineau tells a story of
a dog which was terribly frightened by an echo. Thinking another
dog was barking, he ran forward to meet him, and was very much
astonished, when, as he came nearer the wall, the echo ceased. I
myself once knew a case of this kind, and my dog, when he could
find no enemy, ran back barking, till he was a certain distance
off, and then the echo of course began again. He grew so furious
at last that we had great difficulty in preventing him from
flying at a strange man who happened to be passing at the time.
(*Sound travels 1120 feet in a second, in air of ordinary
temperature, and therefore 112 feet in the tenth of a second.
Therefore the journey of 56 feet beyond you to reach the wall
and 56 feet to return, will occupy the sound-wave one-tenth of
a second and separate the two sounds.)
Sometimes, in the mountains, walls of rock rise at some distance
one behind another, and then each one will send back its echo a
little later than the rock before it, so that the "Ha" which you
give will come back as a peal of laughter. There is an echo in
Woodstock Park which repeats the word twenty times. Again
sometimes, as in the Alps, the sound-waves coming back rebound
from mountain to mountain and are driven backwards and forwards,
becoming fainter and fainter till they die away; these echoes are
very beautiful.
If you are now able to picture to yourselves one set of waves
going to the wall, and another set returning and crossing them,
you will be ready to understand something of that very difficult
question, How is it that we can hear many different sounds at one
time and tell them apart?
Have you ever watched the sea when its surface is much ruffled,
and noticed how, besides the big waves of the tide, there are
numberless smaller ripples made by the wind blowing the surface
of the water, or the oars of a boat dipping in it, or even rain-
drops falling? If you have done this you will have seen that all
these waves and ripples cross each other, and you can follow any
one ripple with you eye as it goes on its way undisturbed by the
rest. Or you may make beautiful crossing and recrossing ripples
on a pond by throwing in two stones at a little distance from
each other, and here too you can follow any one wave on to the
edge of the pond.
Now just in this way the waves of sound, in their manner of
moving, cross and recross each other. You will remember too,
that different sounds make waves of different lengths, just as
the tide makes a long wave and the rain-drops tiny ones.
Therefore each sound falls with its own peculiar wave upon your
ear, and you can listen to that particular wave just as you look
at one particular ripple, and then the sound becomes clear to
you.
All this is what is going on outside your ear, but what is
happening in your ear itself? How do these blows of the air
speak to your brain? By means of the following diagram, Fig. 33,
we will try to understand roughly our beautiful hearing
instrument, the ear.
First, I want you to notice how beautifully the outside shell, or
concha as it is called, is curbed round so that any movement of
the air coming to it from the front is caught in it and reflected
into the hole of the ear. Put your finger round your ear and
feel how the gristly part is curved towards the front of your
head. This concha makes a curve much like the curve a deaf man
makes with his hand behind his ear to catch the sound. Animals
often have to raise their ears to catch the sound well, but ours
stand always ready. When the air-waves have passed in at the
hole of your ear, they move all the air in the passage, which is
called the auditory, or hearing, canal. This canal is lined with
little hairs to keep out insects and dust, and the wax which
collects in it serves the same purpose. But is too much wax
collects, it prevents the air from playing well upon the drum,
and therefore makes you deaf. Across the end of this canal, a
membrane or skin called the tympanum is stretched, like the
parchment over the head of a drum, and it is this membrane which
moves to and fro as the air-waves strike on it. A violent box on
the ear will sometimes break this delicate membrane, or injure
it, and therefore it is very wrong to hit a person violently on
the ear.
On the other side of this membrane, inside the ear, there is air,
which fills the whole of the inner chamber and the tube, which
runs down into the throat behind the nose, and is called the
Eustachian tube after the man who discovered it. This tube is
closed at the end by a valve which opens and shuts. If you
breathe out strongly, and then shut your mouth and swallow, you
will hear a little "click" in your ear. This is because in
swallowing you draw the air out of the Eustachian tube and so
draw in the membrane, which clicks as it goes back again. But
unless you do this the tube and the whole chamber cavity behind
the membrane remains full of air.
Now, as this membrane is driven to and fro by the sound-waves, it
naturally shakes the air in the cavity behind it, and it also
sets moving three most curious little bones. The first of the
bones is fastened to the middle of the drumhead so that it moves
to and fro every time this membrane quivers. The head of this
bone fits into a hole in the next bone, the anvil, and is
fastened to it by muscles, so as to drag it along with it; but,
the muscles being elastic, it can draw back a little from the
anvil, and so give it a blow each time it comes back. This anvil
is in its turn very firmly fixed to the little bone, shaped like
a stirrup, which you see at the end of the chain.
This stirrup rests upon a curious body which looks in the diagram
like a snail-shell with tubes coming out of it. This body, which
is called the labyrinth, is made of bone, but it has two little
windows in it, one covered only by a membrane, while the other
has the head of the stirrup resting upon it.
Now, with a little attention you will understand that when the
air in the canal shakes the drumhead to and fro, this membrane
must drag with it the hammer, the anvil, and the stirrup. Each
time the drum goes in, the hammer will hit the anvil, and drive
the stirrup against the little window; every time it goes out it
will draw the hammer, the anvil, and the stirrup out again, ready
for another blow. Thus the stirrup is always playing upon this
little window. Meanwhile, inside the bony labyrinth there is a
fluid like water, and along the little passages are very fine
hairs, which wave to and fro like reeds; and whenever the stirrup
hits at the little window, the fluid moves these hairs to and
fro, and they irritate the ends of a nerve, and this nerve
carries the message to your brain. There are also some curious
little stones called otoliths, lying in some parts of this fluid,
and they, by their rolling to and fro, probably keep up the
motion and prolong the sound.
You must not imagine we have explained here the many intricacies
which occur in the ear; I can only hope to give you a rough idea
of it, so that you may picture to yourselves the air-waves moving
backwards and forward in the canal of your ear, then the tympanum
vibrating to and fro, the hammer hitting the anvil, the stirrup
knocking at the little window, the fluid waving the fine hairs
and rolling the tiny stones, the ends of the nerve quivering, and
then (how we know not) the brain hearing the message.
Is not this wonderful, going on as it does at every sound you
hear? And yet his is not all, for inside that curled part of the
labyrinth, which looks like a snail-shell and is called the
cochlea, there is a most wonderful apparatus of more than three
thousand fine stretched filaments or threads, and these act like
the strings of a harp, and make you hear different tones. If you
go near to a harp or a piano, and sing any particular note very
loudly, you will hear this note sounding in the instrument,
because you will set just that particular string quivering, which
gives the note you sang. The air-waves set going by your voice
touch that string, because it can quiver in time with them, while
none of the other strings can do so. Now, just in the same way
the tiny instrument of three thousand strings in your ear, which
is called Corti's organ, vibrates to the air-waves, one thread to
one set of waves, and another to another, and according to the
fibre that quivers, will be the sound you hear. Here then at
last, we see how nature speaks to us. All the movements going on
outside, however violent and varied they may be, cannot of
themselves make sound. But here, in the little space behind the
drum of our ear, the air-waves are sorted and sent on to our
brain, where they speak to us as sound.