Download OP40-OP44

JSSS
Teacher Support Material
Sound – Applications
Speaking Tubes
Speaking tubes on ships are made of metal with a funnel at each end. Sound travels along the tube,
even if it’s bent, by total internal reflection, as long as the angle of incidence exceeds the critical
angle. The sound travels more slowly in the air inside the tube than in the metal walls of the pipe.
The situation is analogous to light travelling along optical fibres.
Echoes
Echoes are reflected sounds. They must be heard at least 0.1 s after the original sound to be
distinguishable from it. A surface will reflect sound regularly if it is flat to within a fraction of the
wavelength of the sound. As speech sound waves may have wavelengths several metres long even
rough surfaces such as cliffs can give regular reflection thus giving echoes. In open spaces where
sound spreads out in all directions echoes are fainter than in enclosed spaces such as a tunnel where
the reflected sound cannot escape.
Echoes of ultrasonic waves are used by ships to:
(i) calculate ocean depth;
(ii) detect shoals of fish;
(iii) detect submarines or shipwrecks.
This is called SONAR, i.e. Sound Navigation And
Ranging. The time between sending out a pulse of
ultrasound and receiving the echo can be used to
calculate the position and shape of objects under
the ship. It can show the difference between a large fish and a shoal of small fish. Sonar can also be
used to draw up a map of the seabed which is very important for the laying down of optic fibre
cables for the purpose of transatlantic communication. Echoes of ultrasonic waves are also used by:
dolphins to find objects underwater; bats to locate their prey; visually impaired people fitted with
special equipment to locate obstacles.
Vibrato or Tremolo Effect in the Organ
Large organs may have many thousands of pipes, varying in length from a few centimetres to over
nine metres. Some groups of pipes may have two pipes for each note, with one slightly detuned,
hence producing beats when sounded.
-1-
JSSS
Teacher Support Material
Active Noise Cancellation (ANC)
Noise in cars from the engine and road surfaces which interferes
with music from the car radio can be cancelled out by destructive
interference with its own mirror image using ANC. The noise is
sampled by a microphone attached to a microcomputer and the
original noise and its mirror image are transmitted through two
speakers placed behind the driver’s head and so the noise is
cancelled out.
Stethoscopes
The stethoscope directs sound into the doctor’s ear.
Sound normally spreads out in all directions and
this is how quiet sounds are soon lost. When the
faint sounds of lungs and heart are closely confined
to the path down the tube this enables the doctor to hear them. (Experiment: make one using a short
piece of rubber tubing and a funnel in one end.)
Doctors used to use thin rods with broadened
ends as stethoscopes and, like modern types,
one end was placed against the patient’s chest
and sounds therein were transmitted through the
wood to the doctor’s ear. Motor mechanics may
use wooden rods to trace sounds of knocking in
engines. The sounds are concentrated in the rod
as explained earlier and other noises are blocked
out.
Native American scout listening through the ground for oncoming horse’s
hooves
The hooves strike the ground directly so sound enters the ground where it is produced. The
vibrations that occur cause air molecules
to vibrate also. Hence the sound can
reach the listener’s ears through the
ground or through the air. The sound
travelling through the air may meet
-2-
JSSS
Teacher Support Material
obstacles and even though sound can bend around obstacles, the energy becomes more spread out
and it would not be as loud as if heard directly. Also, if the air near the ground is warmer than that
higher up (usually what occurs in daytime) the waves bend upwards over the listener’s head and the
sound is not heard. The sound that travels through the ground goes directly to the scout’s ear, rather
than having to go around obstacles, compressing a section of the rock which expands and
compresses the next section and so on.
This works particularly well in the American West with its thin rocky soil, with lots of rock outcrops
in contact with the bedrock. Softer soils would absorb a lot of the sound’s energy.
Attracting Fish
Fishermen use transistor beepers in the water which are claimed to attract fish over a radius of 1.5
km. The fact that fish were attracted by the sound of a bell underwater was discovered over 300
years ago by Otto von Guericke who also discovered that sound could not travel through a vacuum.
The Vocal Cords
Vibrations of the vocal cords, which are controlled by
muscular tension, give rise to the human voice and these
vibrations determine the fundamental pitch of the voice.
The vocal cords are bands of connective tissue in the
larynx. Several sets of muscles can pull these cords with
various degrees of tension into the path of air coming
from the lungs. They then vibrate as violin strings
vibrate. Pitch is varied by varying tension in the vocal
cords while volume is varied by varying the force of the
air current. Vocal cords are shorter in women than in men.
-3-