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Sound - Do You Know?
Sound of certain wavelengths shows potential in the non-chemical control of
insects
Adult Indian-meal moths exposed to certain wavelengths during the egg-laying stage had their
reproduction rate reduced by 75%.
Rumbling of thunder is due to reflection of sound from various obstacles
Thunder is caused by lightning, which
occurs when charges move from cloud to
cloud or from a cloud to the ground. These
moving charges heat the air to such very
high temperatures that the air molecules
glow, giving off light. The air is heated so
rapidly that it expands violently outwards
all along the lightning bolt, compressing the air around it. Hence a sound wave is generated as sound
is a compressional wave.
Near the lightning one loud sound is heard due to sound travelling directly to the listener. If you are
further away, rumbling is heard due to sound from different parts of the lightning bolt arriving at
different times as a result of reflections from the ground, hills, or clouds thus prolonging the sound.
Why do we sometimes see lightning but not hear thunder?
The lightning still causes thunder but it doesn’t reach you
even though the lightning is visible because the sound is
refracted upwards and is not audible on the ground. Under
the right conditions lightning can be seen up to 150 km
away but thunder can only be heard up to about 15 km away. The air is cooler higher up in the
atmosphere and sound travels more slowly in cooler air. The section of the wavefront which is lower
down travels faster than that higher up and so the wave is refracted upwards.
How can relatively low sounds (e.g. normal conversation) travel long
distances over water?
On a calm evening the water temperature is lower than the air temperature higher up so it cools the
air next to it. Since cool air is denser than warm air it stays near the water surface provided there is
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no wind. This is the opposite situation to the thunderstorm. Sound is refracted downwards because it
travels faster higher up in the warmer air, and when it hits the water surface it is reflected upwards
only to be refracted downwards again. It continues thus, travelling across the water and does not
lose its energy as quickly as normal as it is not spreading upwards but only in a flattened circle close
to the water and so it can travel great distances. Conversations can be heard up to 1.5 km across a
lake under such conditions. According to the Guinness Book of Records there is a recorded case of a
human voice being detectable at 16.9 km across still water at night.
Why does sound travel further on cool nights than on warm days?
At night the air near the ground is cooler
than the air higher up. Sound travels faster
in the warmer air. The section of the sound
wave higher up travels faster than the
section near the ground so the wavefront is
bent downwards.
During the day the reverse is true. The air
near the ground is warmer than the air
higher up so the wavefront is bent
upwards over the head of the listener.
Why does a stereo system not produce an interference pattern?
A domestic stereo system does not usually show such regular changes in sound level since it
operates over a wide range of frequencies and therefore you wouldn’t get an interference pattern as
for a single frequency. Reflections from walls, ceilings and various objects in the path of the waves
also tend to even out the sound.
The fact that the speed of sound varies with temperature is an important
factor in playing wind instruments
If the temperature of the air column changes then the frequency of the note and hence its pitch will
change (since speed of wave = frequency × wavelength) and the wavelength is determined by the
length of the vibrating air column. It is necessary for the players to warm up instruments before
playing it so that temperature remains constant. When instruments are played outside in cold
weather, breaks between playing cause problems due to instruments cooling down very quickly.
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What happens when an aircraft crosses the sound barrier?
The sound barrier represents a sharp rise in aerodynamic drag that occurs as
an aircraft approaches the speed of sound. It used to be an obstacle to
supersonic flight.
An aircraft such as Concorde is capable of flying faster than the speed of
sound. As it approaches the speed of sound it begins to catch up on the
sound waves travelling in front of it and they are pushed closer and closer
together, Fig. 2.5. A barrier of compressed air is thus formed in front of the
aircraft. As the aircraft reaches the speed of sound it overtakes this high
pressure air which spreads out behind it in a powerful shock wave. In any
time interval the source moves further than the wavefront and a cone (Mach
cone) encloses all the waves emitted by the source (similar to the conical
envelope of the bow wave pattern of a fast-moving boat.) On the ground the
shock wave is heard like a clap of thunder called a sonic boom which may
have enough energy to shatter windows.
Astronauts cannot speak to each other on the moon without the aid of radio
waves because of the lack of air
In one film, when the microphones broke down, the two astronauts put their helmets together, thus
transmitting the sound vibrations through the helmets. Sometimes, in popular science fiction films,
we see and hear an explosion in space which is incorrect as the sound waves, unlike light, cannot
travel through empty space.
The speed of sound may be used to estimate distance from a thunderstorm
Seeing lightning before hearing the thunder is due to the relatively low speed of sound compared to
that of light. Since sound travels approximately 0.3 km in 1 second, by measuring the number of
seconds between the flash of lightning being seen and the thunder being heard, the distance away of
the storm may be estimated. By repeating the procedure it can be determined whether the storm is
approaching or retreating.
A baby’s cry contains frequencies corresponding to the most sensitive part of
the human ear’s frequency range.
Loudness depends on intensity but it doesn’t follow that it is proportional to intensity as the ear
varies in its sensitivity to sounds of different frequency, being most sensitive to frequencies between
2 and 4 kHz, Fig. 2.6. In general the greater the intensity the greater the loudness.
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Driving a car with an open window may generate an infrasonic resonance.
Infrasonic waves are generated whenever a large high-speed flow of air occurs into an enclosed box
or room. The effects on motorists driving with open windows is currently of interest.
Elephants respond to very low frequencies; other animals are sensitive to
very high frequencies.
Elephants use very low frequency sounds produced in their nasal passages which can be heard over
several kilometres by other elephants and are used to coordinate herd movements. Dogs can hear up
to 35 kHz, some bats up to 80 kHz, and whales up to 200 kHz.
Laboratory tuning forks cannot be
used to tune musical instruments.
Most tuning forks found in laboratories are in
scientific pitch which is not the same as the
pitch of corresponding notes on an equally
tempered scale and are thus unsuitable for
tuning musical instruments.
Standard concert pitch is based on a frequency
of 440 Hz for A.
Voice prints can be used to help catch criminals.
Police in Los Angeles are building up files of voice prints. For example a suspected criminal is
asked to repeat some phrase recorded previously, perhaps from an anonymous threatening telephone
call. The recorded voices are converted to digital pulses by the computer and voice prints are
produced which are then compared to see if a suspect’s voice print matches the original. The method
of course is based on the uniqueness of each person’s voice print.
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