When
two sound waves of same
frequency traveling along same
line in same direction,
superimpose upon each other &
the resultant wave shows
alternate variation of intensity
of sound. There are two regions
where sound shows variation of
intensity. One region where
compression of one wave meets
the compression of other, the
amplitude of vibration increases
& intensity of sound is maximum
& this region is constructive
interference. While other region
where compression of one wave
meets the rarefaction of the
other, the net amplitude of
vibration becomes zero and the
intensity of sound is zero &
this region is destructive
interference.
Two
loudspeakers have been installed in an open space to
listen to a speech. When the speakers are in operation,
a person sitting at a place (not far) receives very
feeble sound while another person sitting at another
place receives very loud sound. This is due to the
phenomena of interference of sound. A person sitting at
a place (not far) receives very feeble sound because of
destructive interference of sound & another person
sitting at another place receives very loud sound
because of constructive interference of sound.
32. Can two sound waves of equal
frequency produce beats? Explain.
When two sound waves of
nearly same frequency
traveling along same line in
same direction, superimpose
upon each other & the
resultant wave shows
alternate rise & fall in
intensity. This is known as
phenomena of beats. If two
sound waves of equal
frequency are used, then it
does not produce beats since
the beat frequency obtained
will be zero but
interference of sound
occurs. There are two
regions where sound shows
variation of intensity. One
region where compression of
one wave meets the
compression of other, the
amplitude of vibration
increases & intensity of
sound is maximum & this
region is constructive
interference. While other
region where compression of
one wave meets the
rarefaction of the other,
the net amplitude of
vibration becomes zero and
the intensity of sound is
zero & this region is
destructive interference.
Hence, two sound waves of
equal frequency can not
produce beats.
33. The
reverberation time is longer for an empty hall than for crowded
hall why?
As there are a lots of sound absorbing materials in the
crowded hall than the empty hall, more sounds are
absorbed in the crowded hall and the sound cannot
reflect more as in the empty hall due to being absorbed.
Hence, the reverberation time is longer for an empty
hall than for crowded hall.
34. How are bats able to fly around
even if they don't have eyes?
Bats
produce ultrasonic waves which detect if there is object
or not. The waves of ultrasonic sound produced by bats
are of the high frequency that travels toward the
objects and when it is reached to it , it is reflected
from them and bats analyze the reflected waves coming
from the objects and they can visualize the shape, size,
position and dimension of the object.
35. Why can't we
hear echo in a small room?
When
the sound wave is reflected from any object then it is
called echo. To hear echo clearly, the interval between
original sound and reflected sound must be at least 0.1
seconds, called the persistence of hearing. So, the
minimum distance between the source and the listener
must be 0.05 times the velocity of the sound in the air
i.e. 16.5m. Hence, we can not hear echo in a small room.
36. You are given a watch, and asked
to determine the length of a very large hall. Can you do that?
Explain.
When
the sound wave is reflected from any object then it is
called echo. By counting the echo produced and noting
the time in which the echo is produced, we can easily
find the length of a hall. Let d be the length of a
large hall, v be the velocity of the sound in air and t
be the time at which the echo is produced.
37.
Explain why does the flash of light reaches the earth before the
sound coming from the same thunder?
As the velocity of the light is much more greater than
that of the sound in the air, the lightening effect is
first observed during thunder then later the sound of
the same thunder is heard by the person. Hence, the
flash of light reaches the earth before the sound coming
from the same thunder.
38. Discuss the effect of wind on
the velocity of sound propagation through the air.
EFFECTS OF WIND ON VELOCITY OF THE SOUND:
The velocity of the sound is altered by the effect of
wind. Let the velocity of the sound be v and the
velocity of the
wind
be vw and the direction of the wind makes
angle θ with the velocity of gas then the resultant
velocity of the sound is given by:
conditions:
1. If θ = 00,
then vR = v + vw.
2. If θ =
900, then vR= v.
3. If
θ = 1800, then vR = v - vw.
39. Two people sitting in two boats
in a pond are talking to each other. Why can they hear other
over long distances?
The air
surrounding to the pond is highly humid. As the
density of humid air is less than that of dry air so
the velocity of sound in humid air is greater than
the velocity of sound in dry air. The air close to
the surface of the pond is greatly humid, sound
waves travel faster in it and cover large distance.
Hence, if two people sitting in two boats in a pond
are talking to each other they cannot hear each
other over long distances.
40. Sound can be heard more
distinctly through carbon dioxide than through air. Why?
The
intensity of sound produced as a result of vibration of
particles of a medium varies directly to the density of
the medium. As the density of carbon dioxide gas is
greater than that of the air, the intensity of sound in
carbon dioxide is greater than that in air. Hence, sound
can be heard more distinctly through carbon dioxide than
through air.