Q#1
When you speak to your friend, which of the following parameters have a unique value in the sound produced?(a) frequency
(b) wavelength
(c) amplitude
(d) wave velocity.
Answer: (d)
When we speak there are different frequencies, wavelength, and amplitudes in the sound produced. The frequency and the wavelength changes with the pitch of the sound and the amplitude vary with the loudness. But the wave velocity remains the same in the air when the temperature is constant. Option (d) is true.
Q#2
An electrically maintained tuning fork vibrates with constant frequency and constant amplitude. If the temperature of the surrounding air increases but pressure remains constant, the sound produced will have
(a) larger wavelength
(b) larger frequency
(c) larger velocity
(d) larger time period.
Answer: (a), (c)
The velocity of sound V is directly proportional to the square root of the temperature. So if the temperature increases the velocity of the sound also increases. Option (c).
Since the frequency of the sound producing tuning fork is constant hence the frequency of the sound will also be constant. Also, the wavelength =(Sound velocity)/(Frequency). Thus the wavelength will also increase if the temperature increases. Option (a).
Since frequency is constant, Time period will also be constant.
Q#3
The fundamental frequency of a vibrating organ pipe is 200 Hz.
(a) The first overtone is 400 Hz.
(b) The first overtone may be 400 Hz.
(c) The first overtone may be 600 Hz.
(d) 600 Hz is an overtone.
Answer: (b), (c), (d)
The overtone frequencies may be a simple multiple of the fundamental frequency (ν = nν₀) or an odd multiple of the fundamental frequency {ν = (2n+1)ν₀} depending upon whether it is an open organ pipe or closed organ pipe.
f0 = fundamental frekuency
Overtone frequencies for:
Open organ pipe = nf0, for {n = 2,3,4, . . .}
Closed organ pipe = (2n + 1)f0, for {n = 1,2,3 . . .}
Since it is not given whether the pipe is open or closed, we cannot be sure that the first overtone is 400 Hz. So the option (a) is not true.
The first overtone may be 400 Hz if the pipe is open because it is the first even multiple of the fundamental frequency 200 Hz. Option (b) is true.
The first overtone may be 600 Hz if the pipe is closed because it is the first odd multiple of the fundamental frequency 200 Hz (3*200 Hz). Option (b) is true.
Since 600 Hz is an even as well as odd multiple of 200 Hz, it is an overtone whether the pipe is open one or closed. Option (d) is true.
Q#4
A source of sound moves towards an observer
(a) The frequency of the source is increased
(b) The velocity of sound in the medium is increased
(c) The wavelength of the sound in the medium towards the observer is decreased.
(d) The amplitude of vibration of the particles is increased.
Answer: (c)
When the source of sound moves towards an observer neither the frequency of the sound nor the amplitude of vibration of the particles changes. The velocity of sound is also not dependent upon the speed of the source. Hence the options (a), (b) and (d) are not true.
In such a case, only the apparent frequency of the sound to the observer increases because the same consecutive phase of the sound wave is received earlier hence the wavelength of the sound towards the observer decreases. Thus option (c) is true.
Q#5
A listener is at rest with respect to the source of the sound. A wind starts blowing along the line joining the source and the observer. Which of the following quantities do not change?
(a) Frequency.
(b) Velocity of sound.
(c) Wavelength.
(d) Time period.
Answer: (a), (d)
The sound wave propagate due to the vibrations of the particles of the medium (air), though the particles do not move with the wave. When the wind blows the vibrating particles also move with the wind speed. The velocity of the sound changes accordingly. Option (b) is not true. The frequency and hence the time period of the sound remains the same as that of the source. Options (a) and (d) are true.
Frequency remaining the same the wavelength changes proportional to the sound velocity (𝜆=V/ν), hence (c) is not true.
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