Waves Difficult Questions and Answers | Page - 9

Waves

A source of sound gives 5 beats per second, when sounded with another source of frequency 100/sec. The second harmonic of the source, together with a source of frequency 205/sec gives 5 beats per second. What is the frequency of the source?

1. 95 sec^–1
1. 100 sec^–1
1. 105 sec^–1
1. 205 sec^–1

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Frequency of first source with 5 beats/ sec = 100 Hz and frequency of second source with 5 beats/sec = 205 Hz. The frequency of the first source = 100 ± 5 = 105 or 95 Hz. Therefore, frequency of second harmonic of source = 210 Hz or 190 Hz. As the second harmonic gives 5 beats/ second with the sound of frequency 205 Hz, therefore, frequency of second harmonic source should be 210 Hz or frequency of source = 105 Hz.

Correct Option: C

Frequency of first source with 5 beats/ sec = 100 Hz and frequency of second source with 5 beats/sec = 205 Hz. The frequency of the first source = 100 ± 5 = 105 or 95 Hz. Therefore, frequency of second harmonic of source = 210 Hz or 190 Hz. As the second harmonic gives 5 beats/ second with the sound of frequency 205 Hz, therefore, frequency of second harmonic source should be 210 Hz or frequency of source = 105 Hz.

For production of beats the two sources must have

1. different frequencies and same amplitude
1. different frequencies
1. different frequencies, same amplitude and same phase
1. different frequencies and same phase

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For production of beats different frequencies are essential. The different amplitudes affect the minimum and maximum amplitude of the beats and different phases affect the time of occurrence of minimum and maximum.

Correct Option: B

For production of beats different frequencies are essential. The different amplitudes affect the minimum and maximum amplitude of the beats and different phases affect the time of occurrence of minimum and maximum.

A wave has S.H.M whose period is 4 seconds while another wave which also possess SHM has its period 3 seconds. If both are combined, then the resultant wave will have the period equal to

1. 4 seconds
1. 5 seconds
1. 12 seconds
1. 3 seconds

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Beats are produced. Frequency of beats will be
1 - 1 = 12
3 4 12

Hence time period = 12 s.

Correct Option: C

Beats are produced. Frequency of beats will be
1 - 1 = 12
3 4 12

Hence time period = 12 s.

If the amplitude of sound is doubled and the frequency is reduced to one fourth, the intensity of sound at the same point will be

1. increasing by a factor of 2
1. decreasing by a factor of 2
1. decreasing by a factor of 4
1. unchanged

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Intensity is proportional to (amplitude)² and also intensity α (frequency)². Therefore, intensity becomes
2² = 1 th
4² 4

Correct Option: C

Intensity is proportional to (amplitude)² and also intensity α (frequency)². Therefore, intensity becomes
2² = 1 th
4² 4

Two cars moving in opposite directions approach each other with speed of 22 m/s and 16.5 m/s respectively. The driver of the first car blows a horn having a frequency 400 Hz. The frequency heard by the driver of the second car is [velocity of sound 340 m/s] :-

1. 361 Hz
1. 411 Hz
1. 448 Hz
1. 350 Hz

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As we known from Doppler's Effect
f_apprent = f₀ v + v₀ = 400 340 + 16.5
v - v_s 340 - 22

f_apprent = 448 Hz

Correct Option: C

As we known from Doppler's Effect
f_apprent = f₀ v + v₀ = 400 340 + 16.5
v - v_s 340 - 22

f_apprent = 448 Hz