Waves · Question 14

A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 m s-1. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air = 330 m s-1)

1. 838 Hz
2. 885 Hz
3. 765 Hz
4. 800 Hz

The second overtone of an open organ pipe has the same frequency as the first overtone of a closed pipe L metre long. The length of the open pipe will be

1. L
2. 2L
3. L2
4. 4L

Three sound waves of equal amplitudes have frequencies (n - 1), n, (n + 1). They superimpose to give beats. The number of beats produced per second will be

1. 1
2. 4
3. 3
4. 2

A string is stretched between fixed points separated by 75.0 cm. It is observed to have resonant frequencies of 420 Hz and 315 Hz. There are no other resonant frequencies between these two. The lowest resonant frequency for this string is

1. 10.5 Hz
2. 105 Hz
3. 155 Hz
4. 205 Hz

The fundamental frequency of a closed organ pipe of length 20 cm is equal to the second overtone of an organ pipe open at both the ends. The length of organ pipe open at both the ends is

1. 120 cm
2. 140 cm
3. 80 cm
4. 100 cm

A source of sound S emitting waves of frequency 100 Hz and an observer O are located at some distance from each other. The source is moving with a speed of 19.4 m s-1 at an angle of 60 with the source observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer (velocity of sound in air 330 m s-1), is [figure]

1. 106 Hz
2. 97 Hz
3. 100 Hz
4. 103 Hz

If n1, n2 and n3 are the fundamental frequencies of three segments into which a string is divided, then the original fundamental frequency n of the string is given by

1. 1n = 1n1 + 1n2 + 1n3
2. 1 n = 1 n1 + 1 n2 + 1 n3
3. n = n1 + n2 + n3
4. n = n1 + n2 + n3

The number of possible natural oscillations of air column in a pipe closed at one end of length 85 cm whose frequencies lie below 1250 Hz are (Velocity of sound = 340 m s-1)

1. 4
2. 5
3. 7
4. 6

A speeding motorcyclist sees traffic jam ahead of him. He slows down to 36 km hour-1. He finds that traffic has eased and a car moving ahead of him at 18 km hour-1 is honking at a frequency of 1392 Hz. If the speed of sound is 343 m s-1, the frequency of the honk as heard by him will be

1. 1332 Hz
2. 1372 Hz
3. 1412 Hz
4. 1454 Hz

If we study the vibration of a pipe open at both ends, then the following statement is not true.

1. All harmonics of the fundamental frequency will be generated.
2. Pressure change will be maximum at both ends.
3. Open end will be antinode.
4. Odd harmonics of the fundamental frequency will be generated.