Question

Answer the following questions briefly.
(a) 'A' vibrates 200 times in 5 s, while 'B' vibrates 35 times in 1 s. Which sound will have a lower pitch?
(b) Why do we need two ears?
(c) Why can't we hear the sound made by bats?
(d) When a bee flies, we hear the sound produced by its wings. Why can't we hear a crow flapping its wings as it flies?
(e) How can you raise the pitch of the note of a veena string without changing its length?
(f) The harder you beat a drum, the louder is the sound produced? Explain why this is so.
(g) Why are the walls and ceiling of a cinema hall covered with sound-absorbing material?

Answer 1

(a) Pitch depends upon frequency also.
Frequency = (Number of vibrations) / (Time taken for the vibrations)
Frequency of A = 200 vibrations / 5 sec
= 40 Hz
Frequency of B = 35 vibrations / 1 sec
= 35 Hz

Thus, B has the lower pitch.

(b) We need two ears to locate a sound. If sound is produced on our left, the vibrations of the air reach our left ear just before they reach our right ear. They are heard more loudly in the left ear than in the right ear. The difference in the loudness of the sound in the two ears helps us to identify the direction the sound came from.

(c) We cannot hear the sound made by bats because they produce ultrasonic sound of vibrations more than 20000 Hz, which is not audible to the human ear.

(d) Human beings can hear a sound if its vibrations are within the frequency range of 20 Hz to 20 kHz. The sound vibrations produced by the wings of a flying bee are in this range, so this sound is audible to human. However, the sound produced by the flapping wings of a crow has a very low frequency of less than 20 Hz, so it is not audible to humans.

(e) We can raise the pitch of a note of a Veena string by tightening the string.

(f) The stretched skin on the top of a drum vibrates when it is struck. When we beat a drum harder, vibrations of a large amplitude are produced. The loudness of sound depends on the amplitude of the vibrations. Sounds of large amplitudes are loud.

(g) The walls and ceiling of a cinema hall are covered with sound-absorbing material to prevent the reflection of sound, or echo.