Sound – Important Questions

Q1: A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Ans.
    Given that,
Number of oscillation    = 40

Total time taken        = 4 seconds

Time period                = time taken in one oscillation

                                     = \(\frac{Total  time}{Total   number   of  oscillations}\)

                                     = \(\frac{4 seconds}{40}\)

                                     = \(\frac{1}{10}\) = 0.1 second

Again, frequency       = number of oscillations per second

                                     = \(\frac{Number  of  vibrations }{ Time  taken}\)

                                     = \(\frac{40}{4}\)

                                     = 10 per second = 10 Hz.

Q2: The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Ans.

Number of vibrations per second = 500

Time period = time taken for one vibration

                      = \(\frac{Total  time}{Total   number   of  vibrations}\)
                      = \(\frac{1}{500}\) Hz
                      = 0.002 Hz.

Q3: Identify the part which vibrates to produce sound in the following instruments:

        (a) Dholak            (b) Sitar            (c) Flute

Ans.

(a) Stretched membrane

(b) String of sitar

(c) Air column.

Q4: What do you understand by stringed instruments?

Ans. Those instruments have taut strings, which vibrate when they are plucked, struck, or played with a bow.

Q5: Which object is vibrating when the following sounds are produced?

(i) The sound of a sitar or veena.

(ii) The sound of the tabla.

(iii) The sound of a school bell.

(iv) The buzzing of a bee or a mosquito.

(v) The sound of a bursting balloon.

(vi) The radio.

Ans.

(i) Strings of sitar or veena vibrate to produce sound.

(ii) The skin of the tabla vibrate to produce sound.

(iii) The disc of the bell when beated with hammer vibrate to produce sound.

(iv) Wings of the mosquito or a bee vibrate to produce sound.

(v) Air vibrates to produce sound.

(vi) In the radio speakers vibrate to produce sound.

Q6: What is the normal length of vocal cords in man?

Ans. The normal length of the vocal cord of a man is about 20 mm long.

Q7: What is the unit of the loudness of sound? Explain giving the loudness of few sounds.

Ans. The loudness is expressed in a unit called decibel (dB). It is essentially a ratio if the actual loudness to some base loudness level. The base level is usually defined as that loudness of sound that the human ear can just perceive. The following table gives some idea of the loudness of sound coming from various sources:

At 80 dB the noise becomes physically painful. 

Q8: We hear sound only if its frequency is greater than ……. and lower than ……. .

Ans. 20 hertz, 20000 hertz.

Q9: A tight membrane produces sound of ……. frequency than a loose one.

Ans. Higher.