1. Sound wave and its characteristics:

(i) Sound is produced by vibrating objects.

(ii) Wavelength $(λ)$ is the distance between two consecutive compressions or two consecutive rarefactions. SI unit is metre $(m)$ .

(iii) Frequency $(ν)$ is the number of complete waves produced in one second. SI unit is hertz $(Hz)$ .

(iv) Time period $(T)$ is the time taken by two consecutive compression/rarefaction to cross a fixed point. Its SI unit is second $(s)$ .

(v) Amplitude $(A)$ is the maximum displacement of particles of the medium from their original mean position. Its SI unit is metre $(m)$ .

(vi) The distance travelled by a wave in one second is called speed $(v)$ .

(vii) Some important relations are (i) $T = \frac{1}{ν}$ , (ii) $v = \frac{λ}{T} = ν λ$

(viii) Pitch is that characteristic of sound by which we can distinguish between the different sounds of the same loudness.

(ix) The quality (or timbre) of sound is that characteristic of sound which enables us to distinguish one sound from another having the same pitch and loudness.

(x) Vibration means a kind of rapid to and fro motion of an object. When an object vibrates, it sets the particles of medium around it in vibration.

(xi) The measure of sound energy reaching the ear per second is known as loudness.

(xii) The loudness of a sound is measured in decibel $(d B)$ . It depends on the sensitivity or the response of our ears.

(xiii) The amount of sound energy passing each second through unit area is known as the intensity of sound.

(xiv) The SI unit of intensity is watt per square metre $(\frac{W}{m^{2}})$ .

2. Reflection of sound:

(i) The bouncing back of sound when it strikes a hard surface is known as a reflection of sound.

(ii) The repetition of sound caused by the reflection of sound waves is called an echo.

(iii) The minimum distance between the source of the sound and the reflector should be at least $17 m$ (so that the echo is heard distinctly after the original sound is over).

3. Ultrasonic Sound:

(i) Some animals like bats, dolphins, etc., locate their prey by hearing the echoes of their ultrasonic squeaks.

(ii) In the medical field, echoes of the ultrasound is used to investigate the internal organs of the human body such as liver, gall bladder, heart, etc.

(iii) SONAR stands for sound navigation and ranging.

(iv) SONAR is an apparatus used to find the depth of a sea or to locate the underwater things like shoals of fish, shipwrecks, hills, icebergs, sunken ships and enemy submarines, etc.

(v) SONAR consists of two parts (i) a transmitter and (ii) a receiver. The transmitter produces and transmits ultrasonic waves. The detector converts the ultrasonic waves into electrical signals.

4. Forced and Damped vibration:

(i) If a body vibrates with its natural frequency without the help of any external periodic forces, then it is said to be natural (or free) vibrations.

(ii) The amplitude and frequency of a freely vibrating body remain constant with respect to time.

(iii) When a body vibrates under the influence of an external periodic force with a frequency equal to that of the external periodic force, then its vibrations are called forced vibrations.

(iv) The periodic vibrations of a body of decreasing amplitude in presence of some resistive force are called the damped vibrations.

5. Resonance:

When the frequency of an externally applied periodic force on a body is equal to its natural frequency, then the body readily begins to vibrate with an increased amplitude, this is known as resonance.

Forced and resonant vibrations are the same.

Important Questions on Sound

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