ICSE Physics Class 8 Notes Chapter 7 Sound
→ SOUND : “Is energy which produces in us the sensation of hearing.” It is produced by vibration of body.
→ Sound needs a medium for its propagation. Sound cannot travel in vacuum.
→ Speed of sound is maximum in solids. 5000 m s-1 in steel, in water 1500 m s-1 and in air it is least 330 m s-1 nearly.
→ When a body vibrates, the particles of medium also start vibrating and K.E. of particles changes into potential energy and P.E. into K.E. This is why sound in energy.
→ Sound travels in a medium in the form of wave.
→ Longitudinal wave: When the particles of medium move in the direction of motion of wave by forming compression and rarefaction.
AMPLITUDE : “The maximum displacement of the particle of medium on either side of mean position.”
TIME PERIOD : “The time taken by a particle of medium to complete its one vibration’ ‘t’.
FREQUENCY : “The number of vibrations made by a particle of the medium in one second.”
f measured in Hertz (Hz)
FREQUENCY : f = \(\frac {1}{t}\) or t = \(\frac {1}{f}\)
WAVE LENGTH : “The distance travelled by the wave in one one time period of vibration of particle of medium.”
Or
“The distance between two consecutive compressions or between two consecutive rarefactions.” It is denoted by ‘ λ ’ and S.I. unit of wave length is metre (m).
CHARACTERISTICS OF SOUND :
- Loudness.
- Pitch (or shrillness).
- Quality (or timbre or wave form).
LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be distinguished from a faint sound, both having same frequency and same wave form.
It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) decibel.
PITCH : It depends on number of vibrations per second or frequency: more frequency is high pitch shrilled sound and low frequency is flat sound.
QUALITY : is the characteristic which distinguishes two sounds of the same pitch and same loudness. It depends on wave form.