CBSE Class 9 Physics Sound Notes Set 12

Production and Propagation

Sound is produced by a vibrating body and needs a material medium. In air it travels as longitudinal waves of compressions and rarefactions. Wavelength is the distance between two consecutive compressions, and \( v = n\lambda \). The speed of sound in air is about 330 m/s at room temperature and rises with temperature.

Characteristics

Regular vibrations give a musical note and irregular vibrations give noise. A note has pitch, loudness and quality (timbre). The lowest frequency is the fundamental and the others are overtones, which decide the quality.

Special Effects

Doppler effect: the pitch of an approaching source seems higher and that of a receding source seems lower. Sound refracts when it passes through air layers at different temperatures, because it travels faster in warm air.
A stretched string fixed at both ends forms a standing transverse wave with nodes (zero displacement) and antinodes (maximum displacement).

Resonance Column

Resonance gives the loudest sound when the natural frequency matches the forcing frequency. In a tube dipped in water with a tuning fork of frequency n, the two successive resonance lengths \( l_1 \) and \( l_2 \) give \( v = 2n(l_2 - l_1) \). For n = 512 Hz and \( l_2 - l_1 \) = 32 cm, v = 2 \( \times \) 512 \( \times \) 0.32 = 327.7 m/s.