CBSE Class 9 Physics Waves and Sound Notes Set 15

Wave Motion

A wave carries energy from one place to another without carrying matter. When a stone falls into water, a cork on the surface only bobs up and down while the ripples spread out. Each particle of the medium vibrates about a fixed position and passes the disturbance on to its neighbour.
The speed of a wave depends on the medium, and not on the source. It is the same in all directions if the medium is the same on all sides.

Mechanical and Non-mechanical Waves

Mechanical (elastic) waves need a material medium. Sound is mechanical. Light, radio waves, X-rays and all electromagnetic waves need no medium and travel through vacuum at about \( 3\times10^{8} \) m/s.
The speed of a mechanical wave depends on the elasticity and the inertia (density) of the medium. Sound in water is about 1500 m/s while light in water is about \( 2.25\times10^{8} \) m/s.

Transverse and Longitudinal Waves

Terms Used for Waves

The key relation is \( v = f\lambda = \lambda / T \). Crest to the next trough is \( \lambda/2 \).

Properties of Sound

Pitch depends on frequency: a higher frequency gives a higher pitch. Loudness depends on amplitude. Quality (timbre) lets us tell two sounds of the same pitch and loudness apart.
Human hearing covers about 20 Hz to 20,000 Hz. Below 20 Hz the sound is infrasonic and above 20,000 Hz it is ultrasonic. Ultrasound is used for hunting whistles, by bats to locate prey, and in sonar for finding depths.

Echo and Reverberation

An echo is sound heard after reflection from a distant surface. The ear keeps a sound for about 0.1 s, so the reflecting surface must be at least about 17 m away to hear a distinct echo. Distance to a reflector = (speed of sound × time) / 2.
Repeated reflection in a hall makes sound persist, which is called reverberation. It is reduced by covering walls and roof with sound-absorbing material.
Uses of reflection of sound include the megaphone, the hearing aid, the concave sound board behind a speaker, and sonar.

Simple Pendulum

A small heavy bob hung from a light string that swings in a vertical plane is a simple pendulum. Its time period is \( T = 2\pi\sqrt{L/g} \).
T depends on the length and on g but not on the mass of the bob or the amplitude (for small swings). T squared is directly proportional to L, so a graph of T squared against L is a straight line through the origin. A pendulum with T = 2 s is a second's pendulum; its length is about 1 m on earth.
The bob has maximum kinetic energy at the mean position and maximum potential energy at the extreme positions.