# PSEB 9th Class Science Notes Chapter 12 Sound

This PSEB 9th Class Science Notes Chapter 12 Sound will help you in revision during exams.

## PSEB 9th Class Science Notes Chapter 12 Sound

→ Sound is a kind of energy that produces a sensation of hearing in our ears.

→ Sound is produced due to vibrations of an object, Vibration means the to and fro movement of an object.

→ In human voice, the sound is produced due to the vibration of the larynx.

→ The substance through which sound travels is called medium.

→ Wave is a sort of disturbance.

→ For transmission of sound, a medium is required and the air is generally used as a medium.

→ Sound can not travel through a vacuum.

→ In transverse waves, the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave.

→ The distance between two consecutive rarefactions is called wavelength.

→ The movement of compressions passing through a point in a unit of time is called frequency.

→ The time taken by two consecutive compressions or rarefactions passing through a fixed point is called the frequency of the wave.

→ The relation between velocity (υ), frequency (ν) and wavelength (λ) of the wave is υ = ν × λ

→ The velocity sound depends mainly upon the nature of medium and temperature.

→ To hear an echo the minimum time elapsed between the original sound and reflected sound should be 0.1 seconds.

→ Sound energy passing through a unit area in 1 second is called the intensity of sound.

→ The human audible range is 20 Hz to 20 kHz.

→ Sound travels through a substance in the form of longitudinal waves.

→ Sound travels through air medium in the form of compressions and rarefactions.

→ Compression is a region of high pressure and maximum density.

→ Sound: It is a kind of energy that produces in us the sensation of hearing.

→ Amplitude: The maximum displacement of the particle of the medium on either side of its mean position during vibration is called its Amplitude.

→ Frequency: The number of vibrations made by the vibrating body in 1 second is called its frequency.

→ Wavelength: Whatever time is taken by the particle to complete 1 vibration during the same time the distance travelled by the wave is called wavelength. It is denoted by ‘λ’.

→ Wave velocity: In a medium, the magnitude of the product of frequency and wavelength of a wave is equal to wave velocity i.e. υ = ν × λ

→ Oscillation or Vibration: The to and fro motion of a vibrating particle about its mean position to complete 1 vibration is called oscillation or vibration.

→ Periodic motion: The motion which is repeated after a fixed interval of time is called periodic motion,

→ Rarefaction: In longitudinal waves, the region where the particles of the medium are farther apart than the normal distance is called rarefaction. Here the pressure is lower than normal.

→ Echo: The reflected sound is called echo.

→ Transverse waves: Those waves in which the particles of the medium vibrate about their mean position in a direction perpendicular to the direction of propagation of the wave, such waves are called transverse waves.

→ Longitudinal waves: Those waves in which the particles of the medium vibrate forward and backward in the same direction in which the wave travels then the wave is called a longitudinal wave.

→ Ultrasonic waves: Those longitudinal waves which have a frequency of more than 20000 Hertz (Hz) are called ultrasonic waves.

→ SONAR (Sound Navigation and Ranging): It is a device that measures the time for producing sound waves and reflected waves.

→ Time Period: The time taken to complete one complete vibration (i.e. to travel from mean position to one extreme position than to other extreme position and back is called time period. It is denoted by ‘T’ and expressed in seconds.

→ Compression: Due to the forwarding motion of the longitudinal wave, the region where the particles of medium come close to each other than the normal distance between them is called compression. Here the pressure is higher than normal.