# HBSE 9th Class Science Notes Chapter 12 Sound

Haryana State Board HBSE 9th Class Science Notes Chapter 12 Sound Notes.

## Haryana Board 9th Class Science Notes Chapter 12 Sound

→ Sound is a form of energy which produces a sensation of hearing in our ears.

→ Energy can neither be created nor destroyed.

→ Sound needs a material medium to travel.

→ Sound travels as longitudinal waves through liquid medium.

→ In sound propagation, it is the energy of sound that travels and not the particles of the medium.

→ Sound cannot travel in vacuum.

→ Sound travels as successive compressions and rarefactions in the medium.

→ The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.

→ The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ (lambda).

→ Time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period (T).

→ SI unit of frequency of sound is hertz (Hz).

→ The number of complete oscillations per unit time is called the frequency (v), v = $$\frac{1}{T}$$.

→ The relation among velocity of sound (v), frequency (v) and wavelength (X ) is, v = A.v

→ The speed of sound depends primarily on the nature and the temperature of the transmitting medium.

→ The amount of sound passing each second through unit area is called the intensity of sound.

→ According to law of reflection of sound, The direction in which the sound is incident and reflected make equal angles with the normal to the reflecting surface and the three lie in the same plane.

→ For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.

→ The reflection of sound is used in megaphones, horns, trumpets and shehnais.

→ Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties.

→ The amount of sound energy passing each second through unit area is called the intensity of sound.

→ The audible range of hearing for average human beings is in the frequency range of 20 Hz-20 kHz.

→ Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”.

→ Ultrasound has many medical and industrial applications.

→ The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs, sunken ship, etc.

→ The expanded form of SONAR is Sound Navigation and Ranging.

→ Ultrasonic waves form the image of the heart which is called “echocardiography.’’

→ Ultrasonography is used for examination of the foetus during pregnancy to detect congenial defects and growth abnormalities.

→ Sound: Sound is a form of energy which produces a sensation of hearing in our ears.

→ Mechanical Waves: The characterstics waves produced by the motion of the particles of the medium is called mechanical waves.

→ Compression: A region of high pressure is called compression.

→ Rarefaction: A region of low pressure is called rarefaction.

→ Longitudinal Wave: When the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance, is called longitudinal wave.

→ Transverse Wave: When the individual particles of the medium move in a direction perpendicular to the direction of wave propagation, is called transverse wave.

→ Time Period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave.

→ Pitch: How the brain interprets the frequency of an emitted sound is called the pitch.

→ Amplitute: The magnitude of the maximum disturbance in the medium on either side of the mean value.

→ Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is called wavelength.

→ Velocity of Sound: The distance which a point on wave travels per unit time is called velocity of sound.

→ Intensity of Sound: The amount of sound passing each second throw unit area is called the intensity of sound.

→ Reflection of Sound: When the sound returns back in the same medium after reflection is called reflection of sound.

→ Echo: The reflected sound is called echo.

→ Reverberation: The repeated reflection that results in the persistence of sound is called reverberation.

→ Ultrasonic Sound: Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound.