# Class 9 Sound Notes

Sound Notes

## Sound:-

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

#### Vibration:-

A kind of rapid to and fro motion of an object is called vibration.

• Sound is produced by vibrating objects.

## Wave:-

A wave is a disturbance that moves through a medium when the particles of medium set neighboring particles into motion.

Compression:- It is a region of high pressure.

Rarefaction:- It is a region of low pressure.

• Sound needs a material medium to propagate.

## Mechanical wave:-

A wave which need a material medium to propagate is called mechanical wave.

### Types of mechanical wave:-

• Transverse wave
• Longitudinal wave:-

#### Transverse wave:-

The type of wave in which the direction of movement of particles is perpendicular to the direction of propagation of wave is known as transverse wave.
Ex:- Water wave, wave produced in a stretched string etc.

#### Longitudinal wave:-

The type of wave in which the direction of movement of particles is parallel to the direction of propagation of wave is known as Longitudinal wave.
Ex:- Sound wave, wave produced in a slinky etc.

• Light is a transverse wave but not a mechanical wave.

Trough:- A valley in the graph of a wave is called trough.

Crest:- A peak in the graph of a wave is called crest.

### Wave length (λ):-

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

• S.I. unit:- metre (m)

### Frequency (ν):-

The number of oscillations made by a vibrating body per unit time is known as frequency.

• Frequency = Total number of oscillations/Total time taken
• S.I. unit:- Hertz (Hz)

### Time period (T):-

The time taken by a vibrating body to complete one oscillation is known as time period.

• Time period = Total time taken/Total number of oscillations
• S.I. unit:- Second (s)

Frequency = 1/Time Period
ν = 1/T Or, T = 1/ν

### Amplitude (A):-

The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.

### Pitch:-

How the brain interprets the frequency of an emitted sound is called its pitch.

• Pitch of a sound is determined by its frequency.

### Loudness:-

The measure of the response of the ear to the sound is called loudness.

• Loudness of a sound is determined by its amplitude.

### Quality/Timber:-

The characteristics of a sound which enables us to distinguish one sound from another having same pitch and loudness is known as quality or timber.

• The sound which is more pleasant to the ear is said to be of a rich quality like music.
• The sound which is unpleasant to the ear is said to be of a poor quality like noise.

Tone:- A sound of single frequency is called a tone.

Note:- The sound which is produced due to a mixture of several frequencies is called a note.

### Speed of sound:-

The distance traveled by a point on a wave per unit time is called speed of sound.

• Speed of sound = Wavelength/Time Period
v = λ/T
• Speed of sound = Wavelength x frequency
v = λν

### Speed of sound depends on:-

• State of medium:- Speed of sound is more in solid than in liquid and than in gas.
• Temperature:- Speed of sound increases on increasing the temperature of the medium.

### Intensity:-

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

#### Supersonic speed:-

When the speed of an object is more than the speed of sound than its speed is said to be supersonic speed.

#### Shock wave:-

The wave produced by an object in air moving with supersonic speed is called shock wave.

#### Sonic boom:-

When an object moves with supersonic speed then the shock wave produced by it produces a very loud and sharp sound called sonic boom.

• Shock waves can shatter glass & even damage buildings.

## Echo:-

The repetition of sound due to the reflection of original sound after striking on a large & hard obstacle is known as echo.

#### Persistence of hearing:-

The sensation of sound persists in our brain for about 0.1 s. This time is called persistence of hearing.

#### Minimum distance between source & obstacle to hear an echo:-

Speed of sound in air = 344 m/s
Persistence of hearing = 0.1 s
So, distance traveled by sound in 0.1 s = Speed x time
= 344 x 0.1
= 34.4 m
Hence, minimum distance between source & obstacle to hear an echo = 34.4/2 = 17.2 m

## Reverberation:-

When echoes follow so closely behind the original sound so that the original sound appears to be prolonged then this effect of sound is known as reverberation.

#### Reverberation can be controlled by:-

• covering the roof and walls of the auditorium with sound absorbent materials like rough plaster, compressed fiberboard etc.
• using selective seat material on the basis of sound absorbing properties.
• using heavy curtains.

## Uses of multiple reflection of sound:-

• It is used in megaphones, horns, musical instruments, trumpets, shehanais to send sound in a particular direction.
• It is used in stethoscope.
• Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the halls.

Audible range of hearing for humans:- 20 Hz to 20000 Hz.

## Infrasonic sound:-

Sound of frequency less than 20 Hz is called infrasonic sound.

• Some animals which produce infrasonic sound are rhinoceros, whale, elephant.

## Ultrasonic sound:-

Sound of frequency more than 20 kHz is called ultrasonic sound.

• Some animals which produce ultrasound are dolphin, bat, moth and porpoise
• Children under the age of 5 can hear sounds up to 25 kHz.

#### Uses of ultrasound:-

• It is used to clean parts located in hard to reach places.
• It is used to detect cracks and flaws in metal blocks.
• It is used in echocardiography to form the image of heart.
• It is used to form the images of internal organs such as liver, gall bladder, kidneys, uterus, etc. The technique used in this is called ultrasonography.
• Ultrasonography is also used to examine the growth of foetus during pregnancy.
• It is used to break small stones formed in the kidneys into fine grains.

### SONAR (Sound Navigation And Ranging):-

It is a device used to determine the depth of sea and to locate underwater hills, valleys, submarine, ice bergs, sunken ship etc.

Echo-ranging:- The method used in SONAR to determine the depth of sea and to locate underwater objects is called echo-ranging.

## Human Ear:-

• The outer ear is called pinna. It collects the sound from the surroundings.
• Auditory canal drives the sound to the ear drum (tympanic membrane).
• The middle ear contains three bones hammer, stirrup and anvil which amplify the sound.
• Cochlea turns the pressure variations into electrical signals.
• Auditory nerve sends the electrical signals to the brain.