Properties of Sound | Science Class 9 PDF Download

CHARACTERISTICS OF A SOUND WAVE

When a sound wave travels through a material medium, then the density or pressure of the medium changes continuously from maximum value to minimum value and vice-versa. Thus, the sound wave propagating in a medium can be represented as shown in figure.

Now, we shall discuss the characteristics or quantities to describe a sound wave.

(i) Amplitude : The maximum displacement of a vibrating body or particle from its rest position (i.e. mean position) is called amplitude. In S.I., unit of amplitude is metre (m).

(ii) Wavelength (or length of a wave) : The distance between two successive 1 regions of high pressure or high density (or compressions) or the distance between two successive regions of low pressure or low density (or rarefactions) is known as wavelength of a sound wave. It is denoted by l (read as lambda).

In S.I., unit of wavelength is metre (m).

(iii) Frequency : The number of oscillations or vibrations made by a vibrating body or particles of a medium in one second is known as the frequency of a wave. It is denoted by u (read as Neu). In S.I., unit of frequency is hertz (Hz).

1 hertz = one oscillation completed by a vibrating body or a vibrating particle in one second.

(iv) Time period : Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as time period. It is denoted by T.
In S.I., unit of time period is second(s).
Relation between Frequency and time period Let T = time period of a vibrating body.
Then number of oscillations completed in T second = 1
Number of oscillations completed in 1 second = 1\T
But number of oscillations completed in 1 second = frequency (v)

(f) n = 1\T, frequency = 1\ time period

(iv) Pitch or Shrillness : Pitch is the characteristic (i.e., typical feature) of a sound that depends on the frequency received by a human ear.

A sound wave of high frequency has high pitch and a sound wave of low frequency has a low pitch.

You must have noticed that the voice of a woman has higher pitch than the voice of a man. Thus, the frequency of woman's voice is higher than the frequency of man's voice.

A sound wave of low pitch (i.e. low frequency) is represented by figure (a) and a sound wave of high pitch (i.e. high frequency) is represented by figure (b)

(v) Loudness : Loudness of a sound depends on the amplitude of the vibrating body producing the sound.
A sound produced by a body vibrating with large amplitude is a loud sound. On the other hand, a sound produced by a body vibrating with small amplitude is a feeble or soft sound. Loud sound and soft or feeble sound are represented as shown in Figure (a) and (b) respectively.

Loudness is a subjective quantity : It depends on the sensitivity or the response of our ears. A loud sound to a person may be a feeble sound for another person who is hard of hearing.

(vi) Timbre or quality : Quality or timbre is a characteristic (i.e., a typical feature) of a sound which enables us to distinguish between the sounds of same loudness and pitch.

This characteristic of sound helps us to recognise our friend from his voice without seeing him.
The quality of two sounds of same loudness and pitch produced by two different sources are distinguishable because of different waveforms produced by them. The waveforms produced by a vibrating tuning fork, violin and flute (Bansuri) are shown in figure.

(vii) Intensity : Intensity of a sound is defined as the sound energy transferred per unit time through a unit area placed perpendicular to the direction of the propagation of sound.

That is, intensity of sound

Intensity of a sound is an objective physical quantity. It does not depend on the response of our ears.

In S.I., unit of intensity of sound is joule s^-1 m^-2 or watt m^_2 ( 1Js^-1 = 1W)

We have seen above that sound can travel through solids, liquids and gases. The question which comes to mind is how fast does sound travel? Sound travels at different speeds in different media.

The speed of sound depends on the following factors :

The properties (or nature) of the medium. The order of the speed of sound is

Solids > Liquids > Gases

Temperature

Pressure

In any medium, the speed of sound is increases with a rise in temperature.

As per definition,

Speed of sound

The speed of light in the air (or more correctly in vacuum) is 3 × 10⁸ m/ s, (3lakh kilometre per second).

Conclusion
Speed of sound in solids is greater than the speed of sound in liquids and the speed of sound in liquids is greater than the speed of sound in gases.

Speed of sound in various media

REFLECTION OF SOUND
When a sound wave travelling in a medium bounces back to the same medium after striking the second medium, reflection of sound wave is said to take place. The reflection of sound wave is similar to the bouncing back of a rubber ball after striking a wall or the surface of a floor.

Just like light, sound is reflected by the solid and liquid surfaces. The reflection of sound obeys the laws of reflection.

The laws of reflection of sound are as follows :

(i) Incident angle = Reflected angle and (ii), The incident direction of sound, reflected direction of sound and the normal to the point of incidence all lie in the same plane.

Echo
If we clap our hands while standing at some distance from a high and huge wall or a hill, we hear the clapping of our hands again after some short interval of time. The sound of clap heard by us is known as echo. Echo is produced due to the reflection of sound.

Thus, echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.

Conditions for the production of an echo

1. Time gap between the original sound and the reflected sound

We can hear the two sounds separately if the time gap between these two sounds is more than 1/10 s or 0·1 s. The time interval equal to 0·1 s is known as persistance of hearing. This means, the impression of any sound heard by us remains for 0·1 s in our brain. If any other sound enters our ears before 0·1 s, then the second sound will not be heard by us. Thus, the echo will be heard if the original sound reflected by an obstacle reaches our ears after 0·1 s.

2. Distance between the source of sound and obstacle

Minimum distance between the observer and the obstacle for echo to be heard

Let
Distance between the observer and the obstacle = d

Speed of sound (in the medium) = v

Time after which echo is heard = t

Then,

We know

Speed of soune; in air at 25°C = 343 m s^-1

For an echo to be heard distinctly,

t ≥ 0.1 s

Then d  or d ≥ 17.2 m

Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.

The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25°C is more than 17.2m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.

3. Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle .must be rigid such as a building, hill or a cliff.

4. Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.

Reverberation

The repeated reflection that results in the persistence of sound in a large hall is called reverberation.

Excessive reverberation in any auditorium/hall is not desirable because the sound becomes blurred and distorted. The reverberation can be minimised/reduced by covering the ceiling and walls with sound absorbing materials such as, fiber-board, rough plaster, draperies, perforated carboard sheets etc.

Uses of multiple Reflection of sound
1. Megaphone : Megaphone is a device used to address public meetings. It is a orn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the people (or audience) without much spreading.

2. Hearing Aid : Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.

3. Sound boards : Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e. source of sound) placed at the focus of the sound board as shown in figure.

Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).

4. Stethoscope : Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.

5. Ceilings of concert halls are curved : The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling as shown in figure. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.