Maharashtra Board Class 9 Science Chapter 12 Study of Sound PDF Download

Study of Sound

Sound is a form of energy which creates the sensation of hearing in our ears. This energy is in the form of waves. A medium is necessary for the propagation of sound waves. Sound waves give rise to a chain of compression (place of higher density) and rarefaction (place of lower density) in the medium. The particles of the medium oscillate about their central or mean positions, in a direction parallel to the propagation of the wave. Such waves are called longitudinal waves. On the other hand, in the waves created by dropping a stone in still water, the particles of water oscillate up and down. These oscillations are perpendicular to the direction of propagation of the wave, such waves are called transverse waves.

Teacher's Note

When you clap your hands, you make sound. The sound travels in waves through the air to your ears. This is like when you drop a stone in water and see ripples.

Exam Trick

Remember: Longitudinal waves = sound waves. Transverse waves = water ripples. Sound moves back and forth. Water ripples move up and down.

Points to Remember

Sound is a form of energy.
Sound needs a medium to travel.
Longitudinal waves have compressions and rarefactions.
Transverse waves move up and down.
Sound waves make the particles of the medium move.

A sound wave can be shown in the form a graph. At any moment during the propagation of a sound wave we would find alternate bands of compression and rarefaction of the medium i.e. bands of greater and lesser density. Figure 12.1 A shows the changes in density while figure B shows the changes in pressure. The changes in density or pressure are shown in the form of a graph in figure C.

The wavelength of sound waves is indicated by the Greek letter lambda (\(\lambda\)), while its frequency is indicated by nu (\(\nu\)). The amplitude, which is the maximum value of pressure or density, is indicated by A. The time taken for one oscillation of pressure or density at a point in the medium is called the time period and is indicated by T.

The value of frequency determines the pitch (high or low) of the sound while the value of the amplitude determines its strength or loudness.

Teacher's Note

High frequency means high pitch, like a whistle. Low frequency means low pitch, like a drum. Amplitude means how loud the sound is.

Exam Trick

Remember: Frequency = pitch (high or low). Amplitude = loudness (soft or loud). These are different things.

Points to Remember

Wavelength is shown by the Greek letter lambda.
Frequency is shown by the Greek letter nu.
Frequency decides the pitch of sound.
Amplitude decides the loudness of sound.
Time period is the time for one complete oscillation.

Velocity of Sound

The distance covered by a point on the wave (for example the point of highest density or lowest density) in unit time is the velocity of the sound wave.

\[\text{Velocity} = \frac{\text{Distance}}{\text{Time}}\]

\[\text{Velocity} = \frac{\text{Wavelength}}{\text{Time period}}\]

\[v = \nu \lambda\]

because \[\nu = \frac{1}{T}\]

Any point on the sound wave covers a distance equal to \(\lambda\) (wavelength) in time T (time period). Thus the velocity of sound is given by:

\[v = \lambda \times \nu\]

Thus, the velocity of sound = wavelength × frequency.

In any medium at fixed physical conditions the velocity of sound of different frequencies is very nearly the same. The velocity is highest in solids and least in gases. It increases with an increase in the temperature of the medium.

Teacher's Note

Sound travels fastest in metals like steel and slowest in gases like air. When you tap a pipe, the sound reaches you through the metal before the air.

Exam Trick

Remember: Solid = fastest sound. Gas = slowest sound. The denser the material, the faster the sound travels.

Points to Remember

Velocity of sound = wavelength × frequency.
Sound travels fastest in solids.
Sound travels slowest in gases.
Higher temperature means faster sound.
Different frequencies travel at the same speed in the same medium.

Velocity of Sound in a Gaseous Medium

The velocity of sound in a gaseous medium depends on the physical conditions i.e. the temperature, density of the gas and its molecular weight.

Temperature (T)

The velocity of sound is directly proportional to the square root of the temperature of the medium. This means that increasing the temperature four times doubles the velocity.

\[v \propto \sqrt{T}\]

Density (ρ)

The velocity of sound is inversely proportional to the square root of density. Thus, increasing the density four times, reduces the velocity to half its value.

\[v \propto \frac{1}{\sqrt{\rho}}\]

Molecular Weight (M)

The velocity sound is inversely proportional to the square root of molecular weight of the gas. Thus, increasing the molecular weight four times, reduces the velocity to half its value.

\[v \propto \frac{1}{\sqrt{M}}\]

For a fixed temperature, the velocity of sound does not depend on the pressure of the gas.

Teacher's Note

When you heat the air, sound travels faster through it. This is why sound travels faster on hot days than cold days.

Exam Trick

Remember: Hot air = faster sound. Cold air = slower sound. Temperature is the most important factor that changes sound velocity in gas.

Points to Remember

Sound velocity increases with temperature increase.
Sound velocity decreases with density increase.
Sound velocity decreases with molecular weight increase.
Pressure does not affect sound velocity in gas.
Temperature is the main factor affecting sound in air.

Audible, Infra- and Ultra-sound

The limits of hearing of the human ear are 20 Hz to 20,000 Hz. That is, the human ear can hear sounds of frequencies in this range. These sounds are called audible sounds. Our ears cannot hear sounds of frequencies lower than 20 Hz and higher than 20,000 Hz (20 kHz). Sound with frequency smaller than 20 Hz is called infra sound. The sound produced by a pendulum and the sound generated by the vibrations of the earth's crust just before an earthquake are examples of such sounds. Sound waves with frequency greater than 20 kHz are called ultrasound.

The dog, mouse, bat, dolphin etc have a special ability to hear ultrasonic sounds. Thus, they can sense some noise which are inaudible to us. Children under 5 years of age and some creatures and insects can hear waves with frequency up to 25 kHz. Bats, mice, dolphins, etc, can also produce ultrasound.

Teacher's Note

Bats make ultrasound and use it to find insects in the dark. Dogs can hear sounds that humans cannot hear. This is why dogs bark at things we cannot hear.

Exam Trick

Remember: Audible = 20 Hz to 20,000 Hz (human ear). Infra sound = less than 20 Hz (earthquake sounds). Ultrasound = more than 20,000 Hz (bat and dog sounds).

Points to Remember

Human ears can hear 20 Hz to 20,000 Hz only.
Infra sound has frequency less than 20 Hz.
Ultrasound has frequency more than 20,000 Hz.
Bats and dogs can hear ultrasound.
Earthquakes produce infra sound.

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