Energy, Graphical Representation & Wave Characteristics — lesson. Physics CBSE Live product, Class 9 (2026-27).

Energy of Sound waves:

A sound wave carries energy from one place to another through a medium. The particles of the medium only vibrate about their mean positions; they do not travel along with the sound wave.

When grains placed on a stretched sheet move as the sheet vibrates, it shows that the vibrating sheet transfers energy to the grains. Similarly, a sound wave transfers energy through the medium.

Microphone: Sound energy $\to$ Electrical energy
Speaker: Electrical energy $\to$ Sound energy

Graphical representation of Sound:

Regions of high density and low density

Sound travelling through a medium produces alternating regions of:

Compression: particles are closer together $\to$ high density
Rarefaction: particles are farther apart $\to$ low density

In a Density-versus-Distance graph:

Maximum density $\to$ compression/crest
Minimum density $\to$ rarefaction/trough
Distance between two successive compressions or rarefactions → wavelength
$\lambda\ =\ distance\ between\ successive\ compressions$

$Density-versus-Time graph:$

If we observe the density at one fixed position as sound passes through it, we can plot density against time.

One complete cycle of this graph represents one time period .

Distance vs density graph

Wavelength, Frequency and Time Period:

Wavelength $($\lambda$):$

Wavelength is the distance between two successive points in the same phase, such as two successive compressions or rarefactions.

SI unit: $metre$ ($m$)

Frequency $($\nu$):$

Frequency is the number of complete vibrations produced by the source in one second.

SI unit: $hertz$ (Hz).

For example:

$500\ Hz\ =\ 500\ vibrations\ per\ second$

$Time Period ($T$):$

Time period is the time taken by the source to complete one vibration.

SI unit: $second$ ($s$).

Frequency and time period are reciprocals:

\(\nu=\frac{1}{T}\)

\(T=\frac{1}{\nu}\)

$Phyphox:$

A smartphone application such as Phyphox can be used to analyse sound signals . Students can observe the frequency of different sounds or musical notes .

Different musical notes have different frequencies, which helps us distinguish one note from another.

Amplitude and Intensity:

Amplitude:

In a sound wave, amplitude represents the maximum change in density from the average density.

A larger amplitude means a greater variation in density.

Amplitude is related to the energy carried by the sound wave:

Greater amplitude \(\to\) greater energy

Thus, a louder sound generally has a greater amplitude than a softer sound.

$Sound waves denoting amplitude$

$Intensity:$

Intensity is related to the sound energy passing through a unit area .

As sound travels away from its source, its energy spreads over a larger area. Therefore, the intensity of sound decreases with distance .

Distance increases \(\to\) Intensity decreases

$Speed of Sound:$

The speed of sound is the distance travelled by a sound wave per unit time .

The relationship between speed, wavelength and frequency is:

\(v=\nu\lambda\)

$What Determines the Speed of Sound?$

The speed of sound depends mainly on the medium through which it travels .

Sound can travel through solids, liquids and gases, and its speed is different in different media.

For sound travelling through air, the speed is also affected by:

Temperature
Humidity

In general, the speed of sound in air increases with increase in temperature.

When Sound enters another medium:

The frequency is determined by the vibrating source. Therefore, when sound enters another medium, its frequency remains the same.

However, the speed of sound can change because the medium has changed. Since:

\(v=\nu\lambda\)

if $v$ changes while $\nu$ remains constant, the wavelength also changes.

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1. Energy, Graphical Representation & Wave Characteristics

Theory: