Amplitude and Intensity of Sound Waves

Introduction

Sound waves travel through a medium as alternate compressions and rarefactions. During this propagation, the density of the medium changes periodically.

Two important characteristics of a sound wave are amplitude and intensity. Amplitude tells us how large the density variation is, while intensity tells us how much sound energy passes through a given area in a given time.

Amplitude of a Sound Wave

The amplitude of a sound wave is the maximum change in the density of the medium in a compression or rarefaction compared to the average density.

In simple words, amplitude tells us how much the density of the medium changes from its normal value.

Amplitude = Maximum change in density from average density

Average Density

Average density is the normal density of the medium when there is no sound disturbance.

When a sound wave passes through a medium, the density of the medium increases above average density in compressions and decreases below average density in rarefactions.

Low Amplitude and High Amplitude

A sound wave may have low amplitude or high amplitude.

  • A low amplitude sound wave has a small change in density.
  • A high amplitude sound wave has a large change in density.

Greater amplitude means greater disturbance in the particles of the medium.

Figure

Low Amplitude and High Amplitude Sound Waves Low Amplitude Average density Small amplitude Distance Density High Amplitude Average density Large amplitude Distance Density Amplitude and Energy Small amplitude Less energy Large amplitude More energy Larger amplitude means the sound wave carries more energy.

Explanation of the Figure

The figure compares low amplitude and high amplitude sound waves. In both graphs, the dashed line represents the average density of the medium.

In the low amplitude wave, the density changes only slightly above and below the average density. In the high amplitude wave, the density changes more from the average density.

A wave with larger amplitude carries more energy than a wave with smaller amplitude.

Amplitude and Energy

Amplitude and energy of a sound wave are closely related.

A sound wave with larger amplitude carries more energy. A sound wave with smaller amplitude carries less energy.

For example, when a metal plate is struck gently, it produces a sound of smaller amplitude. When the same plate is struck harder, more energy is transferred to the surrounding medium and a sound of larger amplitude is produced.

Larger amplitude → More energy

Smaller amplitude → Less energy

Activity Example

In the activity with grains placed on a stretched sheet, a loud sound makes the sheet vibrate and causes the grains to move or jump.

If the sound source is struck harder, more energy is transferred to the particles of air. This makes the sheet vibrate with greater displacement, and the grains jump higher.

This shows that a sound wave with larger amplitude carries more energy.

Intensity of Sound

The amount of sound energy passing through a unit area perpendicular to the direction of propagation of sound wave in one second is called the intensity of sound.

Intensity = Sound energy passing through unit area per unit time

Intensity is a measurable physical quantity. It depends on the amount of energy carried by the sound wave and the area over which the sound energy spreads.

Simple Formula for Intensity

If sound energy \(E\) passes through area \(A\) in time \(t\), then intensity can be written as:

\(I=\frac{E}{A\,t}\)

where,

  • \(I\) = Intensity of sound
  • \(E\) = Sound energy
  • \(A\) = Area
  • \(t\) = Time

Intensity and Distance from Source

As a sound wave travels away from its source, it spreads over a larger area.

Since the same sound energy spreads over a larger area, the intensity of sound decreases with distance from the source.

Distance from source increases → Intensity decreases

Figure: Intensity Decreases with Distance

Intensity Decreases with Distance Source Near source High intensity Far from source Low intensity Sound energy spreads over larger area Intensity becomes smaller as distance from source increases.

Explanation of the Figure

The figure shows sound spreading from a source. Near the source, sound energy is concentrated in a smaller area, so the intensity is higher.

Farther from the source, the same energy spreads over a larger area. Therefore, the intensity becomes lower.

Amplitude and Loudness

Humans usually perceive a sound wave of larger amplitude as a louder sound and a sound wave of smaller amplitude as a softer sound.

However, loudness is related to our hearing experience, while intensity is a measurable physical quantity.

Difference Between Amplitude and Intensity

Amplitude Intensity
It is the maximum change in density from average density. It is the sound energy passing through unit area per unit time.
It describes the size of disturbance in the medium. It describes the amount of sound energy received per unit area per unit time.
Larger amplitude means more energy carried by the wave. Greater intensity means more sound energy is received.
It is shown by the height of the wave from the average density line. It decreases as sound spreads over larger area.

Examples from Daily Life

  • A softly struck drum produces a sound of smaller amplitude.
  • A strongly beaten drum produces a sound of larger amplitude.
  • Sound is louder near a speaker because intensity is greater near the source.
  • Sound becomes softer as we move away from the speaker because intensity decreases.
  • A loud sound can make light objects vibrate because it carries more energy.

Important Terms

1. Amplitude

The maximum change in density of the medium from the average density in a compression or rarefaction.

2. Average Density

The normal density of the medium when there is no sound disturbance.

3. Intensity of Sound

The amount of sound energy passing through unit area perpendicular to the direction of sound propagation in unit time.

4. Loudness

The sensation by which we judge a sound as loud or soft.

5. Sound Energy

The energy carried by sound waves through a medium.

Important Points

  • Amplitude is an important characteristic of a sound wave.
  • The amplitude of a sound wave is the maximum change in density from average density.
  • A larger change in density means a larger amplitude.
  • A wave with larger amplitude carries more energy.
  • A wave with smaller amplitude carries less energy.
  • Intensity is the amount of sound energy passing through unit area in unit time.
  • Intensity is a measurable physical quantity.
  • Intensity decreases as distance from the source increases.
  • Sound spreads over a larger area as it moves away from the source.
  • Humans generally hear larger amplitude sounds as louder sounds.

Conclusion

Amplitude and intensity are important characteristics of sound waves. Amplitude is the maximum change in density of the medium from its average density. A larger amplitude means that the sound wave carries more energy. Intensity is the amount of sound energy passing through unit area per unit time. As sound travels away from its source, it spreads over a larger area, so its intensity decreases. Thus, amplitude is related to the energy carried by the wave, while intensity describes how much sound energy passes through a given area in a given time.

en_USEnglish
hi_INहिन्दी en_USEnglish