Sound

Published by: BhumiRaj Timalsina

Published Date: 24 Jan 2022

Sound in Grade-7, Reference Note

Introduction

A sound is a form of energy which gives the sensation of hearing. It is produced due to the vibration of a body. Acoustics is the study of sound waves. We hear various kinds of sound in our daily life. The sound is a type of mechanical wave as it requires a medium for propagation. It travels in all directions. We hear a different kind of sound in our daily life. Sound cannot travel in a vacuum. Flies cannot hear sound at all. Sound travels faster in solid than through air. There is no sound in space as there is no object to travel through or for the propagation of sound. Sound energy is not used for electricity as it produces a very low level of energy. When whales communicate with each other underwater, their sound can travel up to 800km in the ocean.

Sources of sound

Sound is produced due to the vibration of a body. Every vibrating object is the sources of sound energy. There are various sources of sound energy like radio, guitar, piano, human beings, animals, etc. We are able to hear the different sound because as the sound (vibrations) enters our ear, the ear (eardrum) also vibrates. The vibration of eardrum transmits a sound impulse to the brain through auditory nerve. Sound propagates in the form of a wave. The speed of sound is 767 miles per hour.

Wave motion

Sound needs a medium for propagation. It propagates through air, liquid and solid. Sound propagates in the form of waves. A wave is a disturbance that travels through a medium from one location to another location. Wave motion is the transfer of disturbance from one point to another point with no net transport of the material of the medium itself. Simply wave motion means the disturbance in a medium. The motion of the electromagnetic wave, sound wave, etc. are some examples of wave motion. Depending on the direction of vibration, wave motion is of two types which are as follows,

Transverse wave

A transverse wave is a wave in which the particles of the medium move in a direction perpendicular or right angles to the direction of the wave. A ripple on a pond, waves on a string, wave on a rope, etc. are some examples of the transverse wave. Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving motion perpendicular to the propagation of the wave. It consists of two parts crests and troughs. Crest is the point of maximum displacement and trough is the point of maximum negative displacement of a transverse wave.

Source: sciencewaveproperties.weebly.com Fig: Longitudinal wave and Transverse wave

Longitudinal wave

A longitudinal wave is a wave in which particles of the medium move in a direction parallel to the direction of the wave. Soundwave is a common example of the longitudinal wave. Other examples of the longitudinal wave are earthquake wave, tsunami wave, wave produced in slinky. It consists of two parts, compression, and rarefaction. A compression is a region where the particles are closer to each other and rarefaction is a region where furthest apart.

Propagation of sound wave

Sound waves are longitudinal because the direction of vibration of the particles is parallel to the direction of the wave. Sound propagates or travels through solid, liquid and gas. It cannot travel through a vacuum. Sound can propagate through air. The speed of sound is determined by the properties of the air, and not by the frequency or amplitude of the sound. The speed of sound in air is about 332 meter per second. Similarly, it propagates through solid and liquid. The sound wave travels faster in solid than in liquid and gas. The speed of sound in a water medium is about 1498m/s. The speed of sound in steel is 5200m/s.

Speed of sound in different medium

The speed of sound in a different medium is different. Sound travels faster in solid than in liquid and gas. Since the molecules of solid are closely packed the vibrations are transferred easily and fast. The speed of sound is minimum in the air as the molecules of air are loosely packed . The speed of sound in different medium is as follows,

Names of medium Speed of sound
Sea water (at 25 degree celsius) 1533m/s
Glycerol 1904m/s
Mercury 1450m/s
Diamond 12000m/s
Gold 3240m/s
Rubber 1600m/s
Hydrogen (0 degree celsius) 1286m/s

Factors affecting the speed of sound in air

There are various factors which affect the speed of sound. They are given below,

  1. Density
    The speed of sound is inversely proportional to the density of air. When the density of air increases then the speed of sound decreases and vice-versa. The speed of sound in hydrogen is more than the speed of sound in helium as the density of hydrogen is less than the density of helium.
  2. Temperature
    Temperature has a large effect on the speed of sound. Temperature affects the speed of sound because the temperature can affect the “elastic” qualities of different mediums. In the air, the speed of sound increases when the temperature increases and decreases when the temperature decreases. Sound travels faster in hot summer day then in cold winter day.
  3. Humidity
    It is the amount of water vapour present in the air. Humidity decreases the density of air and vice-versa. So, when the humidity of the air increases then the speed of sound decreases and decreases when the humidity of air increases. The speed of sound in moist air is more than that in dry air.
  4. Wind
    The speed of sound is affected by the direction of movement of wind. If the wind is blowing in opposite direction of the sound in which it travels then the speed of sound decreases and vice- versa.
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