Question 1 What is meant by reflection of sound?
Question 2 What type of surfaces are best for reflecting sound?
Question 3 State the laws of reflection?
Question 4 Define the term echo?
Question 5 Why an echo is heard sooner on a hot day?
Reflection of Sound
Sound can also be made to change its direction and bounce back when it falls on a hard surface.
The bouncing back of sound when it strikes a hard surface is called reflection of sound.
Sound is reflected well from hard surfaces like a wall,metal sheet,hard wood,cliff.
Sound waves are much longer than light waves so they require a much larger area of reflection.
The laws of reflection of light are obeyed during reflection of sound
1)The incident,reflected sound wave and normal at the point of incidence,all lie in same plane.
2)The angle of reflection of sound is always equal to angle of incidence of sound.
If we shout or clap near a suitable reflecting object such as tall building or mountain or big empty hall we will hear the same sound again a little later.
The repetition of sound caused by reflection of sound waves is called echo.
First we hear his original sound,after a little while we hear reflected sound.
An echo is simply a reflected sound.A soft surface tends to absorb sound,so there is no echo.
Speed of sound in air is 344 m/s.If we shout at a wall 344 m away,the sound takes 1 sec to reach the wall.The sound reflects from the wall,and takes another 1 sec to return to us.So we hear the echo 2 sec after we have shouted.
If two sounds reaches ours ears within an interval of 0.1 sec then we cannot hear them as separate sound,they appear to be just one sound.
The human ear can hear 2 sounds separately only if there is a time interval of 0.1 sec between two sounds.
We can hear the original sound and reflected sound separately only if there is a time interval of at least 0.1 sec between them.
The minimum distance from a sound reflecting surface which is necessary to hear an echo,can be calculated as:
344 m/s = ———–
The distance travelled by sound in 0.1 sec is 34.4 m.
The distance from a sound reflecting surface to hear an echo is 17.2 m(In order to hear an echo of our shout,we should be at least 17.2 m away from a sound reflecting surface like wall.)
Speed of sound will be more on hot day than on a cold day.Therefore an echo is heard sooner on a hot day.
If there are several reflecting surfaces,then multiple reflections of sound take place and hence several echoes may be heard.
Rolling of thunder is due to the multiple reflections of sound of thunder from number of reflecting surfaces such as cloud and loud.