Reverberation

Question 1 Define the term reverberation?

Question 2 What is a megaphone. Name the principle on which a megaphone work?

Question 3 What is a stethoscope. Name the principle on which a stethoscope work?

Question 4 What is a soundboard. Name the principle on which a soundboard work?

Question 5 Why are the ceiling of concert halls made curved?

Question 6 How can we reduce excessive reverberation in big halls?

Question 7 What will happen if the reverberation time in a big hall is too long?

Question 8 What are infrasonic waves?Name animals which can produce infrasonic waves?

Question 9 What are ultrasonic waves?Name animals which can produce ultrasonic waves?

Question 10 What is the range of frequency associated with infrasound?

Question 11 What is the range of frequency associated with ultrasound?

Question 12 What is the range of frequency associated with audible sound?

Question 13 What is ultrasound?

Question 14 Write few application of ultrasound?

Question 15 How is an ultrasound scan for foetus better than X-ray?

Reverberation
If a sound is made in a big hall,the sound waves are reflected repeatedly from walls,ceiling and floor of hall,and produces many echoes.The echo time is so short that many echoes overlap with original sound.

Due to this original sound seems to be prolonged and lasts for a longer time.The sound made in a big hall persist for a longer time.

The persistence of sound in big hall due to repeated reflections from walls,ceiling,floor of the hall is called reverberation.

In a big hall excessive reverberation is highly undesirable.If the reverberation is too long,then sound becomes blurred,distorted and confusing due to overlapping of different sounds.
Modern concert hall are designed for optimum amount of reverberation.
The excessive reverberation in big halls are reduced by
1)Carpets on floors
2)Heavy curtains on doors and windows.
3)Roof and wall are covered by sound absorbent material like compressed fibreboard,rough plasters.

Uses of Multiple reflection of sound
1)Megaphones,bulbhorn,musical instruments(trumpets and shehnai)are all directed to send sound in a particular direction without spreading all around.In all these a tube followed by a conical opening reflects sound waves repeatedly so that most of the sound waves produced go in the forward direction.

During successive reflection,the amplitude of sound waves adds up due to which loudness of sound increases.

2)Stethoscope:It is used by the doctors for listening the sound produced within the human body,mainly in the heart and lungs.
The sound reaches the doctor’s ear by multiple reflection of sound waves through stethoscope tube.

3)Soundboard
The ceiling of concert hall,conference hall,cinema hall are curved so that sound after reflection reaches all corners of the hall.
A curved soundboard may be placed behind the stage so that sound after reflecting from soundboard spreads evenly across the width of hall.

Range of hearing
The range of frequency from 20Hz to 20,000 Hz is called frequency range of hearing in humans.
The sound which we are able to hear is called audible sound.
The sound of frequencies lower than 20 Hz are called as infrasonic sounds(cannot be heard by human being)

Infrasonic sounds are produced by vibrating simple pendulum,whales,elephants,rhinoceroses(commute of frequency as low as 5Hz),earthquake etc.

Some animals get disturbed and start running here and there just before earthquake occur.This is because before the main shock waves,the earthquake produce low frequency infrasonic sounds which some animals can hear and get disturbed.

The sound of frequencies higher than 20000Hz are called as ultrasonic sounds.Dogs,bats,monkey,deer,cat can hear ultrasonic sounds.Due to its higher frequency ultrasonic sound has much higher penetrating power than ordinary sound.

We can not hear the screams of a bat because its scream  consist of ultrasonic sound having a frequency much higher then 20,000Hz.

Application of Ultrasound

1)It is used in industry to clean hard to reach parts of objects such as spiral tube,odd shaped machines and electronic components : The object to be cleaned is placed in a  solution and ultrasound waves are passed into the solution.Due to their high frequency,the ultrasound waves stir up the cleaning solution.Because of stirring the particles of dust and grease sticking to dirty object vibrate too much become loose get detached from object and fall into solution.The object gets cleaned thoroughly.

2)They are used to detect cracks and flaws in metal blocks : Metal blocks are used in the construction of big structures.There may be some flaws or defects inside the metal blocks which are invisible from outside.These flaws weaken the metal block.Ultrasonic waves are allowed to pass through metal block.An internal crack does not ultrasound to pass through it.It reflects the ultrasound.

3)They are used to investigate the internal organ of human body such as liver,gall bladder,pancreas,kidney,uterus,heart
Ultrasonic scanner is an instrument which uses ultrasonic waves for getting images of internal organ of human body.It helps the doctor to detect abnormalities such as stones in gall bladder and kidney or tumours in different organs.
The ultrasonic waves travel through the tissue of the body and get reflected from a region where there is a change of tissue density.
These waves are then converted into electrical signals that are used to generate image of organ.These images are then displayed on a monitor or printer.
The technique of obtaining pictures of internal organs of body by using echoes of ultrasound pulses is called ultrasonography.

4)It is used for examination of foetus during pregnancy to detect congenial defects and growth abnormalities.

5)It may be employed to break small stones formed in the kidney into fine grains.These grains later get flushed out with urine.