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Reflection of Sound and Echo

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Reflection of Sound and Echo - Lesson Summary



When a sound wave is incident on a solid or liquid surface, it bounces back onto the same medium. This is known as the reflection of sound. Like light, sound too undergoes reflection from any hard surface and obeys the same laws of reflection, which light obeys.

Laws of Reflection of Sound
The direction in which the sound wave is incident and the direction, in which it is reflected, make equal angles with the normal to the reflecting surface, at the point of incidence.
 
The incident sound wave, the reflected wave and the normal at the point of incidence are in the same plane. 

Sound waves need polished or rough surfaced obstacles to get reflected. The incident sound wave, the reflected sound wave and the normal, all lie in the same plane.

Echo
An echo is the repetition of sound that results as a reflection from a surface. Multiple reflection of sound is the successive reflection of sound from various reflecting surfaces. We cannot hear an echo unless the reflecting surface is beyond a specified distance from the source of the sound. The sensation of sound persists for 0.1 second. The minimum distance required to hear an echo is 17.2 metres.
 
Reverberation
The persistence of sound in big enclosures like auditoriums is the result of repeated reflections of sound and is called reverberation.

Reverberation can be minimized by using sound absorbent materials like cardboard, thick curtains and fibre. The process of multiple reflection of sound is applied in:
    •  Stethoscope
    •  Megaphones
    •  Horns
    •  Trumpets

A human e ar can hear sounds with a frequency of 20 hertz to 20000 hertz. This range of frequency is called the audible range. The audible range for different species is different.


Working of Human Ear in the Transmission of Sound Waves to the Brain

The outer ear. is called pinna. It collects the sound from the surroundings. The collected sound passes through the auditory canal, there is a thin membrane called eardrum. When a compression of the medium reaches the eardrum, the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward where a rarefaction reaches it. In this way eardrum vibrates.

The vidrations are amplified several times by three bones in middle ear. The middle ear transmits the amplified pressure variations receievd from the sound wave to the inner ear, the pressure variations are turned into electrical signals by the cochlea.

These electrical signals are sent to the brain via the auditory nerve and the brain interprets them as sound.

Sound waves that have a frequency less than 20 hertz are called infrasonic sound waves or infrasonics. Sound waves that have a frequency greater than 20000 hertz are called ultrasonic sound waves or  ultrasonics.

Ultrasound waves have a wide range of applications in several fields such as health care and industry. Ultrasonography is a technique used to diagnose any defects in internal organs such as liver, uterus and kidney.

SONAR stands for Sound Navigation and Ranging. Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.

Working of SONAR :
A sonar which stands for Sound Navigation and Ranging is technique used for determining the depth and also locating underwater objects such as reefs, submarines etc.
In this method, to find the depth of ocean, a strong ultrasonic wave is sent from the ship towards the bottom of the ocean. This ultrasonic is received back after it is reflected from the bottom of the sea. The time interval t for travel of sound waves from the source to the receiver after reflection is noted.
Then depth d of the ocean is given by the velocity of D = V × t 2   , where V is the velocity of Sound.

Echo-ranging method is used to find the distance travelled by the ultrasound ray, which is given by the equation:  2d = vt

Bats and Porpoises like dolphin are examples of species that uses its personal sonar to obtain its food. The ear converts audible frequencies in air into electric impulses that reach the brain. The human ear is categorised into three main parts, the outer ear, middle ear and the inner  ear.


Echo Versus Reverberation
  • Echo is the reflection of a sound or some other wave off a surface. Reverberation is the sound or the pattern created by the superposition of such echoes.
  •  An echo can be heard only when the distance between the source of sound and the reflecting body is  at least 17 m. A reverberation can occur when sound wave is reflected by a nearby wall also.
  • An echo is usually clear and can be clearly distinguished. A reverb is not a clear replica of the original sound sample.
  • Echo can be used to determine the distance of a reflecting object such as a large building or a mountain, if the ambient temperature is known. Reverberation cannot be utilized for distance measurement applications.
  • An echo can be heard both in open and closed spaces. Reverberation is usually experienced in closed spaces with multiple reflecting objects.

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