Reflection of Sound

           Just like light, sound also travels in form of waves. But unlike light sound requires a definite medium. Sound waves are mechanical waves.  Similar to light, sound waves also get reflected when they hit an obstacle. The geometrical laws for reflection of sound waves are the same as those for light waves. Multiple reflection of sound is the process in which sound waves bounces off obstacles and reflects many times before reaching the destination. Human ear can hear sound waves till the range of 20Hz to 20000Hz.Sounds of frequencies below 20 Hz are called infrasonic sound or Infrasound. Sounds of frequency above 20000 are called ultrasonic sound or ultra sound.

ECHO

                We all have heard echoes when in an empty room or in an empty well or some beautiful valley, but how does echo happen? What is echo actually? Well an echo is a reflection of sound, arriving at the listener some time after the direct sound. That is, when we scream something the same proceeds to us after a delay of time. The sound travels to a reflecting surface, is reflected, and then travels back to the ear. Echo is generally produced by any large surface such as the walls of a building, the sides of a mountain, a cliff, tunnels, deep well or the trees at the edge of a forest.

               

                Sound travels 330 m in one second in air. It takes about one-tenth of a second for the human ear to separate one sound from another. The human ear can hear two sounds separately and distinctly only if there is interval of 0.1s between the two. This is called persistence of hearing. Total distance travelled by sound in one-tenth of a second =330 m/s x 1/10 s = 33 m

                Therefore to hear your echo it is necessary for you to stand at least (33/2) m=16.5m away from the reflecting surface. Sound from your voice will travel 16.5 m to and from the reflecting surface, a total of 33m. At the velocity of 330 m/s, this sound travels for one-tenth of a second. The echo will be clean and distinct because it will reach you one-tenth of a second after you hear your voice for the first time.

APPLICATIONS OF REFLECTION OF SOUND

1. In big theatre, the walls behind the speaker are made spherical and the speaker stands at the focus of that, so that sound is reflected towards the audience and the audience can hear it clearly.

2. The principle of reflection of sound is used in SONAR (sound navigation and ranging). In this technique, impulses of sound are transmitted towards the sea floor and the reflected impulse is recorded. The time for this is measured and the depth of the sea is calculated using the speed of light in that medium.

3. Reflection of sound also has applications in the ultrasound technology, which is used to detect the presence of any foreign object in the body or in the detection of any defects in the development of the foetus. They can be used to detect crack in metal objects and can also be used to remove dust by vibrating metal.

4. Bats cannot see in the dark. So, they use the reflection of sound to find their path and to avoid any obstacles in their path. They use this technique to find food too. They emit sound which is not audible to human ear and as a result of reflection of sound; they come to know about the obstacles in their path.

5. Stethoscope, the symbol of a physician, works on the principle of multiple reflection of sound.

        A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. In an auditorium or big hall excessive reverberation is highly undesirable. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound-absorbent materials like compressed fibreboard, rough plaster or draperies. The seat materials are also selected on the basis of their sound absorbing properties.

-MAYAND  DAVE