Like light waves, sound waves also get reflected when these fall on the surface of an obstacle. But unlike light waves, sound waves do not necessarily require a polished surface for reflection i.e. For reflection of sound waves, the surface may be polished or rough.
The relationship between sound waves and light waves can be demonstrated by the following 2 laws, (which are the same for both sound waves and light waves) :
The angle of reflection (r) is always equal to the angle of incidence (i) i.e.
<r=<i or r=i
2) The incident wave, the reflected wave and the normal (at the point of incidence), all lie in the same plane.
It is a matter of common experience that when we utter a few words in a high domed hall, the words are repeatedly heard on account of reflection from the walls of the dome. This is what we call an echo. An echo is the phenomenon of repetition of sound of a source by reflection from an obstacle. An echo can be distinguished from the original sound, if the obstacle is situated at a suitable distance from the source of sound.
The sensation of sound lasts in our brain for (1/10) of a second. The property is called persistence of hearing . Therefore, to hear a distinct of a second, the time taken by this is sound to reach the listener after reflection should be (1/10) of a sound, the time taken by this sound to reach the listener after reflection should be (1/10) of a second. i.e., let
b = minimum distance of the obstacle from the source (S)
v = speed of sound in air
t = total time taken by the sound to reach the listener (L) after reflection
As total distance travelled by sound = speed of sound x total time 2 d = v x t Substituting v = 344 m/s (speed of sound in air at 20 degrees Celsius) t = 1/10 s, we get 2 d = 344 m/s x 1/10 s = 34.4 m Therefore, d = 17.2 Hence, for hearing a distinct echo, the minimum distance of the obstacle from the source of a sound should be 17.2 m*. * The distance changes with the change of temperature of air!
We have all been in cinemas, theatres etc., can we think of what would happen if sound reflection was not taken care of properly? Would we be able to watch a movie or play as effectively as we are able to do? So, we know come to think about how exactly do they control this reflective effect? Firstly, the ceilings of these halls are curved. This enables the sound to reach all corners of the hall after reflection from the ceiling.
Secondly, a sound board, which is a curved (parabolic or concave) sound reflecting surface, is placed behind the stage. The source of sound is located at the focus of this reflection surface. Sound waves coming from the source become parallel after reflection from the sound board and spread evenly throughout the width of the hall!
Smooth walls have a tendency to direct sound waves in a specific direction.
Subsequently the use of smooth walls in an auditorium will cause spectators to receive a large amount of sound from one location along the wall
There would be only one possible path by which sound waves could travel from the speakers to the listener.
The auditorium would not seem to be as lively and full of sound
Rough walls tend to diffuse sound, reflecting it in a variety of directions.
This allows a spectator to perceive sounds from every part of the room, making it seem lively and full.
This is the phenomenon of reverberation which is often displeasing
It is the phenomenon of persistence or prolongation of audible sound after the source has stopped emitting sound.
The time for which the reverberation persists until it becomes in audible is called reverberation time .
Since it is caused due to repeated reflections of sound waves from the surfaces of auditoriums, it can be reduced by increasing the absorption of sound energy.
Silent intervals between syllables, sounds and filled with reflected energy resulting in a smearing effect of sound, which may be perceived as irritating.
Certain amount of reverberation is desirable.
In order to reduce reverberation, it is necessary to increase absorption. It can be done in the following ways:
By covering the walls with some sound absorbing material like felt, fiberboard, glass wool, etc. or by heavy curtains with folds.
By carpeting the floor.
By upholstering the furniture.
By making a false ceiling of suitable sound absorbing material.
Whales in the ocean "sing" to each other. The sound of their song can travel a distance of 800km.
Sound is used by many animals to detect danger, warning them of possible attacks before they happen.
Our ears vibrate in a similar way to the original source of the vibration, allowing us to hear many different sounds.
The sound of thunder is produced by rapidly heated air surrounding lightning which expands faster than the speed of sound.
Scientists believe that the loudest sounds made by any animal are the whistles of the blue whale, which have been measured at up to 188 decibels — a million times more intense than a jet engine, which is only about 120-130 decibels! (Every 10 decibels represents a 10-fold increase in sound intensity.)
A frequency of 19hz may be responsible for ghost sightings. This frequency is close to the resonant frequency of the eye and would therefore result in ghostly visual illusions in the listener
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