Reflection of Sound, ECHO, REVERBERATION
Reflection
of Sound
The bouncing back of the sound wave on striking a surface
such as wall, metal sheet, plywood, etc. is called the reflection of the
sound wave.
Sound bounces back a solid or a liquid like a rubber ball
bounces back a wall. Sound gets reflected at the surface of a solid or liquid.
Like light, it follows the laws of reflection.
The direction in which sound is incident and is reflected
make equal angels with the normal to the reflecting surface at the point of
incidence. All three are in the same plane surface.
An obstacle of large size which may be polished or rough is
needed for the reflection of sound waves.
Audible sound has a wide frequency range that is from 20 Hz
to 17000 Hz. And also has a wide range of frequency that is from 20 mm to 17 m.
so, the overall nature of the reflection varies according to the texture and
structure of the surface.
The reflection of sound can affect the acoustic
space.
In acoustics, reflection causes echoes and is used in
sonar.
ECHO
Repetition of sound wave caused by the reflection of the
sound wave is called an echo.
If we clap or shout near s suitable reflecting
object like mountain, then we will hear the same sound with little delay. This sound
which we hear is called as an ECHO.
The delay is directly proportional to the distance of the
reflecting surface from the source and the listener.
A true ECHO is a single reflection
of the sound wave. the sensation of the sound persists in our brain for about
0.1s.
To hear a distinct ECHO, the
time interval between the original sound and the reflected sound must be at least
0.1s.
For example, if the speed of the sound to be 344 m/s at a
given temperature, at 22 0C
in air, the sound must go to the obstacle and reach back to the ear of the
listener on reflection after 0.1s.
Hence, the total distance covered by the sound from the
point of generation to the reflecting surface and back should be at least 344
m/s X 0.1s = 34.4 m.
Thus, for hearing distinct echoes, the minimum distance of
the obstacle from the source of the sound must be half of this distance. It means
the distance is 17.2m. this distance will change with the temperature of the
air.
Echoes may be heard more than once due to the
successive or multiple reflections.
Or, the echoes is heard more than once due to
the time difference between the initial production of the sound waves and their
return from the reflecting surfaces.
The rolling of the thunder is due to the successive
reflections of the sound from a number of the reflecting surfaces, such as the
clouds and the lands.
REVERBERATION
Reverberation is a persistence of the sound or
echo after a sound is produced.
A sound is produced in an auditorium or an empty hall or a
big hall. And it persists by repeated reflection from the wall until it is
reduced to a value where it is no longer audible. The repeated reflection that occurs
in the persistence of the sound is called reverberation.
In the big hall or an auditorium, the excessive
reverberation is undesirable. To avoid or to reduce the such reverberation, the
roof and walls of the auditorium or big hall are generally covered with sound-
absorbent materials like compressed fiber-board, rough plaster or draperies. The
seat materials are selected on the basis of their sound absorbing properties.
Reverberation is frequency dependent (length of the decay,
or reverberation time).
Reverberation occurs naturally when any one sings, talks or
plays an instrument acoustically in a hall or performance space with
sound-reflective surfaces.
Reverberation is a significant source of mistakes in
automatic speech recognition.
Dereverberation
Dereverberation is the process of reducing the level of
reverberation in a sound or signal.
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