Sound energy is result of vibrations in objects.
The sound vibrations produce waves of pressure that travel through a medium such as air.
Sound wave keeps going until they run out of energy.
Sound energy is a form of mechanical energy.
Fast vibrations create high note where as slow vibrations create low note.
Sound produce by humans:
Humans are capable of producing sound by presence of their voice box or larynx.
When lungs force the air through the slit, result in vibration of vocal cords and produces the
sound.
Sound needs medium for propagation. In vacuum sound cannot travel.
Human Ears:
Sound waves travel into the ear canal with aid of funnel shaped ear pinna until they reach the
eardrum and there is vibration of eardrum which gives us sensation of sound by cascading
mechanism performed in inner ear of human.
The number of oscillations per unit time is the frequency of the sound wave.
Its SI unit is hertz (Hz).
The Amplitude of the sound wave depends upon the force by which an object is made to
vibrate.
Characteristics of sound waves:
The amount of sound energy passing each second through unit area is called the intensity of
sound that is “Loudness”
Loudness is directly proportional to square of the amplitude of the vibration producing the
sound. It is expressed as decibel (dB).
Above 80 dB the noise become intolerable.
Amplitude and Frequency:
The loudness of sound depends on its amplitude.
The frequency determines the shrillness or pitch of sound.
Higher the frequency the sound has high pitch where as low frequency result in low pitch.
Audible and Inaudible sounds:
• Sound of frequency less than about 20 vibration per second that is 20 Hz cannot be
sensed by the human ears. This sound is called as Inaudible sound.
• Sound of frequency of 20,000 vibration per second that is 20 kHz are also not audible
to human ear.
The range audible sound frequencies is roughly from 20 to 20,000 Hz.
Noise and Music:
Noise is unpleasant to the ear whereas Music is pleasant to hear and is of rich quality.
Noise pollution:
Presence excessive or undesirable sound in environment is collectively termed as
“Noise pollution”
A person who exposed to loud sound continuously may get temporary or may even
permanent impairment of hearing. Permanent impairment is rare is usually from birth itself.
Robert Boyle’s classic experiment:
✓ Initially one can hear the sound of the ringing alarm kept inside glass jar.
But after the pumping out some air from the jar by usage of the vacuum pump,
it is observed that the sound of the ringing alarm decreases if one keeps on pumping
the air out of the glass-jar ,
✓ Then at one point the glass jar will be having none of any air at this moment no sound
can be heard from the ringing alarm now.
We conclude that a vacuum is produce which not allowing sound to travel through it.
This indicate that sound needs a material medium for its propagation.
Reflection of Sound:
Sound bounces or reflected off from surface or a wall.
Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of
Reflection.
The directions in which the sound is incident and is reflected make equal angles with the
normal to the reflecting surface, and above mentioned three lie in same plane.
An obstacle of greater size which may be smooth or rough is needed for the reflection of
sound waves.
Echo:
If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we
will get sensation of hearing the same sound again a little later. This sound which we hear is
called an “echo”.
The sensation of sound last for or persist in brain for about 0.1 s.
To hear a distinct echo the time interval between the original sound and the reflected one
must be at least 0.1s.
The speed of sound to be 344 m/s at a given temperature, consider at 22 ºC in air, the sound
must go to the obstacle and hear back 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) × 0.1 s = 34.4 m.
Thus, for hearing distinct echoes, the minimum or least distance of the obstacle from the
source of sound must be half of this distance, that is, 17.2 m.
This distance will change with the temperature of air.
The echoes may be heard more than once due to multiple bouncing off or reflections.
Eg. The rolling of thunder
Reverberation:
A sound created in a massive hall will last or persist by repeated and multiple reflection from
the walls until it is reduced to a value where it is no longer audible.
The repeated reflection that results in persistence of sound is called “Reverberation”
In an auditorium or big hall excessive reverberation is highly undesirable for conduction of
any event.
To reduce undesirable impact due to reverberation, the roof and walls of the auditorium are
generally covered with sound-absorbent materials like compressed fiber-board, rough plaster
or draperies. The seat materials are also selected according to their sound absorbing
properties.
Uses of multiple reflection of sound:
- Medical instrument used for carefully listening sound within the body , mainly in case
of heart or lungs , the stethoscope works on principle of multiple reflection of sound. - In movie theater , ceilings and corners are somewhat curved so that sound
undergoes multiple reflection and resulted sound has comparatively greater impact.
Speed of sound in different media :
Sound travels with distinguished speeds depending on from what it is traveling through.
Sound waves travel the slowest through gases, faster through liquids, and fastest through
solids.
The speed of sound in a medium depends also on temperature and the pressure experienced
in the medium.
The speed of sound in air is 331 m/s at 22oC . But at 20oC considering it is room temperature,
sound travels at 343 meters per second.
Sonic boom:
When the speed of any object crosses or get exceeded the speed of sound , is said to be
traveling at supersonic speed.
Bullets, jet aircrafts often travel with such great speed.
Sonic boom generated by supersonic aircrafts may result in shattering of glass or even
damage to building as there is formation of shock waves which are very sharp and loud ,
which carries large amount of energy
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