Sound

Sound is a common part of everyday life .Few sounds give us a pleasant experience such as good music or singing of birds etc and sometimes we feel sound is annoying or unpleasant.

Sound is defined as any pressure variation that an ear can detect ranging from the weakest sounds to the sound levels which can damage hearing.
When we try to produce sound air molecules are compressed and extended or in simple words pressure changes occur.

For an example let us consider the sound of the alarm clock that wakes us up. In order to wake us from a deep sleep, many of us set the alarm to a high volume .The clock here is the source of sound energy which causes rapid wave oscillation and high pressure thus causing ear drums to vibrate and transmit the signals to inner ear. Thus we are able to hear the sound produced by the clock.

In the above example we have seen that waves are propagating but the important point to notice here is that it is propagating in air. Sound waves require a medium for propagation and here the medium is air. It needs a medium or to be precise molecules to transfer the vibrations and generate pressure changes. Sound waves are thus mechanical waves.

Now let us go through the famous electric bell jar experiment:

• An electric bell i.e. electric current will set the bell into vibration, is suspended inside an airtight glass bell jar connected to a vacuum pump.

• If the air is slowly removed from the bell jar by using a vacuum pump, the intensity of sound goes on decreasing and finally no sound is heard when all the air is drawn out.

• We would be seeing the hammer striking the gong repeatedly.

• This clearly proves that sound requires a material for its propagation.

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  Sound can damage and destroy too. A sonic boom can shatter windows or shake plaster off walls .But the most delicate case is when sound damages the mechanism designed to receive it-The Ear. So there is a need for us to measure the sound energy and take appropriate precautions. Before getting into the details of how sound energy is measured let us get familiarised with few terms.
  Amplitude, Time period and frequency of vibration:
  Amplitude is the maximum disturbance or the height from its undisturbed position.
  Time period –the time it takes before it starts repeating the cycle.
  Frequency-The reciprocal of Time period, measured in HZ.
  Wave Length-How long the wave moves from the top of the wave to another top of the wave.
  
  
  
  So now how do we measure sound energy?
  The sound pressure level measures how forceful the wave is .It is measured in decibels or dB.
  The sound pressure level, Lp (in dB)= 20 log p/p0 ,
  Where p is the measured value in Pa
  p0 = standardised reference level of 20 micro Pa the threshold of hearing.
  0 dB is the softest level that a person can hear. Normal speaking voices are around 65 DB and the maximum is 130 dB.
  
  
  Frequency is also used in the measure of sound waves. Frequency is measured in the number of sound vibrations in one second. A healthy ear can hear sounds of very low frequency, 20 Hertz (or 20 cycles per second), to a very high frequency of 20,000 Hertz.
  
  
  Few facts:
  
  
  • Our ears vibrate in a similar way to the original source of the vibration, allowing us to hear many different sounds.
  • The scientific study of sound waves is known as acoustics.
  • Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas (not just fluids).
  • A sonic boom occurs when an object breaks the speed of sound. The sound waves from behind and in front of the object crash into each other and creates the boom.
  • Sound travels at a speed of about 330m/s
  • Sitting in front of the speakers at a rock concert can expose a person to 120 dB, which will begin to damage hearing in only 7-1/2 minutes.
  • The cochlea (inner ear) is about the size of a pencil eraser.
  
  
  
  
  
  
  -Keerthi Kuthati