sound wave basics

A sound wave is a mechanical longitudinal wave produced by a vibrating object. The traveling sound wave consists of alternating compressions and rarefactions (decompressions) which are detected as changes in pressure. In other words, the vibrating object sets off a pressure wave which alternately compresses and decompresses molecules of air or whatever medium the wave is traveling through. The basic characteristics of a sound wave include the following:
            
              Amplitude ~ A
              Wavelength ~ l
              Frequency ~ f
              Phase ~ f

  ·The amplitude is proportional to the maximum distance a particle is displaced.

   ·The wavelength is the distance the wave travels in one cycle of vibration, for a sound wave this is the distance between to compressions or rarefactions.

   ·The frequency is the rate of vibration of the particles given in units of cycles per second or Hertz (Hz).

           Frequency and wavelength are related to the speed of sound by the following:
               
                                      lf = v where v is the speed of sound
  
   ·The phase of the wave is important when considering the construction of a complex sound from several waves. For the      purposes of this discussion we will leave the phase alone.

The intensity of a wave is proportional to the amplitude squared. For a sound wave the intensity is given by:
           
                                      I = 2p r v f^2 A^2 where r is the density of the medium

Instead of speaking of the intensity of the sound wave (which requires the use of very large numbers) it is easier to use a logarithmic scale. Here is where the ever so familiar decibel comes to play. The sound intensity level is given in decibels and is defined:

                                       b = 10 dB log (I/I0)          where I0 is the standard reference

We now see that the decibel scale is dependent upon the reference intensity. For sound waves in air
I0 = 10^(-12) W/m^2, and for sound waves traveling in water the reference intensity is 6.7 ´ 10^(-19) W/m^2. The decibel scale is relative to the medium of travel; some care must be taken when comparing the sound level intensity of water to that in air.

Humans can hear a range from 20 Hz to 20000 Hz. Sounds below this frequency are infrasonic and sounds above this frequency are ultrasonic.

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