Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio

Chapter Two
AV Physics

Page 2

Batting practice for the audio/video pro and a primer for the novice

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Acoustical Energy

ound is the compression and rarefaction of sound waves traveling through a medium such as air.  An initiating disturbance compresses air molecules which then rarefact and then compress in a continuing cycle until it terminates in our ears or fades ... This is the cycle of sound.

                Handbook Note:
                  Hertz (Hz) = a unit of measurement of cycles per second.
                  One thousand = one kilo or K
                  KHz = one thousand Hertz
                  One million = one Meg, Mega or M
                  MHz = one million Hertz

          Sound engineers have translated sound waves into measurable illuminated lines on the screen of an oscilloscope.   The modulating  lines illustrate the amplitude, wavelength, and frequency of a sound wave.

          The oscilloscope reveals a ten-octave frequency range of sound that spans from
20Hz to 20KHz.  The area of 20Hz to about 200Hz includes low frequency bass.  
The human voice and many musical instruments occupy the region of approximately 200Hz to 1KHz.  The balance of the frequency range includes the harmonics of sound such as the reverberations of a guitar, piano, bell, cymbal, etc.

Handbook Note: The speed of sound = 1130 ft/second

          The wavelengths of sound can be computed by dividing the speed of sound by the frequency.  For example; 20 cycles per second has a wavelength of 56.5 feet (113020) and
20KHz has a wavelength of about 0.678 of an inch (113020K).

          The amplitude or objective volume level of sound is measured in the logarithmic units of sound pressure called a decibel (db).  It is a scale that extends from 0db to 194db.
          A logarithmic scale is employed because of the immense range of the human ear that spans from the threshold of hearing to beyond the threshold of pain.  

          The threshold of hearing is the level created by a molecule of air landing on the surface of the eardrum.  You read that right.  The threshold of pain is at about the sound pressure level created by a large aircraft during takeoff at close range.
          In the 1930s, Bell Labs researchers Fletcher and Munson observed that the subjective response of the human ear differed from the objective measurement of sound pressure level. They determined that the human ear is most sensitive to the range of 1KHz to 5KHz. Frequencies above and below must be increased in volume to be perceived as loud.   The effect becomes more prominent as the volume is decreased.  The effect diminishes as the volume is increased.   Fletcher and Munson defined this subjective or psycho-acoustical response to sound as loudness.

      Handbook Notes:
doubling of loudness = about a 9 to 10 decibel change in sound pressure level.

      A doubling of amplified power creates an increase of about 3db.
      Speed of sound = 1130 ft/second at sea level.
      Frequency range of audible sound = 20 Hz to 20KHz
      Range of audible wavelengths = 56.5 ft. to 0.678 in.
      Threshold of hearing = 0 db
      Threshold of pain = approximately 120 db
      Sound can also travel through other media such as water, wood, steel, etc., but it cannot travel            
      through a vacuum such as the vacuum of outer space.

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018