Sound

Analysis of sound usually treats sound as waves; in many ways, sound waves are analogous to ocean waves or ripples in water caused by dropping a stone in a pond or puddle. Sound has the following characteristics and properties which affect its perception:

• Wavelength. This is the period between waves of sound and is also refered to as pitch or frequency. Expressed in Hertz (Hz), cycles per second, the human ear perceives frequencies ranging from 20 Hz to 20,000 Hz, although as humans age they tend to lose their ability to hear high frequency sounds. Hearing ranges for some animals include:

  domestic cats	100-32,000 Hz
  domestic dogs	40-46,000 Hz
  African elephants	16-12,000 Hz
  bats	1000-150,000 Hz
  rodents	70-150,000 Hz



• Amplitude. The volume, or loudness of a sound.

  

In this graphical representation of a sound, the x axis is time and y axis is amplitude, or the strength of the signal. A louder sound produces higher waves, but does not change the distance between the waves. The wave makes repeated full cycles; the number of cycles completed per second is the frequency in Hertz. When a microphone or recording is played into an oscilloscope, sound is represented in this way.
• Intensity. The energy of the sound, measured in watts per square meter. Intensity is a function of the square of the amplitude, corrected for the density of the medium and the at which the sound travels in the medium.
• Dissipation. The loss of intensity as sound travels. One important factor in loss is the effect of spreading; as the sound spreads out over a larger area, less energy is present at any one point. Mathematically, this dissipation is described by the inverse-square law. Another cause of loss is absorption and scattering by the medium in which the sound is travelling.
• Reflection and refraction. When sound hits a denser medium, some of the energy bounces, or echoes, off the surface of the denser medium. Echoes can be useful, as in the echolocation system of bats, or can result in confusing noise. When sound moves from a medium of one density to a medium of a different density, it refracts, or changes wavelength, as well. This results in distortions of the sound.

Sound perception

Infrasound (5-50 hz) perception of infrasound

The human hearing range, 20-20,000 hz

Ultrasound (above the human hearing range, greater than 20,000 hz) perception of ultrasound

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copyright ©2001 Michael D. Breed, all rights reserved