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:
- 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.
In general, a body part, such as a hair or a membrane, vibrates in sympathy with
the movement of molecules in the surrounding medium (air, water, or soil). This
starts the chain of events that leads to transduction. While the principles
underlying perception of sound are basically the same for different frequencies,
sounds below the normal human range of hearing (infrasound) or above it (ultrasound)
sometimes differ in their perception more details can be found by following the
copyright ©2001 Michael D. Breed, all rights reserved