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The Transmission of Sound Waves and Intensity

Longitudinal Waves

When a tuning fork is struck, it pushes air molecules into each other, which creates a steadily moving area of collision called a compression (C). When the tine moves back, it creates a region of emptiness called a rarefaction (R). In sound waves, a compression is an area of higher than normal air pressure and a rarefaction is an area of lower than normal air pressure.

Wave Motion

Density of Air Molecules


Properties of a Longitudinal Wave

Wavelength - distance between midpoints of successive compressions or rarefactions.
Amplitude - describes the displacement of the air molecules from the rest position.
(All other terms used previously to describe waves are used to describe sound waves)

Intensity of Sound


Intensity of Sound - Formula Intensity - the average rate of energy flow per unit area across a surface perpendicular to the direction of propagation.


 Click here to listen to different intensities of sound.



Measuring the Intensity of Sound

Intensity, or loudness is a more difficult property to measure accurately because it involves a small amount of energy and the potential range of sound intensity is great. The decibel is the unit used to measure the intensity of sound.

Waveform

  Here is a representation of a sound that goes from a low intensity to a   high intensity. To the left is the waveform for the sound.


The intensity of a sound wave can be a factor of bothe frquency and amplitude of the wave as shown in the two diagrams below:

QUIET SOUND

Louder Sound

The first decibel value, 0 dB, is fixed near the threshold of hearing. The decibel scale does not increase with uniform gradations. A sound 10 times more intense than 0 dB equals 10 dB. A sound 100 times more intense than 0 dB equals 20 dB. A sound 1000 times more intense equals 30 dB, so on and so forth.

The following chart shows decibel (dB) values for various sources of sounds or noise.

Also, as the distance between the source and the person increases the intensity of sound decreases.

Notes to be Made

  1. Know the difference between compression & rarefaction and how it relates to molecular density of the sound transmitting medium.
  2. Define amplitude and wavelength in terms of compressions and rarefactions.
  3. What is sound intensity and how is it measured and what units are associated with intensity.
  4. Explain the decibel (dB) scale of sound intensity.
  5. According to the provincial Occupational Health & Safety Act what are unacceptable sound intensity levels per timed exposure.
    Why are government regulations regarding sound intensity stipulated in the workplace?
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