Physics - Sound: acoustic power
The intensity of a sound wave in watts per metre squared is a measure of
the number of joules per second passing through a square metre. That is:
intensity = power / area
Intensity can be measured at any location out from a sound source. If
intensity is measured at a known distance from a sound source, the total
acoustic power of the source can be calculated.
Power = intensity x area covered by the sound wave:
An ideal source is considered to produce a perfectly spherical wave
travelling outward in all directions. Since the surface area of a sphere is
given by A = 4 pi r, then
Power and energy
Power in watts is the rate of use of energy
Decibel and intensity calculation shortcuts.
* If one mosquito is at a distance of 10 cm from your ear the intensity
would be equal to 10 W m(0 dB). Calculate the sound level of 1000 mosquitos
* The noise output from one speaker has an output of 60 dB. A second
speaker is turned on, what is the new sound level?
Hence the new sound is 2 x 10 W m or 63 dB.
A shortcut is to realise that whenever an intensity doubles this
corresponds to a +3 dB rise.
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