Basic Documentation

Sound Measurement Parameters
Figure 2 illustrates another important rule regarding sound pressure levels and distance.
Whenever the distance between a sound source and a receiver is doubled, the sound
pressure level at the receiver is reduced by 6 dB from its previous value. This is a very
important relationship. For instance, if a sound source produces a sound pressure level of 40
dB at a receiver 15 feet away, the sound pressure level would be reduced to 34 dB if the
distance away were doubled to 30 feet. (Note that this is a non-linear relationship and the
results cannot be interpolated. Therefore, in this example, it would be incorrect to assume
that the dB level drops at 2 dB for every 5 foot increase in distance.
If the distance were again doubled from 30 feet to 60 feet, the sound pressure level at 60 feet
would be 28 dB. This relationship continues each time the previous distance is doubled.
If the sound pressure level was 60 dB at a 12 foot distance between the sound source and
the receiver, what would the sound pressure level be at a distance of 100 feet? Using the
distance doubling rule, the sound pressure levels at various distances would be:
• 60 dB at 12 ft
• 54 dB at 24 ft
• 48 dB at 48 ft
• 42 dB at 96 ft
• 36 dB at 192 ft
Since 100 feet is slightly more than 96 feet, the dB level would be perhaps just a bit less than
42 dB.
Figure 2. Sound Pressure Level Decrease Due to Distance.
Octave Bands
Sound can vary in pitch or frequency from a very low base sound to a very high pitch sound
such as a squeak. In terms of actual frequency, human hearing ranges from about 20 cycles
per second (Hz) at the low end to around 20,000 Hz at the high end. The actual frequency
span of hearing varies from person to person and tends to decline somewhat as we age with
the upper frequency end of our hearing being the portion mostly affected by age.
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