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Audio in the 21st Century - Part 9
In part 9 of Scott Janus' "Audio in the 21st Century" the introduction to sound technology continues. In this installment we explore Decibels and Sound Pressure through a series of examples.
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By
Scott Janus
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Page 1 of 2
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Audio DesignLine
(10/31/2006 8:41 AM EST)
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Editor's note: This article and the subsequent articles provide an introduction to Sound. It is excerpted from the book Audio in the 21st Century published by Intel Press. This excerpt is from Chapter 2 of the book. All material is copyright Intel.
Digg this article
SPL provides sound intensity by comparing the measured pressure against a reference pressure associated with the quietest sound a human can hear (known as the threshold of hearing), as shown in Equation 2.10.
Equation 2.10 Sound Pressure Level
SPL = 20 log (P1/P0), where P0= 20 µ Pa
To clarify that a particular decibel measurement is of the sound pressure level, the number is often annotated as dB-SPL.
Example
A handheld sound level meter indicates that a nearby jackhammer has a sound pressure level of 85dB-SPL. What is the sound pressure of the jackhammer?
Solution
SPL = 20 log (P1/P0)
(SPL/20)= log (P1/P0)
10(SPL/20)= P1/P0
P1= P0* 10(SPL/20)
P1= 20 µPa * 10(85/20)
P1= 20 µPa * 10(85/20)
P1= 347 milliPascals
Remember that the SPL is dependent on distance from the sound source, as well as the geometry and location of reflective and refractive objects in the environment. Keeping that in mind, Table 2.3 provides a list of the SPL of some common sounds in typical situations. Note that prolonged exposure to sounds above 100 dB result in permanent hearing loss.
Table 2.3 SPL of Some Common Objects
Sound SPL (dB)
Whispering 10-20
Quiet Library 30
Ordinary conversation (at 1.5 feet) 65
Vacuum cleaner 75
Heavy traffic 85
Siren (at 100 feet) 100
Rock Concert 120
Threshold of Pain 120
Jet engines (at 100 feet) 140
Decibels Revisited
Now that we've introduced Sound Power Level (PWL) and Sound Pressure Level (SPL), let's spend some more time getting familiar with measuring them in decibels.
Remember that decibels are logarithmic ratios. As such, you cannot simply add them. For example, a 15dB-PWL sound source combined with another 15dB PWL sound source does not result in a PWL of 30dB. Instead, the combined PWL is 18dB, as we shall see in the following example.
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