Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
6
Sound Approximate Air
Pressure Amplitude in
Pascals
Ratio of Sound’s Air
Pressure Amplitude to
Air Pressure
Amplitude of
Threshold of Hearing
Approximate
Loudness in
dBSPL
1. Threshold of
hearing
0.00002 = 1 0
2. Breathing
0.00006325 =
3.16 10
3. Rustling leaves 0.0002 = 20
4. Refrigerator
humming
0.002 = 40
5. Normal
conversation
0.02 =
60
6. Vacuum
cleaner
0.06325 = 70
7. Dishwasher 0.1125 = 75
8. City traffic
0.2 =
80
9. Lawnmower 0.3557 = 85
10. Subway 0.6325 = 90
11. Symphony
orchestra
6.325 110
12. Fireworks 20 = 120
13. Rock concert 20+ = + 120+
14. Shotgun firing
63.25 =
130
15. Jet engine
close by
200 = 140
Table 4.1 Loudness of common sounds measured in air pressure amplitude and in decibels
Now let’s see how these observations begin to help us make sense of the decibel. A
decibel is based on a ratio that is, one value relative to another, as in . Hypothetically,
and could measure anything, as long as they measure the same type of thing in the same units
e.g., power, intensity, air pressure amplitude, noise on a computer network, loudspeaker
efficiency, signal-to-noise ratio, etc. Because decibels are based on a ratio, they imply a
comparison. Decibels can be a measure of
a change from level to level
a range of values between and , or
a level compared to some agreed upon reference point .
What we’re most interested in with regard to sound is some way of indicating how loud it
seems to human ears. What if we were to measure relative loudness using the threshold of
hearing as our point of comparison the , in the ratio , as in column 3 of Table 4.1? That
seems to make sense. But we already noted that the ratio of the loudest to the softest thing in our
table is 10,000,000/1. A ratio alone isn’t enough to turn the range of human hearing into
manageable numbers, nor does it account for the non-linearity of our perception.
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