Part 05 · Oscillations and waves · Card 11 of 11
Sound intensity level
Loudness measured in decibels (dB), a log scale where +10 dB means ten times the intensity.
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Analogy
Earthquake scales also climb by powers of ten.
Example
Decibels
dB = 10 log(I / I0)I0 = 1e-12 W/m^2dB = 10 log(1e6) = 60 dB
In practice
An intensity of 1e-6 W/m squared is 60 dB.
Practice
Flashcards
- How many dB is a tenfold rise in intensity?10 dB.
Questions people ask
Why do we use decibels?
Our ears respond to sound intensity on a roughly logarithmic scale. The range from a whisper to a jet engine is a factor of 10 to the 12. Decibels turn this huge range into numbers from 0 to about 120, which is easier to work with.
How does intensity fall with distance?
For a small source spreading evenly, intensity equals power divided by 4 pi r squared. Doubling the distance cuts intensity to one quarter, which is about 6 dB quieter. This inverse square law is why a distant conversation sounds much softer than a nearby one.