The
intensity of a sound is a measure of the
strength of the sound wave as it is indicative of the energy transported by the wave to a specific unit of area.
Recall that sound intensity is proportional to the square of the amplitude of the sound wave
Recall that the amplitude of a sound wave is a measure of the pressure changes associated with the wave.
The commonly used measure of
sound level is the
decibel (dB).
Sound level in decibels is calculated using the
sound intensity (
I) relative to a
reference intensity (
I0).
I0 is commonly the lowest (or threshold) intensity of sound that a person with normal hearing can perceive at 1000 Hz (10
–12 W m
–2).
The logarithmic relationship and the factor of 10 are used because our ears are not very sensitive to changes in intensity.
Note from the table below that, for example, we perceive a loud rock concert to be twice as loud as a loud conversation whereas the ratio of the sound intensities is actually a factor of 1 million.
| Sound level β** | Intensity I/W m–2 | source of sound |
| 0 | 10–12 (I0) | threshold of hearing |
| 10 | 10–11 | rustle of leaves |
| 20 | 10–10 | whisper at 1 m |
| 30 | 10–9 | noise in a quiet home |
| 50 | 10–7 | soft music |
| 60 | 10–6 | loud conversation |
| 70 | 10–5 | busy traffic |
| 90 | 10–3 | inside a subway |
| 120 | 1 | loud rock concert |
| 160 | 104 | bursting ear drums |
| **To calculate values in dB subtract the exponent on I0 from the exponent on I and multiply the result by 10. |