Loudness: measured in decibels

As with many biological 'instruments' the ear registers loudness as an approximate log function of intensity. To accommodate this observation, the decibel (loudness) scale is logarithmic for intensity.

 Sound

 Intensity (Watts/m2)

Decibels 

Human heart beat

Classroom noise

Rock band

Jet liner at take-off

 10-12

10-7

10-1

1

 Zero

50 dB

110 dB

120 dB

At the threshold of hearing, ones own heart beat has an intensity of about zero dB. The noise level in a normal classroom may approach 50 dB and the noise under a jet airliner taking off is ~120 dB. Too much of that causes permanent hearing loss.