Sounds are measured in decibels. Zero 0 decibels or 0 dB is the softest sound a person with normal hearing can hear at least 50% of the time. Here are decibel levels of everyday sounds:

0 dB - Threshold of hearing
10 dB - Rustle of leaves
20 dB - Water dripping
30 dB - Whisper
40 dB - Quiet radio in room
50 dB - Moderate rainfall
60 dB - Conversation, dishwasher
70 dB - Busy traffic, vacuum cleaner
80 dB - Alarm clock
90 dB - Lawnmower
100 dB - Snowmobile, chainsaw
120 dB - Rock music
130 dB - Jet plane takeoff

The important thing to know about decibels is this: if a sound increases by 10 decibels, it doubles in loudness as we perceive it. It sounds twice as loud to us. If a sound decreases by 10 decibels, it seems as if the sound has been reduced to half the volume.

Perceived Change in Loudness by Decibel Level

This chart shows the change in decibel sound level and how it is perceived by the human ear:

+/- 1 dB - Not perceptible
+/- 3 dB - Threshold of perception
+/- 5 dB - Clearly noticeable
+/- 10 dB - Twice as loud or 1/2 as loud
+/- 20 dB - Four times as loud or 1/4 as loud
+/- 30 dB - Eight times as loud or 1/8 as loud