Understanding decibels

Decibels (db) is the name given to measure the intensity of a sound. The db scale can be confusing because the human ear is very sensitive. Your ears can hear everything from a snail climbing a wall to a loud jet aircraft. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound.

On the decibel scale, the smallest audible sound is 0dB. A sound 10 times more powerful is 10dB. A sound 100 times more powerful than near total silence is 20dB. A sound 1,000 times more powerful than near total silence is 30dB. Here are some rule of thumb sounds and their db ratings:

25db normal speech can be easily overheard
30db loud speech can be heard clearly
35db loud speech can be distinguished (normal conditions)
40db loud speech can b heard but not distinguished
45db loud speech can be heard faintly but not distinguished
50db loud speech or shouting can only be heard with great difficulty

The above ratings are taken while standing near the sound. Any sound above 79dB can damage your hearing, and the loss is related both to the power of the sound as well as the length of exposure. You know that you are listening to an 80dB sound if you have to raise your voice to be heard. Eight hours of 90dB sound can cause damage to your ears; any exposure above 140dB sound causes immediate damage including intense pain. Sound pressure levels in dB can't be simply added together, like other quantities, because adding decibels is equivalent to multiplying the numbers.

If N sources generating the same sound pressure level, are combined, the overall sound pressure level will increase by 10 log N dB. For example 60dB + 60dB = 63dB and not 120dB as many people expect. The greater the difference in sound level between two sources, the less effect there is on the combined level. When the difference between two sources is more than 6dB, the combined level will be less than 1dB higher than the louder source alone.

When 2 different sound are present, the sound level of the combination of the two cannot be more than 3dB greater than the higher level. Locating a piece of equipment that has a noise level of 85dB in an area that already has a noise level of 85dB will result in a combined noise level of 88dB (85 + 3).

When on noise level of a combination is greater that the other, the resultant can be estimated from the following table:

As a general rule of thumb, depending on the sensitivity of the listener, most people will perceive a 3dB improvement as just noticeable, a 5-6dB improvement as very noticeable and an 8-10dB difference easily a halving of the sound level.