The decibel (abbreviated dB) is the unit used to measure the intensity of a sound. The decibel scale is a little odd because the human ear is incredibly sensitive. Your ears can hear everything from your fingertip brushing lightly over your skin 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. That's a big difference.
On the decibel scale, the smallest audible sound (near total silence) 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 common sounds and their decibel ratings:
- Near total silence - 0 dB
- A whisper - 15 dB
- Normal conversation - 60 dB
- A lawnmower - 90 dB
- A car horn - 110 dB
- A rock concert or a jet engine - 120 dB
- A gunshot or firecracker - 140 dB
You know from your own experience that distance affects the intensity of sound -- if you are far away, the power is greatly diminished. All of the ratings above are taken while standing near the sound. Any sound above 85dB can cause hearing loss, 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 85dB sound if you have to raise your voice to be heard by somebody else. Eight hours of 90dB sound can cause damage to your ears; any exposure to 140dB sound causes immediate damage (and causes actual pain).
Sound pressure levels in dB cannot simply be 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 two individual 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).
It is also worth noting, as we get asked all the time, what results one can expect when adding one or more products to an existing system/construction. Just because the data sheet states a product alone may have an acoustic rating of say 25dB, that doesnt mean that adding a single layer to a floor will improve things by 25dB because the floor is already a 'something' not just an open void. If the floor was already achieving say 35dB then adding something rated at 25dB won't give an end result of 60dB.
Acoustic underlays are also somethng else we get asked about a lot. The majority of underlays have a Delta figure which is basically a test before and after on a concrete floor which works out how much of an impact improvement the underlay gave. This figure doesnt really convert accurately to a timber based floor and by the time a timber overlay has been installed ontop of said underlay then the results wont be anywhere near the stated Delta Rw value. It's all information there to mislead customers into thinking one product is far superior to another, when in fact there is very little difference between any of them in the real world. .
When building acoustic walls, especially for office meeting rooms/partitions etc a general rule of thumb would be:
- Rw 25dB normal speech is easily overheard
- Rw 30dB loud speech is easily overheard
- Rw 35dB loud speech ican be understood under normal conditions
- Rw 40dB loud speech is heard but not easily understood
- Rw 45dB loud speech can be heard faintly and not easily distinguished
- Rw 50dB loud speech / shouting can be heard with quite a bit of difficulty