At a distance of meters, sound pressure level (SPL) is dB.
Then the sound pressure level at meters, is dB.
CLARIFICATIONS
¿What is a decibel?
As SPL is a pressure, it is measured in Pascals (Pa). The sound pressure that is considered as a limit for the human audition is 2 · 10-5 Pa. On the other hand, human ear pain threshold
is about 200 Pa. It is a very wide range, so, to handle these quantities more easily the unit usually used is the decibel (dB), which is a logarithmic scale relative to a reference level
What is the difference between sound pressure and power pressure?
Power pressure (Lw or SWL) is related to the level of sound emmited by the noise source. Sound pressure (Lp or SPL) represents the sound measured at a certain distance. So when we give a
sound pressure level, we have must mention the distance. Power level does not depend on distance it is an intrinsic characteristic of the machine or noise source.
¿What is the difference between dB and dB(A)?
Not all the frequencies are perceived equally by our ears. For instance, we are much more sensitive to noise between 1kHz and 4kHz than to higher frequencies.
This is the reason why sometimes we use
filters, so that the filtered values are more representative of the levels perceived by a human ear. One of the most used filters is type A, and the corresponding scale is dB(A) or dBA.
Rules of thumb
The power of a sound wave varies with the square of the pressure. If we divide by two the sound pressure level, the power level will be reduced by 3dB.
If we double the power level, sound pressure level will be increased by 3dB.
If we double the distance, sound pressure level is reduced by 6 dB.