How Does a Sound Meter Interpret Different Frequency Ranges?

[fisico30]

Hello Forum,

I am planning to buy a noise meter.
This instrument receives a signal that carries a certain total power. A spectrum analyzer tells us how this total power is distributed among the various frequency components.

When the sound meter gives a reading in the dB (A) scale it is practically applying a filter to that frequency spectrum and giving us the area (power) contained only in those frequencies. For example, if 100 dB SPL of sound hit the noise meter the reading will probably be less dB than 100 because the meter only cares about a certain frequency range...

dB (C) focuses on the low frequencies...

Question: If the meter is in front of a noise source, the dB reading on the dB (C) scale can be larger than the reading on the dB (A) scale. The dB(A) and dB(C) reading don't sum up to 100 dB SPL? why not? For instance, could it be that we read 60 dB(A) and 90 dB(C) even if the sound hitting the meter is 100 dB SPL...
Thanks,
fisico30

 

[anorlunda]

It is basically a pressure measurement.

Sound power level $P=20log(\frac{P}{P_{ref}})$

Where Pref is the reference or hearing threshold pressure.