How can I calculate audio dB level ?

Question

I want to calculate room noise level with the computer's microphone. I record noise as an audio file, but how can I calculate the noise dB level?

I don't know how to start!

Answer 1

You can't easily measure absolute dB SPL, since your microphone and analogue hardware are not calibrated. You may be able to do an approximate calibration for a particular hardware set up but you would need to repeat this for every different microphone and hardware set up that you plan to support.

If you do have some kind of SPL reference source that you can use then then it gets easier:

• use your reference source to generate a tone at a known dB SPL - measure this
• measure the ambient noise
• calculate noise level = 20 * log10 (V_noise / V_ref) + dB_ref

Of course this assumes that the frequency response of your microphone and audio hardware is reasonably flat and that you just want a flat (unweighted) noise figure. If you want a weighted (e.g. A-weight) noise figure then you'll have to do rather more processing.

Answer 2

You can't. dB is a relative unit, IOW it is a unit for comparing two measurements against each other. You can only say that measurement A is x dB louder than measurement B, but in your case you only have one measurement. Therefore, it simply isn't possible to calculate the dB level.