[SoX-users] Calibrate soundlevels recorded from microphone with data measured from sound meter

[Peter Holm]

I have to do a long term meeting of soundlevels. I use a condenser
microphone and a cheap soundmeter.

My plan is frpm one speaker 1 meter from the microphones send sine
waves and place the sound meter beside the microphone. Record the
sound then adjust the line in level so the in-signal is the same as
measuered on the soundmeter.

The results from the following measuremt will be 1 minute long wav
files and a plot from the file.
I plan to record the files once each ten minutes.

So : How do I compare data from recorded files and calculate the
correct decibel-levels.?

And why I not use a better sound-meter plugged in to the computer, it
is becasue they all have to bad range in frequenzy..

[Peter Holm]

To be more clear. The fist recording will be for calibration. The next
step is to use the calibratd data to determine soundelevels from my
long-term meetiong and show that in a plot.

Post by Peter Holm
I have to do a long term meeting of soundlevels. I use a condenser
microphone and a cheap soundmeter.
My plan is frpm one speaker 1 meter from the microphones send sine
waves and place the sound meter beside the microphone. Record the
sound then adjust the line in level so the in-signal is the same as
measuered on the soundmeter.
The results from the following measuremt will be 1 minute long wav
files and a plot from the file.
I plan to record the files once each ten minutes.
So : How do I compare data from recorded files and calculate the
correct decibel-levels.?
And why I not use a better sound-meter plugged in to the computer, it
is becasue they all have to bad range in frequenzy..

[fmiser]

Post by Peter Holm
I have to do a long term meeting of soundlevels. I use a
condenser microphone and a cheap soundmeter.
My plan is frpm one speaker 1 meter from the microphones send
sine waves and place the sound meter beside the microphone.
Record the sound then adjust the line in level so the in-signal
is the same as measuered on the soundmeter.

I'm a little bit confused.

Audio is measured with decibels (dB). It does not matter if it is
sound in the air (dB SPL, or Sound Pressure Level) or in a wire
(dBu where 0 dB is 0.77 VRMS, disregarding impedance loading), or
in a digital system (dB FS where 0 dB is the maximum encodable
level)

Once you have recorded a reference is should be just simple math.
For example, if the SPL meter shows 80 dB SPL and your recording
is at -15 dB FS, then 0 dB FS will equal 95 dB SPL.

!!!! EXCEPT !!!!

dB SPL measurements are a bit more complex. There is frequency
filtering (A-weighted, C-weighted), and two time factors (fast,
slow).

This means that if your measured sound is different in frequency,
or different in "crest factor" (average level compared to peak
level) it _will_not_be_accurate_.

Post by Peter Holm
The results from the following measuremt will be 1 minute long
wav files and a plot from the file.
So : How do I compare data from recorded files and calculate the
correct decibel-levels.?

Plot the recording with a dB scale, and add or subtract the values
you calculated from the test recording.

That is all.

If you want average level, or peak level, or minimum level SoX can
get that for you with stat or stats.

Post by Peter Holm
And why I not use a better sound-meter plugged in to the
computer, it is becasue they all have to bad range in frequenzy..

dB SPL is supposed to be frequency weighted. A-weighting and
C-weighting have a different frequency response. That means that
a dB SPL meter should NOT EVER have a flat frequency response.

Despite all I wrote, I suggest you try it. If you are wanting
approximate values, not "traceable, suitable for law work", the
results will probably be just fine.

If you want values that can be defended in a court of law, or deal
with legal sound exposure limitations, it probably isn't going to
be accurate enough, but it can still show approximate ranges.