# REW .cal file correction factor + SPL meter reading = true SPL?



## Chrisbee (Apr 20, 2006)

Hello

Forgive the intrusion but could I trouble you for a moment? 

If I'm taking readings from my Galaxy 140 using C-weighting and adding the .cal file correction factor am I reading the true SPL? 

For example: I play REW generated sinewaves through my sub: 

I read 106dB @ 10Hz @ 1 metre (39.4") on my Galaxy 140 SPL meter. 

I add the 13.45dB correction factor (taken from the REW text .cal file) to my 106dB(C) meter reading and then assume that I am actually outputting 119.45dB measured at 1 metre @ 10Hz.

Is the assumed ~120dB @ 10Hz true or false? (I'm ignoring any harmonic distortion components for the moment)


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## wadesi (Jan 22, 2009)

Good question...


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## brucek (Apr 11, 2006)

> If I'm taking readings from my Galaxy 140 using C-weighting and adding the .cal file correction factor am I reading the true SPL?


Yes. The end result of applying the cal file is to create a flat mic response. If the meter conformed to a perfect C-Weight response, then we wouldn't need a cal file, and we could simply check the C-Weight box in REW and it would apply a reverse C-Weight to the readings, and the result would be flat. That's a perfect world, but in the real world, no meter seems to exactly conform to the C-Weight curve, and especially at the extremes. So, we apply a curve (cal file) that more closely matches the meter over the wide extreme low frequencies we desire, and the result hopefully is flat.

brucek


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## Chrisbee (Apr 20, 2006)

brucek said:


> Yes. The end result of applying the cal file is to create a flat mic response. If the meter conformed to a perfect C-Weight response, then we wouldn't need a cal file, and we could simply check the C-Weight box in REW and it would apply a reverse C-Weight to the readings, and the result would be flat. That's a perfect world, but in the real world, no meter seems to exactly conform to the C-Weight curve, and especially at the extremes. So, we apply a curve (cal file) that more closely matches the meter over the wide extreme low frequencies we desire, and the result hopefully is flat.
> 
> brucek


Perfect! It seemed so obvious that I didn't quite believe it.

In a moment of doubt over an unlikely reading I feared the .cal corrections were unique to REW's inner workings. 

I wanted to be certain before I accepted the results of my testing as genuine.

Many thanks. 

Chris


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