# REW's SPL METER ACCURACY ??



## randyc1 (Jul 20, 2011)

How accurate is the SPL Meter in REW, without the use of another SPL meter for reference. 

In other words when creating a sound , how close to the actual SPL would the (REW SPL) be ??


----------



## Moonfly (Aug 1, 2008)

Without a point of reference by which to calibrate the internal spl meter, it is pretty much useless. You also need a mic of decent accuracy, so a laptops mic is not really of any use either, nor any other mic unable to measure real world spl's. Once calibrated though it functions properly, but exactly to what degree of accuracy I think John will have to answer.


----------



## SAC (Dec 3, 2009)

It all depends what you are doing.

Few purposes require an absolute SPL gain scale. If you need that, then you will need to either provide a Calibration reference, or you will at least want to bring REW into agreement with some other device (such as a RS SPL meter), but I would not consider that calibrated, nor would it be acceptable in more formal settings such as are required for NLA (noise level analysis) studies.

I am not sure if REW accommodates reference by output voltage, but this link provides instructions regarding the approach that might potentially be applied or at the very least explains one method by which this is commonly performed.

For many applications, a calibrated level is not required, as you are generally comparing relative levels - meaning you only care about the differences between two or more signals, such as in comparing direct and indirect signals in an ETC to evaluate reflections in a room. In this case the differences in gain between the various signals is sufficient to determine if the particular energy arrival is above a certain differential gain threshold to present problems, or. conversely, below accepted thresholds so as to avoid creating psycho-acoustic problems.

So, a calibrated measurement may be insufficient or perfectly adequate depending upon the application and the context of its intended application, which makes it difficult to provide a simple answer your question, as a specific application has not been defined.


----------



## randyc1 (Jul 20, 2011)

Yes i understand Sac ,.. I just wanted to get an idea of what level ( db ) , i am acustomed to listening movies at ??

From what i could tell , the Rew Spl Meter is INITIALLY way off ... With Pink noise I could'nt even get one speaker alone, to play even close to 75 db with more than 3/4 vol ??


----------



## SAC (Dec 3, 2009)

Randy, for that it would be sufficient to simply 'sync' it with some reasonably reliable independent source, such a the proverbial RS SPL meter.

That would provide you with a reasonably accurate indication of gain.

When I get a chance I will try to do a bit of digging to see if REW has a voltage reference option where you could use the voltage output as a source to establish gain level...unless John or someone else can jump in confirm or deny that option.

And if it is not available, that would be a very handy functional feature to add to REW, as it would make such gain calibration very easy!


----------



## JohnM (Apr 11, 2006)

Because REW has no control over, or knowledge of, the sensitivity and gain settings of the input chain (mic, preamp, soundcard) it has to be told what a given input signal corresponds to as an SPL level. Until that is done the absolute figures REW shows are meaningless. If input chain gains are later changed the SPL level cal needs to be repeated to re-establish the correct reference.


----------



## randyc1 (Jul 20, 2011)

Ok ,.. thanks guys for the explainations !


----------



## Saturn94 (Jun 8, 2013)

JohnM said:


> Because REW has no control over, or knowledge of, the sensitivity and gain settings of the input chain (mic, preamp, soundcard) it has to be told what a given input signal corresponds to as an SPL level. Until that is done the absolute figures REW shows are meaningless. If input chain gains are later changed the SPL level cal needs to be repeated to re-establish the correct reference.


Is this still the case today using a USB mic like the UMIK-1?

I used the UMIK-1 with REW to set all channels to 75db using my processor's internal test tones and found it pretty much matched my RS meter except the sub reading on the RS meter was a couple db or so lower than the UMIK-1.

I didn't go through the calibration process in the REW guide because it said this wasn't necessary with the UMIK-1.


----------



## JohnM (Apr 11, 2006)

With UMIK-1 the cal file includes a sensitivity figure, V5.01 beta can read that and the input volume setting and so convert the signals from the mic into actual SPL.


----------



## Saturn94 (Jun 8, 2013)

JohnM said:


> With UMIK-1 the cal file includes a sensitivity figure, V5.01 beta can read that and the input volume setting and so convert the signals from the mic into actual SPL.


My apologies, but I don't quite understand. Are you saying my readings are accurate (I think you are)?


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

Yes, that's precisely what he said. 

Regards,
Wayne


----------



## Saturn94 (Jun 8, 2013)

Wayne A. Pflughaupt said:


> Yes, that's precisely what he said.
> 
> Regards,
> Wayne


Thanks.

I was a little embarrassed to ask, but what the ....there's no harm in asking.


----------

