# checking levels



## chinni123 (Jan 8, 2012)

I see following in the check levels window .

Adjust the AV processor volume (preferred) or the Sweep Level (if connected directly to an equaliser, subwoofer or amplifier) to obtain the desired SPL reading on your personal SPL meter. Generally this will be 75db.
_I understood setting to 75db. _


* Next, adjust the Input Volume either using the REW Input Volume control or your soundcard's mixer or OS audio level controls so the input level is between -12 and -24dB FS, ideally around -18dB FS. If you are using a mic preamp, the level control on it may need adjusting. Note that the VU meter is heavily filtered to make it easier to read, it will react slowly to changes in volume setting so allow time for it to settle.*

What is this second step. Where is this REW input volume control and where should I see input level between -12 and -24db fs? I am using ecm 8000 microphone and TASCAM 144.

Thanks for your help.


----------



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

Assuming you’re using Version 5, under Preferences / Soundcard, next to the meters you’ll see “Input Volume.”

Regards, 
Wayne


----------



## EarlK (Jan 1, 2010)

If you are using JAVA drivers this is similar to what you'll see in the preferences window ( if you are also using the lastest REW beta version ) .










:sn:


----------



## aackthpt (Jan 24, 2011)

On the US-144, the "input L" and "input R" knobs are used to adjust the input gain. Unless they don't have enough range to the low or high, I'd just use them to set the level. In the picture that EarlK posted, you will set the "in" bar (middle one of the three on the upper right) to -18 dBFS. The "Ref In" is only needed if you are using the loopback timing reference; I usually set it to match the dBFS of the "out" bar (it doesn't really need headroom).

These bars, and the sound needed to use them, don't become active until you select "check levels" and then "next", IIRC.


----------



## chinni123 (Jan 8, 2012)

aackthpt said:


> On the US-144, the "input L" and "input R" knobs are used to adjust the input gain. Unless they don't have enough range to the low or high, I'd just use them to set the level. In the picture that EarlK posted, you will set the "in" bar (middle one of the three on the upper right) to -18 dBFS. The "Ref In" is only needed if you are using the loopback timing reference; I usually set it to match the dBFS of the "out" bar (it doesn't really need headroom).
> 
> These bars, and the sound needed to use them, don't become active until you select "check levels" and then "next", IIRC.


Thanks. It is interesting that the bar names changed in beta. I remember seeing "out", "left" and "right". Now it is "out", "in" and "Ref in". The help is REW seems does not reflect the new change of labelled meters. 

So, basically adjusting levels intended to set normal listening volume from receiver (75db) as well as setting enough input level (~-18dBFS ) so that measurements are accurate.

I assume that "check levels" from preferences window or from measurement window are same tests.


----------



## aackthpt (Jan 24, 2011)

chinni123 said:


> Thanks. It is interesting that the bar names changed in beta. I remember seeing "out", "left" and "right". Now it is "out", "in" and "Ref in". The help is REW seems does not reflect the new change of labelled meters.
> 
> So, basically adjusting levels intended to set normal listening volume from receiver (75db) as well as setting enough input level (~-18dBFS ) so that measurements are accurate.
> 
> I assume that "check levels" from preferences window or from measurement window are same tests.


Help in REW taught me a lot (I love the discussion of minimum phase) but yes, parts of it can be incomplete, out of date, or difficult to understand sometimes. Plow through and get it done! 

Volume from the receiver (or whatever combination of speaker/amp/etc is sending out the stimulus signal) needs to be high enough to give a reasonable signal to noise ratio over the ambient noise. Higher is theoretically better but we also want to keep the system operating in a linear range (not pushing it too hard which causes distortion) so 75dB is typically a good compromise. I usually run my sweeps at 80dB; sometimes they are run higher especially if they are for specific reasons like subwoofer compression testing. Recently Wayne seems to be telling people they should be 50dB above the ambient noise with their sweep (80dB probably is not for me actually) so I do wonder if we should have a protocol to check the ambient noise first. For the moment 75-80dB generally works fine though.

Input level needs to be high enough to maximize dynamic range of the measurement but low enough that it doesn't clip. So, same as on the stimulus/output side we are trying to keep the signal to noise ratio high but also avoid distortion. But on the input side REW will warn you if there is clipping I believe. It's also entirely possible to set levels at, say -12.5 dBFS and have it clip. If that happens you just set it a bit lower and run again, until you have a non-clipped measurement (then use that setup future runs).

Yes, the two "check levels" are the same. But the measurement window one is only meant as a quick check to verify it's OK before measuring which is why it doesn't last long. I use the one in the preferences pretty much every time I re-set up my measurement rig in case I have bumped the dials on the interface.


----------



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

aackthpt said:


> Recently Wayne seems to be telling people they should be 50dB above the ambient noise with their sweep (80dB probably is not for me actually) so I do wonder if we should have a protocol to check the ambient noise first. For the moment 75-80dB generally works fine though.


That’s what I’ve been told by acoustics experts, but it’s only an issue if you’re interested in generating a waterfall graph. For a standard frequency response measurement, it’s not necessary to get that high above the noise floor. The only ambient-noise issue related to frequency response graphs is that you not have the level so low that room noise “pollutes” the graph.

Regards, 
Wayne


----------



## chinni123 (Jan 8, 2012)

aackthpt said:


> Help in REW taught me a lot (I love the discussion of minimum phase) but yes, parts of it can be incomplete, out of date, or difficult to understand sometimes. Plow through and get it done!
> 
> Volume from the receiver (or whatever combination of speaker/amp/etc is sending out the stimulus signal) needs to be high enough to give a reasonable signal to noise ratio over the ambient noise. Higher is theoretically better but we also want to keep the system operating in a linear range (not pushing it too hard which causes distortion) so 75dB is typically a good compromise. I usually run my sweeps at 80dB; sometimes they are run higher especially if they are for specific reasons like subwoofer compression testing. Recently Wayne seems to be telling people they should be 50dB above the ambient noise with their sweep (80dB probably is not for me actually) so I do wonder if we should have a protocol to check the ambient noise first. For the moment 75-80dB generally works fine though.
> 
> ...


Thanks a lot John. It is excellent explanation. It always help to know what exactly it means rather than levels should be -18db etc. Basically, signal/noise ratio should be high and dynamic range should be large to get proper measurements.

Thanks again for sharing your knowledge.


----------



## JohnM (Apr 11, 2006)

chinni123 said:


> The help is REW seems does not reflect the new change of labelled meters.


Are you referring to the help accessed from within REW or the online files and pdf? Online and pdf are for the last release version (V5.0), help in the beta version is updated as I go along but if you spot things that are out of date let me know which sections so I can update them in the next release.


----------



## chinni123 (Jan 8, 2012)

JohnM said:


> Are you referring to the help accessed from within REW or the online files and pdf? Online and pdf are for the last release version (V5.0), help in the beta version is updated as I go along but if you spot things that are out of date let me know which sections so I can update them in the next release.


Thanks for reply John. I was referring to help from the tool. I thought I saw "Out, Left, right" meters. May be I got confused with the pdf document. I am not able to find now. It seems up to date.

Sure. I will let you know if I find any sections from the help of tool that does not reflect beta version.

Edit:
VU meters show "left" , "right"


----------

