REW & varying Volume offers Different peaks & valleys

[CZ Eddie]

So I know that I'm supposed to setup my SPL meter at my listening position and adjust the test tones so my SPL meter reads 75db. Then calibrate REW to reflect 75DB also. But I still get messages about my level being to low (-35db or thereabouts).

So I wind up adjusting volume so my SPL meter reads 85db or like today, so that it read 100db.

And I noticed that as I ramp up the volume or lower it, the graphs that REW puts out can be wildly different.

Like if I setup my EQ so that level is about 100db, then I create a fairly flat response with that level. If I then lower the volume so the level is about 85db and I run REW again, I get a bunch of valleys and peaks that weren't there before.

Why is that?

And why do I keep getting an error when I post that says my tags are too short? I don't know what a tag is or why I have to put one in the thread, so I just made up a bunch of words.

[thewire]

Interesting. I don't have the correct answer but I have some input. Probobly like me it's your soundcard. Try a loopback of your right output into your right input. Does the distortion level get lower the higher level you play sine waves while looking in the RTA tab? Does ringing show on your graphs higher the lower the decibal level? Same for measurements in your room? Might try finding an appropriate sweep level and test that the loopbacks also measure flat at the level your testing your sub, make sure the SPL is calibrated proper and such. I usually use the recommended level around 5 which sometimes results in 85dB, or sometimes 95dB. All depends on what stage I am in during treatments etc and the sweep level. -18 sweep I might get near 85dB and for -15dB sweep I might get 95dB. Do not fear to check soundcard loopbacks if you suspect something isn't right IMO. Then I check a disk for a sanity check later on, and sometimes redo a measurement based on my findings. 100dB might be a pretty high level to be doing sweeps. Something like that might knock a picture off the wall, or damage a subwoofer in some circumstances.

[DrWho]

Did your microphone (SPL meter) move at all inbetween changing the volume? Moving as little an inch can introduce some wildly different results...

[Wayne A. Pflughaupt]

We'd probably have to see your graphs before we could make any recommendations, Eddie.

Regards,
Wayne

[Lonely Raven]

Not being able to dial in 75db means you probably need more gain on the microphone. What are you using as a preamp for it?

[JohnM]

If you are measuring a subwoofer make sure the end frequency of the sweep is not higher than 200Hz. If you still have problems with the input level to REW being too low increase the soundcard line input volume. If you increase the SPL too much your meter will start to clip, the flatter response at 100dB is a result of clipping within the SPL meter.