Need help with REW - can't get reasonable measurements

[Tobias]

Hi there

I just bought an BFD and wanted to use REW to measure the proper filters to apply. But I can't really get any reasonable measurements. The curve acts very weird, growing linearly below 30 hz (being flat at 75 dB above 30hz), and certainly something is wrong since I have more than 100 dB at 2 hz

The only thing I can think I'm doing wrong is the connections to my soundcard. I was a bit confused of which inputs/outputs to use. My soundcard got black, green, pink, blue and yellow inputs/outputs, and I came up connecting my receiver to the green one, and the RS SPL meter to the blue one.

I then followed the manual calibrating the soundcard, and I got a completely flat curve at 0 dB as in the help file. Then I loaded the RS calibration file which I downloaded from this forum, and checked levels, setting my receivers master volume such that the sound level meter showed about 75 dB at the 80 dB setting.
Finally I clicked the calibrate SPL button in mic/meter settings. The meter showed 75 dB and i just clicked finished without changing anything.

Then I measured on the 512kb setting with end frequency 150 hz and tried to measure, and I got this weird curve, clearly not showing the proper response..

I'm completely new to this program and would be happy if someone could help me and try figure out what I am doing wrong?

Thanks in advance

[Tobias]

I just tried clicking the "SPL meter" button in REW and clicked the red circle, and if I made sounds at the SPL meter it didn't show up on the computer, it just stayed at 0 dB - I guess it is supposed to show the SPL reading on the meter? :P

[brucek]

Sounds like you've done most things correct.

Set the end frequency to 200Hz.

Set your Graph Axis limits to a vertical scale of 45dB to 105dB and horizontal scale of 15Hz to 200Hz. This is the standard we usually use for most subwoofer graphs. You don't want to look at anything down to 2Hz because your meter and its calibration don't go that low.



Be sure to set the target up by selecting subwoofer and clicking the Set Target Level button (that doesn't look like a button) as the last thing you do before Measuring.



Post a graph and I can likely tell you what's wrong.

brucek

[brucek]

I guess it is supposed to show the SPL reading on the meter?

Yep. You did use stereo connectors at the soundcards line-in and line-out?

brucek

[Tobias]

brucek wrote:

Yep. You did use stereo connectors at the soundcards line-in and line-out?

brucek

Yeah I did

And thanks for the quick replies. I'm gonna give it some more tries later to see if I can get something reasonable

edit: Btw, should I unplug my main speakers, so it's only my sub playing..? Can't turn them off in my receiver, so I would have to simply unplug them, it's quite troublesome to get them connected again , so if I could skip this it would be preferred

[brucek]

and if I made sounds at the SPL meter it didn't show up on the computer, it just stayed at 0 dB

Have you tested the SPL meter by using a standard RCA to RCA connector and plugging it into the AUX or CD of your receiver and sing. It's a microphone. Be sure it works.

Btw, should I unplug my main speakers, so it's only my sub playing..?

Most people do unplug the mains, so they're just looking at the sub with its crossover. Set the receiver to stereo mode, so only the mains are playing and not the other speakers. Then you'll only need to disconnect the mains (with the power off). Once the sub is equalized, add the mains to check the area around the crossover for dips or peaks.

brucek

[Tobias]

brucek wrote:

Have you tested the SPL meter by using a standard RCA to RCA connector and plugging it into the AUX or CD of your receiver and sing. It's a microphone. Be sure it works.

I tried that, and it works

[Tobias]

I was wondering why my BFD keeps blinking lots of red dots in the display? Have I missed doing somethig or will it just go away when I am ready to add the filters (when I get the measurements to work )?

[brucek]

was wondering why my BFD keeps blinking lots of red dots in the display?

Those are filters that need to be turned to the OF mode before carrying on.

Read the manual on how to turn them all off first.

brucek

[Tobias]

Ah ok I have turned them off now.

But I tried to measure it all from scratch now, and when I was about to calibrate the soundcard, with the loop connected, having the output/input device and output/input set to default, I didn't get any reading on the input bar on the right in the settings window... I experienced this before, but I just tried using other settings than default device input/output etc. and I got input reading, if I then tried setting it back to default, I got a reading again..

I must be doing something wrong somewhere..

[brucek]

having the output/input device and output/input set to default

Don't set them to default. Set them to Line-out (speaker) and Line-in......

brucek

[Tobias]

I get both input and output responses when I do as you proposed. But the measurement of the soundcard looks a bit odd

Here's a graph:


edit: Even though I got some flat measurement earlier, i cannot reproduce it.. but this one I get every time I try now.. dunno what happened earlier

[brucek]

But the measurement of the soundcard looks a bit odd

Before running Soundcard calibration routine.

Clear C-Weight switch.
Clear microphone calibrate file.
Clear Soundcard calibrate file.

Run routine and save file.

brucek

[Tobias]

I had no calibration files loaded, and unticking of the C weighting SPL meter in mic/meter didn't help

[Tobias]

anyone know what could be wrong?

[brucek]

anyone know what could be wrong?

I suspect your soundcard software has some equalizer/tone control setting turned on that boosts the bass and treble a bit and you end up with that response.

Is the graph you post the response of the loopback cable after running a measurement with the soundcard cal file in place, or is that the dotted line showing the soundcard cal file itself?

brucek

[Tobias]

The graph I posted is the one I get after measuring without any calibration file loaded..

[brucek]

The graph I posted is the one I get after measuring without any calibration file loaded..

I'm a bit confused.

The chain of events should be to install a loopback and clear all calibration files and then execute the Make Soundcard calibrate file.

This file is saved.

Then a measurement is made before removing the loopback cable to ensure the inverse Soundcard calibration file is applied to get a flat response of the cable. You don't take a measure of the cable without any calibration file loaded?

brucek

[Tobias]

Oh.. when I say measurement I mean the graph I get from pressing the "measure.." button in the soundcard settings This is the graph I have posted.
In the REW help file they call it a measurement, so I just called it the same..

[brucek]

OK, so that's the soundcard calibration file. It's not so bad really. Note the scale on the graph. The peak at ~25hz is only a ~1.5dB high. If you changed the vertical axis, it would look flat as a pancake. This file will compensate inversly when you measure.

So, if you then measured a loopback cable (with the soundcard file loaded) to verify everything is fine, I suspect you'll get a flat line.

brucek

[Tobias]

So, now I tried measuring the sub.. I got this graph:



It looks like it's jumping very much up and down.. could this be the right graph, or do you think I got something wrong with the measurements? If you think it's the real stuff, what would you propose me to try changing?
I can recognize the 45 hz peak, which I hear very clearly when I make the 100hz LFE sweep from the AVIA dvd, so that would indicate it's the real curve i get...

I think I will try make a measurement with the sub on some different locations.

And btw, would it be ok just to unplug one of the main speakers? (Since the output on the soundcard is only connected to L or R on the receiver) That would save some time since the cables I use is quite annoying to work with

[brucek]

It looks like it's jumping very much up and down.. could this be the right graph

Yeah, that's not too good. That's one of a dip between 20Hz and 40hz. Try some different spots for the sub if that's possible, just to be sure something else isn't wrong. Maybe just move the mic around to some other spot and get a good reading, so you know you're on the right track.

Looks like you took a good measure of the sub only, so I have to think you've done everything correct. Looks perfectly normal above 50Hz, but not so good below that....... You must be in some sort of null. You do have your receiver in stereo I presume (and not some 5.1 mode).....

Yeah, you can just disconnect the one main, if that's the only one playing. You just want to get the sub alone with its crossover. Always be sure those speaker cables don't short by mistake....

brucek

[Tobias]

I just tried a quick measurement without unplugging my speakers (will unplug them later for another measurement) and moving my sub to some other location. I got this graph this time:



Haven't regulated the phase knob or gain for the new location.. but It does look better now ?

[brucek]

Try unplugging those mains and see how the sub looks. Can't tell until you do that.......

brucek

[Tobias]

I just unplugged it and now I got this one



When I do the measurements I notice that the input reading on the meter is a bit low - just around -24dB.. though I did setup up the volumes prior to the measurement..

What would you propose me to try do now to get a better response?