Some Beginner Questions

[mgarkie]

Hi!

I just discovered REQ, and I have installed it and calibrated my sound card - a M-Audio Transit USB. My computer is a 1.3 GHz Athlon w/ 512 MB RAM. Windows XPSP2, Java v6 update 5. On the analog side I have a Behringer ECM 8000 and MX602 Mixer.

I have a 20' x 20' room with a 12.5' vaulted ceiling. My goal is to build a subwoofer system to use with my Paradigm Studio/20 monitors. I would like to experiment with speaker placement and room treatment. It looks like Room EQ Wizard is a fantastic tool for the job!

But I am having a problem getting started ...


The results of my sound card cal are consistent with the documentation.



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However, when I take a measurement of the loopback cable, I get this:



Any input as to what I might be doing wrong would be greatly appreciated!

Mike Garkie
Denver, CO

[JohnM]

Move the graph down a bit, your measurement is just above the top.

[brucek]

However, when I take a measurement of the loopback cable, I get this:

Your graph is displaying the REW target line and also your soundcard cal file. The measurement line is off the graph as already mentioned.

The proper graph axis to use is vertical = 45dB-105db and horizontal = 15Hz-200Hz for a subwoofer. These axis will result in your 75dB measurement being directly in the middle of the plot.

You set the axis by clicking the Graph Axis Limits icon button in the top right corner of REW.

brucek

[mgarkie]

If I adjust the plot to match the 'standard' ones in the forum, I get this (looks much better - thanks!)



I guess I don't understand the significance of the different plots.

The green plot at +80 dB is my measured response? Relates to headroom?

The dotted line at 0 db my soundcard calibrated response/frequency window?

The blue curve - a built in filter profile?

I'll spend some more time reading the documentation ... is there a step-by-step tutorial on taking a near-field speaker measurement?

Thanks in advance for your help!

- Mike

[brucek]

The green plot at +80 dB is my measured response? Relates to headroom?

The green line is your measurement of a straight cable to verify that the soundcard correction file is working - and it is indeed because your line is straight. The level of that signal is unimportant. You could have changed it to +75dB by doing a Calibrate SPL routine and set it to 75dB, but it matters not.

The dotted line at 0 db my soundcard calibrated response/frequency window?

Yeah, it simply shows the response of the soundcard cal file. you don't need it on the plot, so uncheck the box to take it away.

The blue curve - a built in filter profile?

Yep, it tracks a target for setting filters in keeping with the crossover setting you put into the target setting (i.e.80Hz)

I'll spend some more time reading the documentation

Good idea. Hopefully you read through some stuff here

is there a step-by-step tutorial on taking a near-field speaker measurement?

No, but they're no different than taking a regular listening postion measurement, other than where you place the speaker and the mic.

brucek

[mgarkie]

Thanks for the quick replies!

Here is my first crack at a near-field plot (1 M) on-axis of one of my speakers! I set my mixer so the input level matched the output level. I then set the speaker level so the SPL was ~= 75 dB on my RadioShack 33-2050 analog meter placed at the mic position.



How does it look? I don't have anything to compare it to, so does it look like my set-up is working so far?

What is the significance of the deep spikes in the measured responses in the higher frequencies? Can I filter the data to get a smoother response?

REW does not support level controls on my M-Audio Transit - is there a more convenient alternative to Windows Volume Control for setting levels?

I'll read through the material at the link provided! thanks brucek!

Sorry for all the questions, but I'm excited about playing around with REW!

- Mike

[kamui5785]

Enable "1/3 Octave smoothing" from the drop down menus up top. That will make it, well, ... smoother

If you want to measure just your sub, disconnect your mains and change the graph scaling to 15-200Hz.

-Brian

[kamui5785]

From looking at that graph so far it looks like you have a huge dip at the crossover point.

What are your speaker / subwoofer settings?

-Brian

[brucek]

As Brian says, we have to determine what you're attempting to measure.

If it's a mains speaker, then turn off all other speakers except the one you're measuring.

That jaggyness is comb filtering and can be relieved by turning on 1/3 octave smoothing option.

If you have the speaker in the middle of the room as far away from hard reflective surfaces as possible (proper near-field tactic), then you probably won't have as much comb filtering as you have, but either way, turn on smoothing for full range speakers (not for subs).

If it's a sub you're near fielding, then turn off all other speakers also and use the axis 15Hz-200Hz.

You can uncheck the soundcard cal file display, and turn on full range speaker in place of subwoofer in speaker type under target settings when measuring full range. This will give you a nice flat indicator target.

brucek

[mgarkie]

I measured each one of my speakers using this setup (from the old TEF days!), the speakers are laying on their backs in the middle of the room. The mic is 1 M above them, pointing down.

--- The left speaker


-- The right speaker

- MIke

Overall, a pretty good match -- is there a way to superimpose plots in EAW?

There is still that troubling hump in the lower midrange ... I check and see what the crossover frequencies of the speakers is ....

Could it be room modes? Can you gate the response in the time-domain to remove room echoes?

-- Mike

[brucek]

the speakers are laying on their backs in the middle of the room.

Mmm, I don't know if that's such a good idea. The reflections from the very close floor behind them won't be good. Best to stand them up.

is there a way to superimpose plots in EAW?

Yes, select the Measured tab and turn on and off any measurements to overlay with a checkbox.

Can you gate the response in the time-domain to remove room echoes?

Yes, select the IR Windows icon button in the top right hand corner of REW, and select to reduce the Right hand window, paying attention to the resulting resolution.

brucek