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Guide to Speaker/Room Correction Using Freeware and JRiver
mwillems:
Room correction:
If you want to go the next step of attempting to correct the room response (in addition to the speaker response), you can apply some of the same techniques covered above with a few modifications and caveats. For example, you can (after doing your speaker correction), take measurements with frequency dependent windowing at each of the listening positions in the room, export them to REW, average them and then generate additional correction filters for the averaged response. However, you may find that the measurements at the various listening positions are different enough that even the average may not be that helpful. However they are likely to agree on at least some things, and those are things that may benefit from EQ. A few things to keep in mind:
1) Room nulls or boundary cancellation effects (large dips in the response caused by the room) can't usually be corrected by EQ. Attempted corrections are not likely to work, and will compromise the sound somewhere else in the room even if they do work. By contrast, peaks or room resonances (large raised lumps in the response) often can be usefully EQed out. If you want to account for this and “play it safe” in generating your correction, you can set the maximum boost in REW to 0dB, so that it only applies cuts and doesn't even attempt to boost. You can also limit boost to one or two dB as a less risk-averse compromise solution.
2) Because the wavelengths of high frequency sound are very short, attempts to EQ room behavior at high frequencies are a moving target. At very high frequencies, it's hard to even position the microphone consistently enough to get consistent measurements off axis. For this reason I'd recommend limiting attempts to EQ room behavior to frequencies below 1 Khz or so (this can be achieved by setting the top of the “match range” in REW to 1 KHz).
3) Some of the non-linearities you see in some listening positions may not even result from the room, but rather from the directivity of your loudspeakers. Loudspeakers are engineered to be as flat as possible on axis, but as you rotate off axis, the frequency response of the loudspeaker will begin to change, and will tend to change differently at different frequencies because the directivity of a loudspeaker tends to vary with frequency response. If your listening positions are significantly off-axis (i.e. more than 30 degrees off axis) from either speaker you will get nonlinearities that cannot meaningfully be corrected without messing things up for other listening positions because they are a consequence of the polar pattern of your loudspeaker. If all of your listening positions are significantly off-axis, you'd be better off rotating the speakers or otherwise repositioning them.
As you might've gathered, those caveats are pretty significant. The best way to test whether your proposed room correction is a net plus or minus is to re-measure at each listening position, and on axis with the tweeter at 1 meter. The listening positions should each look better (or at least not worse), and the on axis response shouldn't look completely crazy. If the correction fails those tests, you know you've done too much and need to scale it back.
Conclusions:
This obviously just begins to scratch the surface on a lot of issues, but this is a method that has produced repeatable positive results for me. There's always more to learn, and always more tweaking to be done, but hopefully this has been a useful beginning for some folks. Here are a few additional links that might be helpful to folks trying to feel their way through more advanced measurement and correction topics:
Megathread on Holm over at Diyaudio (where the software author posts once in a while): http://www.diyaudio.com/forums/software-tools/144984-holmimpulse-measuring-frequency-impulse-response.html
Very detailed REW Manual: http://www.hometheatershack.com/roomeq/REWV5_help.pdf (the hometheatershack forums are also a great resource on REW)
Mojave's guide to using REW as a real-time analyzer with JRiver (very useful when you're tweaking your settings by hand): http://yabb.jriver.com/interact/index.php?topic=69725.0
RePHase megathread over at DIYAudio (where the software author posts frequently):
http://www.diyaudio.com/forums/multi-way/221434-rephase-loudspeaker-phase-linearization-eq-fir-filtering-tool.html
Feel free to offer corrections, suggested improvements, or questions below ;D
JimH:
mwillems,
Thanks very much for your work on this.
Jim
mojave:
Thanks for writing this up. 8) I have a few questions, but want to reread your posts and think about them some more.
I noticed you mentioned using MME output in HOLM. MME is for legacy software prior to Windows XP. MME only supports 16 bit and 44.1 KHz output. Although HOLM shows other sample rates, the Wikipedia article says MME resamples everything to 44.1. I would recommend at least DirectSound. DirectSound will still use the loopback and has lower latency.
Where did you find examples for how to do a convolution config file? I've been wanting to do this for a while, but never got around to it. REW lets you export the filters' impulse response as a wav. If not using Rephase, one could export their REW filters and create a config file rather than manually entering in all the filters into JRiver. Since a lot of people just EQ the subwoofer channel, the config file needs either only reference channel 4 or show the other channels as blank.
mwillems:
--- Quote from: mojave on February 17, 2014, 01:58:29 pm ---Thanks for writing this up. 8) I have a few questions, but want to reread your posts and think about them some more.
I noticed you mentioned using MME output in HOLM. MME is for legacy software prior to Windows XP. MME only supports 16 bit and 44.1 KHz output. Although HOLM shows other sample rates, the Wikipedia article says MME resamples everything to 44.1. I would recommend at least DirectSound. DirectSound will still use the loopback and has lower latency.
--- End quote ---
That's a very good point, I had been using MME, but direct sound is more flexible. I'll edit it to recommend direct sound.
--- Quote ---Where did you find examples for how to do a convolution config file? I've been wanting to do this for a while, but never got around to it. REW lets you export the filters' impulse response as a wav. If not using Rephase, one could export their REW filters and create a config file rather than manually entering in all the filters into JRiver. Since a lot of people just EQ the subwoofer channel, the config file needs either only reference channel 4 or show the other channels as blank.
--- End quote ---
That's adapted from the config file that I personally use for two-channel convolution on my office stereo (which is the one I was using as the example in the guide). This guide is pitched towards getting the most out of a pair of bookshelf speakers, so I figured the config file should match that goal. I'm planning to do a later guide to deal with subs and active crossovers specifically, and your point about the subs is a good one. I clarified that the config file is only for a 2-channel setup, and I'll make sure to provide less generic configs in the later guide.
I do briefly mention above that REW allows for WAV export, but I don't spend much time on it, because I don't personally recommend it (even if one isn't using RePhase). By using PEQ for the correction (instead of convolution), one can use the bulk of one's speaker correction with loopback, DVD sources, video games, etc. with no trouble. That's not possible if you export the filters as a WAV and use them in the convolution module. So from my perspective, the preferred approach is to copy the filters into PEQ so that folks can get the most use out of the correction they've generated. I'll beef up the export discussion a little, though, so the option is clearer.
Thanks very much for the feedback; keep it coming ;D
mojave:
--- Quote from: mwillems on February 17, 2014, 02:11:38 pm ---That's adapted from the config file that I personally use for two-channel convolution on my office stereo (which is the one I was using as the example in the guide). This guide is pitched towards getting the most out of a pair of bookshelf speakers, so I figured the config file should match that goal.
--- End quote ---
I'm really asking, "Where on the internet does one find the proper format for a convolution configuration file?"
Edit: I think I found it here: Config file
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