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Discussion Starter · #1 ·
Hello,

I'm a newbie at REW and room equalization. I'm still reading various material and haven't really tried REW yet. I have a couple of questions based on what I've read so far. From what I gather, the emphasis on room equalization is on the Subwoofers and Fronts. I have a 7.1 system, and I'm wondering why it isn't important to equalize the center channel, surrounds and rears? Did I misread something?

Also, when using REW, do you run the Subs and Fronts separately? If so, does it matter whether you do the Subs first or the Fronts first? On the Fronts, do you run Left and Right separately or together?

I'm using two sets of speakers for my Fronts because I didn't like my Mains upper response (so one set of speakers are my Front "Mains" and the other my Front "Assists". So, coming out of my AV Processor's Front output, I'm using a "Y" cable to two separate equalizers and amps. For the Mains, I'm using a SAE 2800 equalizer (parametric) and a Carver TFM-35 amp. For the Assists, I'm using a Numark 2600 equalizer (1 octave graphic) and a Carver TFM-15CB amp. Again, I have about the same questions as above: Do I adjust the sets separately using REW, or do I treat the pair of speakers (per side) as a single Front speaker? I hope this question makes sense...

Thanks in advance,
Kix
 

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Typically if only the subwoofers are being equalized, it’s best to first generate graphs running the subs only. Then another with the main speakers added, typically only the front pair, to see if there are any problems in the region of the crossover frequency.

People do indeed equalize the main speakers. Most people these days have receivers that have auto-EQ functions, but there are old-school “die hards” that run outboard equalizers and equalize them manually - i.e., based on their interpretation of REW graphs.

For EQing the main channels, typically you want to measure them independently one at a time.

As far as what to do about your dual speaker set up, if the deficiencies you’re hearing from your main speakers are coming directly from the speakers themselves and not room acoustics, I would think your SAE equalizer would be able to take care of it. Maybe with REW measurements you could see exactly where they are deficient and set filters to adjust the issues much more precisely than you have been by ear, and then you could retire the “assist” speakers

Regards,
Wayne
 

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Discussion Starter · #3 · (Edited)
Awesome information. I'm up to chapter 20, reading the REW EQ Wizard help files (out of 40) and this fills in most of the gaps I wasn't understanding. As far as a sub equalizer goes, I'm going to take your advice given on my "Assist" speakers above and wait until I take my first REW measurements before adding/removing anything.

I'm still curious about the center, side, and rear speakers on a 7.1 system. Maybe I just haven't read far enough yet. Why haven't I read anything about equalizing them into room acoustics and why wouldn't someone want to equalize them? I read your papers on "House Curves" and wonder how the center, side, and rear channels factor into that as well.

Thanks for helping me understand,
Kix


Btw: I guess I'm in the "old die-hard" camp. Aside from my center channel speaker and its equalizer, everything I have audio-wise is vintage 1978-1989. But, I'm an old dog that can still learn new tricks...
 

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Personally I think equalization of the center and surrounds is important as well (assuming it is needed), and I’ve done it with mine. The main reason people don’t use outboard equalization for these channels is that it gets expensive real quick. You can’t add equalizers to a common AVR as there is no provision for connection. So, it requires the addition (and cost) of outboard amplifiers, as well as the equalizers themselves. And while you can get away with a cheap equalizer for subwoofers, equalizers for the main channels need to be high quality, ultra-quiet units –read “not cheap.” All told, you can easily rack up a $1000+ premium to obtain outboard equalization for all channels, even if you go with used equipment.

But just because you don’t see outboard EQs in someone’s equipment list that doesn’t mean they aren’t using any. Most receivers these days have built-in equalization, and pretty powerful and effective ones at that. Most AVRs these days have fairly effective auto EQ systems, and some have equalizers with manual adjustments on top of that. I have a Yamaha RX-V2500 in my bedroom system that’s several years old, and it came with a pretty effective parametric EQ for the main channels. It only had 1/3-octave resolution, but it was capable enough to get what my speakers needed. My center channel would be virtually unlistenable without it. If they ever move up to 1/6-octave resolution, I’ll be giving serious thought to ditching my outboard EQs on my main system.

Regards,
Wayne
 

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Discussion Starter · #5 ·
Thanks for the reply Wayne.

Well, I got started setting up REW and thought that I would get a little farther along than I have before needing advice. I got the soundcard calibrated. It's not what I expected, with the IR slightly curving up above 600Hz. It looks a little like the example of someone forgetting to turn off the C-weighting, but not on both ends. I'm using a pretty cheap PC from Walmart that I bought for my kids and stuck in the livingroom. So my first thought and question is, is my soundcard good enough?

Here's what it looks like:
Text Line Design Pattern Pattern


So, next up was the "Check Levels". I disconnected the RCA cable feeding soundcard output into its input, turned back on C-weighting, and plugged the output into a Y RCA connector feeding my AVP. I had turned off all EQ's and amps, so was only using AVP and two sub-woofers. I immediately began hearing a "noticeable" hum from the SW's without even starting the test. I unplugged the SW's, turned on the EQ's and amps to the mains, sides, etc and was still hearing the hum (just a little crisper). Next I turned all external EQ's and amps off again, and hooked the PC directly to the sub-woofer and no hum at all.

The hum sounds like 60Hz to me. I measured my room's ambient SPL and measured 53dB with everything turned off (except furnace, it's Iowa...). Measured again with my PC connected to the AVP and two SW's and measured 66dB. So, the hum is about 13dB.

So, I have two questions. First, does my soundcard & cal look like it's okay? Second, any ideas why I'm getting this humming noise (ground-loop?). and where I might start looking to correct it? System sounds fairly good when it's not connected to the PC soundcard...

Thanks in advance,
Kix
 

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Discussion Starter · #8 ·
Thanks for the replies, Wayne and John.

Took the better part of the night to read through 315 posts in the link above, but it was worth it (I hope). Since I don't have any pro equipment yet, the unbalanced/balanced cable converter probably isn't going help me. I still got some good ideas reading though, and it gave me a plan of attack....

I did a little more testing tonight. I think I may have 2-3 sources that are producing the hum. The biggest source is my cable connection to my TV. Second biggest, is when I connect my PC to the AVP. Last, even with no cable or PC connected, I have a very faint hum.

The right way to fix the problem seems to be to figure out why the grounds are different for these sources. Unfortunately, it's -12F outside and I'm 54 years old. So, the right way will need to wait until Spring. So, I'm going to have to do it the Kix way for a bit (patch) until Spring arrives.

I know this isn't the right thread for this type of discussion, so I'll just leave the subject right here. Still hope to make my first measurements yet this week or weekend...

I just wanted to reply and say "Thanks again for the guidance".

Regards,
Kix
 

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Discussion Starter · #9 · (Edited)
Okay, I must be doing something wrong. I recalibrated my soundcard, no problem. Now I'm trying perform the Check Levels procedure and either I've set something wrong in REW, or I'm not getting a signal out of my SPL Meter and into my Line In on the soundcard.

I'm using a Galaxy CM140. I've adjusted the volume on my AVP so that the SPL meter reads 75dB. I'm using an mono 1/8" connector to the CM140 and a single RCA plug on the other end. On the Line In on the soundcard, I'm using a stereo "Y" connector with two RCA jacks on one end and 1/8" connector to the soundcard. I'm using the "red" RCA connector on both line in/out (white RCA connector is unused).

This is what I'm seeing in REW after pressing Check Level button:

(Please check the following reply for the screen-shot. It didn't like my first attempt, and I can't see the attach button in edit mode)

I'm expecting to see something in the graphic panel under "In", but there isn't anything (-99dB FS). I'm unable to adjust Input Volume or Output Volume and both are set to 0 (which makes sense why I'm seeing -99db FS in the graphic area. If I uncheck the two "Control Input/Output Volumes", it still doesn't allow me to adjust them. Although they are no longer grayed and are enabled, they "try" to adjust, but automatically go back to 0.

Any thoughts on what I might be doing wrong? I used my DMM to make sure the cable is good from SPL meter 1/8" plug to the plug on the soundcard "Y" connector and it's good. I guess I could reconnect my speakers to the soundcard out and validate "something" is coming out of the CM140.... It's a new meter that I got a couple weeks ago. But it seems strange that I can't adjust REW's Input Volume, as that seems like what the purpose of this step is, make the dB reading in REW match the SPL meter.

Thanks in advance,
Kix
 

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Discussion Starter · #13 ·
Pretty odd to have the levels set to zero. If you uncheck REW's control can you adjust the levels using the Windows recording and playback volume controls?
Thanks John and Wayne,

Wayne was spot on. After calibrating the sound card, I didn't like the way the RCA to 3.5MM adapter fit into my soundcard. The adapters were thick and the line in/out connectors were close together, so the two adapters were somewhat forced in. So, I bought some adapter cables to replace them. Long story short, I was basically using a mono cable into the line-in instead of just into the right channel.

Waiting for different cables to come in....

Kix
 

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Discussion Starter · #14 · (Edited)
Got the new adapters for the soundcard, which fixed my issue of not having an input signal from the SPL meter. Also got an isolator for my TV's cable line, which fixed the majority of my ground-loop problems (at least until Spring and I can fix the root-cause of the problem). There's still some faint humming going on....

Just for kicks and grins until cables come in for my Sub EQ, I played around with REW and took my first measurements of my subs without EQ. Not quite sure what I'm seeing, but I either wouldn't expect that dip at 85Hz on SPL then bounce back up to continue it's decline, or I would expect it to dip beginning at 80 and just keep falling from there. I have the cross-overs on the subs set to 80Hz. I'd also expect it to remain flat at 80db until it got to 80Hz, instead of starting to decline at 45Hz. Any thoughts?

I also have no clue what the phase is telling me, aside from being close to 180. Does that mean something's 180 out of phase?

Text Line Plot Diagram Pink


I'm also curious about Gain Structure... I probably ought to post these questions on Wayne's thread about Gain Structure, but if it's not applicable in my case, I don't want to pollute his thread. Is that something that needs to be done "before" taking measurements in REW? After reading the articles, is it even applicable to my set-up? I'm using a consumer AVP and consumer amps, but a pro EQ. I believe the EQ has a +4 dBu/-10 dBv switch on it. Should the output of the AVP be tested, just to determine the max I want to turn up it's volume?

I appreciate any advice,
Kix
 

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Just for kicks and grins until cables come in for my Sub EQ, I played around with REW and took my first measurements of my subs without EQ. Not quite sure what I'm seeing, but I either wouldn't expect that dip at 85Hz on SPL then bounce back up to continue it's decline, or I would expect it to dip beginning at 80 and just keep falling from there. I have the cross-overs on the subs set to 80Hz.
Part of the problem is the way you have the graph scaled vertically – see the post linked below. With proper scaling you’d see a steeper decline. It’s an easy adjustment you can make in REW, no need to re-measure.

Getting Graphs Ready to Post


I'd also expect it to remain flat at 80db until it got to 80Hz, instead of starting to decline at 45Hz. Any thoughts?
You expected virtually ruler-flat response all the way up to the crossover frequency? Have you studied the graphs from any other threads? :D


I also have no clue what the phase is telling me, aside from being close to 180. Does that mean something's 180 out of phase?
You can ignore the phase trace. Better yet, just turn it off for future graphs uploaded, it just muddies up the graphs. :T


I'm also curious about Gain Structure...
Is that something that needs to be done "before" taking measurements in REW? After reading the articles, is it even applicable to my set-up? I'm using a consumer AVP and consumer amps, but a pro EQ. I believe the EQ has a +4 dBu/-10 dBv switch on it. Should the output of the AVP be tested, just to determine the max I want to turn up it's volume?
There is no gain structure issue in your situation. As stated in Part 9 of the gain structure article, “If you have a pro audio equalizer or other processor, just connect it between the AVR and amp (either consumer or professional) and you’re done. The processor has no level-adjustment requirements (as thoroughly documented in Parts 4 and 5).”

As far as the switches on the equalizer, you can start with them in the -10 dBV setting. If the EQ has level meters and you see them hitting red, then you can switch to the +4 dBu setting. Or, you could just start with +4 and be done with it. :T

Regards,
Wayne
 

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Discussion Starter · #16 · (Edited)
You expected virtually ruler-flat response all the way up to the crossover frequency? Have you studied the graphs from any other threads? :D
Well, of course I expected it to be flat all the up to 80Hz. :D Like everything else, I start with high expectations and lower them if needed, versus the opposite. LOL I try to read every new post on this forum, but haven't had a chance to read much further back than a couple weeks from when I joined. I'll see if I can't find some other sub-only SPL's for comparison.

BTW. Thanks for the reply Wayne. I really do appreciate it.

I changed the vertical scaling and dropped the phase as suggested, and you're right, the drop became steeper (and looks worse to these newbie eyes). Here's the results:

Text Line Plot Purple Pink


Also, the Mic/Meter seems strange. A 5dB drop from 100Hz to 30Hz seems like a lot, so does another 2.5dB drop 30Hz to 20Hz. Is that normal?

I'm not going to do anything yet, but I'm SO tempted to raise the cross-overs on my subs so they don't drop at 45Hz like they are, or at least that low.

Even though I don't need to deal with Gain Structure, does it make sense to find out where my AVP begins clipping, just to avoid hitting it or going above it? It seems like it should matter, since I'd want to avoid sending a clipped signal to the EQ, then Amps (regardless if commercial or pro). Or, am I still missing a point in your articles?

Please, if anyone else has an opinion, chime in.

Thanks,
Kix
 

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Also, the Mic/Meter seems strange. A 5dB drop from 100Hz to 30Hz seems like a lot, so does another 2.5dB drop 30Hz to 20Hz. Is that normal?
It is for a SPL meter mic. Most budget meters don’t have an option for flat (i.e. Z-weighted) response, only for A or C weighting. C-weighting is flatter than A, but nevertheless rolls out below 100 Hz. So naturally the calibration file is compensating for that.


I'm not going to do anything yet, but I'm SO tempted to raise the cross-overs on my subs so they don't drop at 45Hz like they are, or at least that low.
Can’t hurt to try, but I doubt it’ll make a difference. If it does it will mean your crossover is grossly inaccurate.


Even though I don't need to deal with Gain Structure, does it make sense to find out where my AVP begins clipping, just to avoid hitting it or going above it? It seems like it should matter, since I'd want to avoid sending a clipped signal to the EQ, then Amps (regardless if commercial or pro).
Not really necessary, but it might be useful to determine at what volume setting on your pre-amp that distortion sets it. I expect you will find it to be much higher than you’d ever use.

Regards,
Wayne
 

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Discussion Starter · #18 · (Edited)
Well, you were right about a couple things Wayne. First, playing with the crossovers on the Subs or via AVP didn't really make much difference in freq response. I also tried moving the subs closer to the wall, moving further away from the wall, and rotating them slightly inward and outward with not much change.

Also, from what I can tell, I was never able to hear where my AVP begins clipping. I tried several times and couldn't hear any overtones with the 1KHz signal as I ramped up to full volume. Output voltages from the AVP at max volume was:

FR 2.069vac
FL 2.049
SR 1.914
SL 1.979
Ctr 2.002
Sub7.61
RR 2.014
RL 2.079

I was a little surprised the Sub was so much higher than everything else. But it makes me wonder if that's contributing to an issue that I'll describe below in the charts. I used a Carver TFM15CB amp. Not pro, but has input gain controls.

So, back to measuring with REW... I definitely must have something funky going on with my room. From the LP, I'm still dropping out at 45Hz. However if I place the Mic about 12 inches in front of the sub, it improves dramatically. I have all speakers in my AVP set to small, with XO's set 80Hz.

This first graph is overlaying my subs from the LP and also 1 foot in front of one of the subs. LP is green trace. It's definitely showing some change above 50Hz when mic is 1 ft away from the sub..

Edit: Refer to graph, second from the bottom of this post. The title is wrong!!! It says Front Mains - LP and 1Foot, it's really Subs - LP and 1Foot. Sorry, but I haven't figured out how to edit an attachment.

View attachment 86818

Okay. So, here are my Fronts using the same "Use subwoofers to set levels" level as in my Subs chart:

Edit: Refer to graph, at the bottom of this post.
View attachment 86826

Now for 15-22KHz.... Besides the "spike at 60Hz ground-loop problem" that is evident, why is the SPL so low when measuring my Fronts compared to my Subs between 20-45Hz? I mean, between the Subs and Fronts charts, it's like it falls off the edge of the earth above 45Hz. Is it the ground-loop issue at 60HZ? Surely, it can't be that my AVP Sub out is 7.4vac and so much higher than all others (my levels are ~75db either way)???

Text Plot Line Font Slope


Text Line Font Plot Pink


I'm limited as to where I can place two subs. One, I can swap positions of the Fronts and Subs. Currently on the front wall, Subs are between Fronts and Center. Two, I can place the left sub in the rear left corner of the room, and the right sub I can slide up to about half of the right wall. I plan to play around with this, this weekend. If this isn't the correct forum, please feel free to move.

Any thoughts as to my next approaches or what seems to be going on with with what I'm seeing in REW?


Regards,
Kix
 

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Hey Kix,

So, back to measuring with REW... I definitely must have something funky going on with my room. From the LP, I'm still dropping out at 45Hz.
You’re not really dropping out below 45 Hz. You just have a peak in that area, and response naturally has to fall on either side of a peak. You actually have good extension out to about 21-22 Hz, which is pretty respectable actually...


However if I place the Mic about 12 inches in front of the sub, it improves dramatically.
Well sure, to a large extent you’ve minimized the effect of the room with close mic placement.


This first graph is overlaying my subs from the LP and also 1 foot in front of one of the subs. LP is green trace. It's definitely showing some change above 50Hz when mic is 1 ft away from the sub.
As above. :T


Edit: Refer to graph, second from the bottom of this post. The title is wrong!!! It says Front Mains - LP and 1Foot, it's really Subs - LP and 1Foot. Sorry, but I haven't figured out how to edit an attachment.
You can’t - you have to go back to REW and re-save your graph (i.e. no need to re-measure). Before you do that you can change the title.


Now for 15-22KHz.... Besides the "spike at 60Hz ground-loop problem" that is evident, why is the SPL so low when measuring my Fronts compared to my Subs between 20-45Hz?
You have to re-do REW’s SPL calibration routine before switching from subs to mains measurements, and vice-versa. With a given signal, subs will always measure a higher SPL than the main channels, especially if you’re measuring only one speaker.


I'm limited as to where I can place two subs. One, I can swap positions of the Fronts and Subs. Currently on the front wall, Subs are between Fronts and Center. Two, I can place the left sub in the rear left corner of the room, and the right sub I can slide up to about half of the right wall. I plan to play around with this, this weekend. If this isn't the correct forum, please feel free to move.
You have a nasty drop between 45-85 Hz that’s so severe it can’t be fixed with EQ. Try any other locations you have available. The closer to a corner the better, and full in-corner is usually best. Not necessarily perfect, but response that can easily be cleaned up with equalization is good enough. And remember, you don’t have to separate the subs. If you only have one location that gives best results, they can be stacked. Not pretty, admittedly...

Regards,
Wayne
 

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Discussion Starter · #20 ·
Thanks for the input, Wayne. You're a tremendous help (along with so many others here).

As soon as my living room is "freed up" tonight (read, wife goes to bed), I'll focus a little more on my Mains & the SPL Cal and see if I can't raise the signal level some. Might make some measurements of each Main separately too. Saturday, I can wire up a YDP2006 to the Subs and try to find a better location for them. My XLR to RCA adapters came in today. :D

Kix
 
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