Comb Filtering Confusion

[Jack N]

In my old house, I had a full 8.1 set-up with a Yamaha RX-V3300 (rated at 130wpc, but it doesn’t sound like that much) controlling everything. I had some older stereo components laying around not being used, so one day I got the itch to try and integrate them into the system. I ran the pre-outs from the receiver over to the Yamaha power amp (M85 - which has 2 separate mono-blocks and power supplies, one for each channel, 270wpc) which in turn powered another pair of identical speakers up front (same manufacturer, same model, etc.), so I had one pair being powered by the receiver and one pair being powered by the M85. The sound quality improved noticeably. I know it wasn’t just a perceived thing as everyone (not one exception) that heard it commented on how good it sounded. So, following the typical male logic of “if one is good, two is better…..” proceeded to buy more M85’s and more identical speakers to match their respective brothers in each channel. So by the time I was done, I had two full systems all being controlled by the one receiver. One system being powered by the receiver, the other system being powered by 4 M85s. The sound was amazing in that now horrendously over crowded small living room. I had two people tell me that they’ve never heard anything better, ever. (Although they’re not audiophiles, it’s still nice to hear.)

Then I sold my house and I now have everything packed away, waiting while I finish my new dedicated home theater room. It will be approx 15 x 45. In the mean time I’ve discovered some articles and discussions about comb filtering, which has me confused, and has led me to starting this thread. There seems to be three sides to this issue. One side says absolutely under no circumstances should you use two speakers per channel. Another side says that in real world applications comb filtering, if not impossible, is next to impossible to hear. And the third side says that it’s strictly luck of the draw – that there’s just too many variables to predict if your system will be compromised or not – you just have to set up your system and find out for yourself to see how the variables all come into play.

Well, with a substantial amount of money already invested, I’m going to be biased towards the “impossible to hear” side. Although with what I’ve read, I can certainly see where the “impossible to predict” side would have a legitimate argument. I have a strong tendency to shy away from the “absolutely don’t use multiple speakers” side because their argument seems to be based mostly on pure sine waves, which I don’t listen to except during calibrations.

This is where you come in. Is the big change in room size and layout going to act as a detriment ? I’m wondering if you think it’s going to be worth the extra expense and the extra time of basically setting up two separate audio systems in my new home theater. Which side do you adhere to, and why ?

 

[billnchristy]

Personally I would set up the reciever outputting to the amps outputting to the speakers.

Dont bother with the reciever power, use those amps and run a single set of each.

Set the other stuff up in another room or something.

Reality is, if you like it, it doesnt really matter what the "experts" say, set it all up, if it still blows your mind keep it, if not trim down.

Definitely keep the amps though, they will always sound better than reciever amps.

 

[tbewick]

If you're happy with the sound that your system produces, then really that's the most important thing. The problem with using multiple speakers would probably be getting an uneven frequency response in the different listening positions. What this would result in is reduced sound quality, in the sense that spatial imaging and detail would be reduced. You would get a more uneven frequency response, with all kinds of phase effects that would reduce the sound quality.

I would compare it to listening in one of those 'five/six channel stereo' modes which some receivers have, where you have two channel put out through all your speakers. It sounds good to begin with, but after a while it sounds flat, because you lose the spatial effects that are audible in the original two channel mix.

Changing the room could have a considerable effect on the sound. I think that if you are investing in new equipment, then you'll get the most improvement when you upgrade your speakers. Better speakers usually have a flatter amplitude response with a concomitant improvement in impulse/transient response, and smoother dispersivity off-axis, helping to produce a more convincing stereo effect. Speaker placement will have a major effect on the sound quality.

 

[Jack N]

Looks like the general consensus so far is – “it’s impossible to predict”. I guess I’m starting to take on that attitude too. I’m thinking that with the room being 45 feet long, and the front and rear speakers being so far apart, I can’t help but wonder - just what will it sound like ? And now that I think about it some more, I’m really wondering how it will sound using so many bi-polar speakers in a room that large. Hmmmm. Not sure what to think now. Any more thoughts ?

 

[tbewick]

Looks like the general consensus so far is – “it’s impossible to predict”. I guess I’m starting to take on that attitude too. I’m thinking that with the room being 45 feet long, and the front and rear speakers being so far apart, I can’t help but wonder - just what will it sound like ? And now that I think about it some more, I’m really wondering how it will sound using so many bi-polar speakers in a room that large. Hmmmm. Not sure what to think now. Any more thoughts ?

Sorry, I thought that you were using more than one of the front speakers, i.e. you had two centre speakers connected. I would have thought that with surround speakers you should have less trouble, because they're mainly used for ambience.

Maybe the best thing to do would be to set up your system and use the test tones on a DVD like Digital Video Essentials. You can use the subwoofer crossover test to check if you've got any problems with phase differences cancelling out the bass. The THX Optimizer also has some speaker phase tests which might be worth trying. Video Essentials has some other tests to check surround performance.

 

[Jack N]

I’ve actually grown rather fond of the all-channel stereo thing. I didn’t care much for it when it was just the receivers’ amp powering things – I think the power supply is too small which causes a somewhat flat dynamic range. But after I added the external amps, it really brought things to life. So consequently, I use full towers in the 4 corner channels. I know most people don’t use the rear channels for music, however because of my listening preferences I pay just as much attention to the rears as I do the fronts.

For calibration, I use a minimum of 3 different test CDs along with a white/pink noise generator. I compare the results of all of them, and use 14 band per channel equalizers (4 stereo eqs each with two 14 band channels per eq – all identical models) to get things balanced out to a good average. It’s really a huge pain to calibrate, so I usually avoid it like the south end of a skunk headed north. That’s one of the reasons why I started the thread. I’m trying to gather opinions about this set-up – to see if it’s really worth all the calibration work and constant tweaking. So please keep the feedback coming !!!

 

[Buckle-meister]

Reality is, if you like it, it doesnt really matter what the "experts" say...

I both agree and disagree. I agree in the sense that nobody can tell another what they like. However I disagree in the sense that experts usually are experts for good reason, and more than likely they'll be able to make changes which will result in a sound the odds of which are that you will like better.

...I’ve discovered some articles and discussions about comb filtering, which has me confused...One side...says that in real world applications comb filtering, if not impossible, is next to impossible to hear...Well, with a substantial amount of money already invested, I’m going to be biased towards the “impossible to hear” side...

The effects of comb filtering may readily be heard just by comparing the sound from having the listening position close to, or worse; hard up against any boundary in a room such as a wall versus having the listening position remote from any walls nearby. It's for exactly this reason that the listening position (and speakers) should always be remote from a room's walls.

Take a listen to the Comb Filtering clip on this page to better appreciate its effect.

 

[tbewick]

For calibration, I use a minimum of 3 different test CDs along with a white/pink noise generator. I compare the results of all of them, and use 14 band per channel equalizers (4 stereo eqs each with two 14 band channels per eq – all identical models) to get things balanced out to a good average. It’s really a huge pain to calibrate, so I usually avoid it like the south end of a skunk headed north. That’s one of the reasons why I started the thread. I’m trying to gather opinions about this set-up – to see if it’s really worth all the calibration work and constant tweaking. So please keep the feedback coming !!!

I'm not really surprised that you get good sound after all that work! From what I've read, the most important thing is that your system sounds good with high quality material after calibration. I'm sure you'll know that the ear can perceive when the direct sound from the speakers is uneven. Accordingly, when adjustments are made using time-invariant equalisers, this can sometimes lead to a reduction in sound quality. To account for this, calibration is ideally done with measuring equipment and equalisers/DSP's that fully take into account the room's impulse response.

 

[Jack N]

Buckle-meister

Great video clip. Thanks for the link. Man, a video sure is a lot more effective at conveying important information. Although I knew walls had an impact on sound, I didn’t realize just how much until I viewed the video. It’s alarming as far as I’m concerned, and will definitely have an impact on the final lay-out.

The guitar simulation was interesting also. My old set-up didn’t sound anything like the phasing that was going on there, but now I’m more aware of what to be listening for in the new set-up. Thanks again for the link.

Tbewick

Unfortunately my equipment is 100% manual which is why it takes so long to calibrate. Also, at present I don’t have any way of measuring the timing. For right now I have to rely on the manually selectable delays in the receiver. It must be at least reasonably accurate as I didn’t readily detect a timing problem in my old set-up.

 

[Buckle-meister]

In my old house, I had a full 8.1 set-up...

What're the 7th/8th speakers? 6.1 with additional front left/rights?

...my new dedicated home theater room...will be approx 15 x 45...Is the big change in room size and layout going to act as a detriment? I’m wondering if you think it’s going to be worth the extra expense and the extra time of basically setting up two separate audio systems in my new home theater. Which side do you adhere to, and why?

The change in room size will, without a doubt have an effect on the sound compared to what you were used to in your previous house though I can't say whether or not it'll be worse. As to the second question, I can understand why you like the sound from doubling up certain (all?) speakers as you're probably perceiving a much bigger, fuller sound filling the room. However, I personally am of the mind that if a system is correctly set up and room treatment and/or parametric EQ and/or additional subs are used, that there'd be no need (and probably no desire) to double up on speakers for any channel (you did ask ).

I’m thinking that with the room being 45 feet long, and the front and rear speakers being so far apart, I can’t help but wonder - just what will it sound like?

You don't have to place the rear surround and surround back channels at the rear wall. There's nothing to stop you placing them further into the room so as to better 'enclose' the listening position. In fact, as you've previously indicated that your rear speakers are towers, they'd automatically lend themselves to this setup layout.

For calibration, I use a minimum of 3 different test CDs along with a white/pink noise generator. I compare the results of all of them, and use 14 band per channel equalizers (4 stereo eqs each with two 14 band channels per eq – all identical models) to get things balanced out to a good average. It’s really a huge pain to calibrate...

I'll bet it is. How do you compare the results?

Thanks for the link.

You're welcome.

 

[Jack N]

How are the channels laid out ? - There are the traditional 3 channels across the front, and the same across the back. There are also 2 “presence” channels, as Yamaha calls them, behind and above the 2 main front channels. They add depth to the front sound stage. Reasonably effective.

The opinion(s) about using a second set of speakers is what I was looking for from this thread. After watching the other videos in the link, returning to a single set may be the way I’ll go. However I think I’ll use the receiver strictly as a pre-amp and retain the external amps. The dynamic range is noticeably better.

Pulling the rear channels in was something that I was already considering. The biggest problem to doing this is where to place the rear center channel without having it physically in the way. I’m not too keen on the idea of having it sitting out in the middle of the room. Dark speakers in a darkened room sound like an accident waiting to happen.

How do I compare ? Strictly on a relational basis. I don’t use any one test CD or noise generator as an absolute because each method yields different volume levels, and no one CD/DVD covers all the frequencies. I look for similarities such as low/high spots. I’m in agreement with the videos that straight tones yield better results. Thus I give more credence to the test tones than to the noise generators. I don’t like to completely ignore them though because they give a more generalized curve. Hope that makes sense.

 

[Buckle-meister]

There are the traditional 3 channels across the front, and the same across the back. There are also 2 “presence” channels...

Ah, ok. Now I can picture your setup. Thanks.

The opinion(s) about using a second set of speakers is what I was looking for from this thread. After watching the other videos in the link, returning to a single set may be the way I’ll go.

That'd be my recommendation.

...no one CD/DVD covers all the frequencies. I look for similarities such as low/high spots...

(looks slyly left and right) There is an easier way.

I use a program called ETF. You wire up a computer's line outputs to your system and take the line out from a microphone, in my case Radio Shack's SPL meter placed at the listening position to the computers line inputs. ETF sends out a full range (20Hz to 20kHz) frequency sweep over 5 (five!) seconds and picks up the impulse response via the microphone. From the impulse response you can obtain all sorts of useful information but of greatest relevance to the above quote would be the frequency response to 1Hz resolution over the 20Hz to 20kHz domain.

If you're interested, a trial version of ETF maybe downloaded here. The trial version doesn't have all the features of the full version, but it'd let you see your room's frequency response. In an untreated, un-EQ'd and un-multiple-sub'd room it'll almost certainly look horrendous, but you'd be much better off for having seen it, as at least then you'd know, in detail, what you'd have to work with.

I’m trying to gather opinions about this set-up – to see if it’s really worth all the calibration work and constant tweaking.

It's absolutely worth taking the time to calibrate your system, time consuming though it may be. This was my (2 channel) low frequency response at the listening position before experimentation with placement of room treatment and more especially my sub. This was the response after.

The frustration of taking measurement after measurement after measurement was, with hindsight, totally worthwhile, as the sound is now spectacular.

 

[Jack N]

Thanks for another good link. I’ve downloaded the trial version (at least I hope it’s the trial version ! ) and will give it a whirl when I get the room set up. I noticed that this program was referred to in one of the videos.

Although it looks like the Real Traps stuff works good, I’m afraid the prices of the panels are just too high. I know they would make a remarkable difference, but with the quantity of them that I’d need for that size room, I just can’t justify the price. I’ll probably have to do some research and figure out how to make some home-made versions. I’d like to incorporate them right into the walls and ceiling for a cleaner look.

 

[Buckle-meister]

I’ve downloaded the trial version (at least I hope it’s the trial version ! ) and will give it a whirl when I get the room set up.

If I can help at all, just ask/send a P.M.

Although it looks like the Real Traps stuff works good, I’m afraid the prices of the panels are just too high...I’ll probably have to do some research and figure out how to make some home-made versions.

Sploo (member) did exactly that. Help maybe found here. Or, there are other manufacturers whose products cost appreciably less than RealTraps. GIK's springs to mind.

I’d like to incorporate them right into the walls and ceiling for a cleaner look.

1. Absorption spaced from a wall absorbs to a greater extent that if it were hard up against the same wall.
2. The lower the frequency to be absorbed, the greater the thickness of absorption/space from a wall (within reason) the absorption needs to be in order to be effective.
3. Absorption placed in corners is primarily used to tackle low frequencies, which for most folk's rooms, is probably the region most in need of sorting out.
4. Absorption placed elsewhere, i.e. against side walls or the ceiling, is primarily used to tackle (by intercepting) higher frequencies being reflected from those surfaces.

 

[Nick250]

Just this past week I installed just one GIK Tri Trap in the only 90 degree corner in my listening room and the improvement is dramatic. 98% of boomyness is gone! Truly astounding. I ordered four of them, one may be all I need. A huge "wow" factor for me on the improved room acoustics.

http://www.gikacoustics.com/product_info.html

Nick

 

[Jack N]

Wow. More good links. Thanks.

$50 - $60 per panel is more like it. I can afford to buy a few of those.

Nick250 - The reviewers weren't too happy with the appearance, especially the overlap. What do you think?

 

[Nick250]

As you can see from GIK link, it's pretty big. It matches the color of my walls so that helps me. I have no problem with the appearance, but it's big so it's noticeable from a size perspective, but not unduly so IMO. I don't know what your are referring to in regards to overlap.

Nick

 

[Jack N]

I've never tried inserting a short cut, so I hope this works. I copied it from a previous post.

http://www.audioholics.com/productreviews/avhardware/GIKacoustics244panels.php

Well, I guess that doesn't work. At any rate, if you go to that page there's a GIK review there where they mention the overlap. Actually, I guess they call it a seam.

Thanks for the feedback too. Judging by your response, the seam isn't all that noticable.

 

[Buckle-meister]

Judging by your response, the seam isn't all that noticable.

You'll never notice the seam because you'll never want to leave your listening position.

 

[Jack N]

LOL ! Good point !