Comb Filtering Confusion - Home Theater Forum and Systems - HomeTheaterShack.com

Reply
 
Thread Tools
post #1 of 32 Old 10-06-06, 09:32 PM Thread Starter
Shackster
Jack N
 
Join Date: Oct 2006
Location: Minnesota
Posts: 97
Comb Filtering Confusion

I was lead to this site for the first time less than an hour ago (following a download lead). It said I was free to download what ever I want for free (money wise), but the fee was to do some posting at this site. No problem. I like free ! So here goes. I started a thread at another forum yesterday but have only gotten one response so far so I thought I'd post it here and see what happens.

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 ? Is the only reason why it sounded good was because I was pouring a large amount of power into a confined space ? 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 ?
Jack N is offline  
Sponsored Links
Advertisement
 
post #2 of 32 Old 10-06-06, 11:14 PM
Elite Shackster
 
Sonnie's Avatar
 
Join Date: Apr 2006
Location: L.A. (Lower Alabama)
Posts: 22,577
My System
Re: Comb Filtering Confusion

Hello Dain and welcome to the Shack!

Quote:
It said I was free to download what ever I want for free (money wise), but the fee was to do some posting at this site.
Who told you this? We charge 100 bucks for all our downloads. No, really, just kidding. While we don't require posting for the downloads we do encourage it. After all, posting is what helps us keep the site going.

I think we've got some guys who understand your issues and can address them.
Sonnie is offline  
post #3 of 32 Old 10-06-06, 11:32 PM
Senior Shackster
Dan
 
Join Date: May 2006
Location: Australia
Posts: 292
Send a message via MSN to Danny
Re: Comb Filtering Confusion

Comb Filters occur when an identical sound emerges from 2 (or more) loudspeakers in different positions. It causes cancellation of some frequency's while amplifing others. To hear the difference wire two speakers in Mono and carefully align them so that they are in the same position (see picture below), then while you listen get a friend to move one backwards so that they are no longer aligned, you should hear a difference in sound quality. Comb filters are pretty much inevitable in any sound system however their effects can be minimised by careful placement of the speakers
Attached Thumbnails
Comb Filtering Confusion-comb-filters.jpg  

Danny is offline  
 
post #4 of 32 Old 10-08-06, 10:06 PM Thread Starter
Shackster
Jack N
 
Join Date: Oct 2006
Location: Minnesota
Posts: 97
Re: Comb Filtering Confusion

Good practical suggestion. I’ll definitely try that when I get everything set up.

Well, between the two forums it sounds 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 ?
Jack N is offline  
post #5 of 32 Old 10-09-06, 12:34 PM
HTS Senior Moderator
 
Wayne A. Pflughaupt's Avatar
 
Join Date: Apr 2006
Location: Corpus Christi, Texas
Posts: 9,177
Send a message via Yahoo to Wayne A. Pflughaupt
Re: Comb Filtering Confusion

Hi Dain,

You might want to check the last two posts at this thread (by Mitch and John). It’s more than just non-time-aligned speakers. John says that room reflections cause comb filtering, too. So, essentially there is no escaping it.

Even without the room, comb filtering is probably inevitable with any multi-channel system. Even if you minimize it by careful placement, anyone in your HT sitting outside the “sweet spot” will be “affected.” Not that that’s all bad. I’m sure you’ve sat outside the sweet spot in your system at one time or another. How did it sound? Aside from the obvious imaging penalty, I’ll bet it sounded just fine.

I recall experimenting with this a little back in the 80s, and I remember being pleased with the results as well. I’m guessing that the perceived improvement is caused by the slight delay, which is probably adding a sense of “spaciousness.” However, I wasn’t too enamoured with the prospects of having so many big speakers in my living room – which back then only meant two stereo pairs.

You might want to look into Yamaha’s Digital Soundfield Processing. It comes standard with most of their receivers. The processing mimics actual acoustical venues and spaces, so you can get the spaciousness without all the extra hardware and speakers you’re using now. You’ll want one of their upper-line models that utilize a second set of front speakers, which greatly enhances the effect.

Regards,
Wayne
Wayne A. Pflughaupt is offline  
post #6 of 32 Old 10-09-06, 09:39 PM
Ilpo
Inactive
 
Posts: n/a
Re: Comb Filtering Confusion

Comb filtering is a common term to a measurable amplitude performance. Two identical signals summed with delay result in cancellation at frequencies where the delay is an odd multiple of half wavelengths. Even multiples add constructively and result in 6 dB addition of amplitude. Resulting amplitude response has deep dips, hence it visually resembles a comb. Audibility depends on many issues, frequency, delay etc. Ear does not integrate the two signals in a same way as microphone and measuring gear, but tends to localize the sound to the closest source.
The presented issue of having speaker pair for a single signal will mean that the directivity of each channel's combined speaker increases at all but lowest frequencies. Listening even slightly off axis of the pair (=channel) will have audible effect due to the directivity changes, and even more so if the speakers are not identical. So, although the results may be "impressive", they will not be truthful to the original signal.
post #7 of 32 Old 10-10-06, 01:00 AM Thread Starter
Shackster
Jack N
 
Join Date: Oct 2006
Location: Minnesota
Posts: 97
Re: Comb Filtering Confusion

Ilpo,

"will mean that the directivity of each channel's combined speaker increases at all but"....

I haven't heard that before. Food for thought to be sure. I'd be genuinely interested in seeing a graphic representation that would reinforce that statement, if you feel like supplying one.

Wayne,

I hadn't given much thought to the room adding to the effect, but it does make sense if you think about it. You're right in that every system out there suffers from it to a certain degree. It's just a matter of to what extent, and to what extent each person does to try and control it.
Jack N is offline  
post #8 of 32 Old 10-10-06, 06:42 AM
Ilpo
Inactive
 
Posts: n/a
Re: Comb Filtering Confusion

About directivity; imagine you have two radiators for the same signal. Given that the radiators are identical, on the axis of the pair the radiated signals are in phase. Move your observation point sufficiently off axis so that the distance to the radiators is no more the same. At some frequency the path length difference is half wavelength and the signals cancel. You have first null in your radiation pattern. At very low frequencies the wavelength is so long that a small distance between radiators means practically nothing in this respect. (Example 100 Hz, 3,4 m wavelength, speakers are 0,5 m apart, at 90 degrees off axis the difference is still 0,5 m which is about 1/7 of wavelength.) Imagine you have only one speaker and do the same thinking. As the size of your radiator now is the size of your drive unit, this phenomenon does not happen (or to be exact, it happens but at much higher frequency, and then we are observing radiation from the opposite edges of the drive unit).
What if your speakers are not identical? The differences in the phase response mean that even on axis of the pair the signals are not in phase and the main lobe of the radiation pattern will depend on frequency.
So, you will hear interesting responses.
You will find the graphics on any acoustics textbook.

BR,

Ilpo
post #9 of 32 Old 10-10-06, 02:05 PM
Senior Shackster
 
Join Date: Jun 2006
Posts: 365
Re: Comb Filtering Confusion

Quote:
Ilpo wrote: View Post
About directivity; imagine you have two radiators for the same signal. Given that the radiators are identical, on the axis of the pair the radiated signals are in phase. Move your observation point sufficiently off axis so that the distance to the radiators is no more the same. At some frequency the path length difference is half wavelength and the signals cancel. You have first null in your radiation pattern. At very low frequencies the wavelength is so long that a small distance between radiators means practically nothing in this respect. (Example 100 Hz, 3,4 m wavelength, speakers are 0,5 m apart, at 90 degrees off axis the difference is still 0,5 m which is about 1/7 of wavelength.) Imagine you have only one speaker and do the same thinking. As the size of your radiator now is the size of your drive unit, this phenomenon does not happen (or to be exact, it happens but at much higher frequency, and then we are observing radiation from the opposite edges of the drive unit).
What if your speakers are not identical? The differences in the phase response mean that even on axis of the pair the signals are not in phase and the main lobe of the radiation pattern will depend on frequency.
So, you will hear interesting responses.
You will find the graphics on any acoustics textbook.

BR,

Ilpo
This is correct. I however would not use two identical speakers for a single channel because the sound will not be even over a number of seats. I certainly would not use two speakers for the center channel where dialog intelligibility is of the upmost importance. Comb filtering does add diffusion or the sense of spaciousness, but this works at the peril of clarity. You would be far better off just using a single speaker per channel IMO
Sir Terrence is offline  
post #10 of 32 Old 10-10-06, 09:56 PM
Ilpo
Inactive
 
Posts: n/a
Re: Comb Filtering Confusion

Well, this is the only realistic conclusion. I just tried to explain the reasons why only one speaker per channel should be used.

Ilpo
Reply

Bookmarks

Tags
comb , confusion , filtering

Quick Reply
Message:
Options

Register Now




PLEASE COMPLETE ALL REQUIRED FIELDS BELOW... THANKS!

REQUIRED FIELDS ON THIS PAGE
YOU MUST COMPLETE ALL OF THESE

Username
Password
Confirm Password
Email Address
Confirm Email Address
Random Question
Random Question #2




User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
PLEASE READ BELOW PRIOR TO ENTERING AN EMAIL ADDRESS!

ATTENTION!

YOU MUST ACTIVATE YOUR ACCOUNT!

Activation requires you reply to an email we will send you after you register... if you do not reply to this email, you will not be able to view certain areas of the forum or certain images... nor will you be able download software.

AN INVALID EMAIL ADDRESS WILL CAUSE YOUR ACCOUNT TO BE DELETED!

See our banned email list here: Banned Email List

We DO NOT respond to spamcop, boxtrapper and spamblocker emails... please add @hometheatershack DOT com to your whitelist prior to registering or you will get nowhere on your registration.


Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML is not allowed!
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 


For the best viewing experience please update your browser to Google Chrome