# How do you handle the surrounds with 2 seating rows?



## jpeter1093

I have a longish room; planning a 106" diagonal 16:9 acoustically transparent screen. Fronts and sub will be behind it.

First row will be about 12 feet from the screen and the second row 19 feet from the screen. The back wall of the room is 21 feet from the screen.

I plan on mounting 2 speakers at the 12 foot location and 2 more at the 21 foot location. Would I be better off sending the same surround signals to both pair? 2 right rears and 2 left rears (similar to a commercial theater) or send the surround signal to the 12 foot speakers ONLY and a back rear signal to the pair at the 21 foot pair (behind the second row). I'm just thinking if I only send the surround signal to the 12 foot pair, the second row, being 7 feet further back, wouldn't be able to enjoy the 12 foot pair.

I ended up trying a 7.1 setup with the surrounds coming out of the 'middle' row speakers and the back coming out of the back wall row speakers. I then switched out the middle row of speakers and used the back wall row only speakers with a 5.1 setup. I ran MCACC both times and watched a number of test scenes; both movie and music/concert videos.

I thought having the surrounds in front of me (I sat in the couch against the back wall) and the back speakers on either side of me caused a 'muddier' sound in 7.1 than the 5.1 setting without the middle speakers and the back speakers acting as the normal rear surrounds. It also sounded ok from that middle row.

So, a new question. I'm going to leave the back row speakers on the side walls near the back wall. Does it make sense to mount true back speakers even though they'll only be a couple of feet (and on the back rather than the side wall) from the surrounds? Or should I just bite the bullet and leave the 5.1 setup? Opinions, please


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## bpape

I would still do the rears on the rear wall. This will give the best sound IMO for the primary front row seating. Not going to get any rear surround field for the back row that close to the wall anyway for the most part regardless of where you put them so you might as well optimize for the front row.

Bryan


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## Quijibo

this is sounding very very similar to my room (I'm still doing some Demo, and finalizing decisions on speaker placement.
how wide is your room?


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## spartanstew

My first row is 12.5' from a 126" screen and my second row is 18.5'.

My rears are on the back wall and my sides at are about the 14' mark. 

Sounds great.


PS. I am using Axiom (quad-poles) speakers.


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## igl007

Quijibo said:


> this is sounding very very similar to my room (I'm still doing some Demo, and finalizing decisions on speaker placement.
> how wide is your room?


My room is 21' long as well. Im curious as to the width too. I am still building everything, but was thinking about going just 5.1 with the surround speakers on the back wall. It just seemed weird to have have side speakers almost right next to the 1st row. Then like someone said, you have surrounds in front of you for the 2nd row. I know in a normal large theater the speakers are on the side and on the back wall, but in small theaters (21'x12'x7 in my case) does it still make sense? I don't really want to optimize for just 1 row.


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## Owen Bartley

The space I'm planning to put my theatre in is a bit smaller than yours I think, and I always thought I'd do a set of side surrounds for each row, and the rear surrounds on the back wall. I figured that the levels on the side surrounds would be a bit lower, since they were so close to the seating, so it wouldn't interfere too much.


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## SAC

“I don't really want to optimize for just 1 row.”

“I always thought I'd do a set of side surrounds for each row, and the rear surrounds on the back wall. I figured that the levels on the side surrounds would be a bit lower, since they were so close to the seating, so it wouldn't interfere too much.”

Ah, were things so simple.

You have two choices.

You wither address the entire space in an ‘averaged’ manner, meaning all locations will be slightly compromised in exchange for an attempt to increase coverage, or you optimize for a localized listening region and accept the fact that periphery locations will be slightly less optimal.

Spaced sources will be near the same gain, and as the spatial dispersion will overlap, the result (due to the combining of signals called superposition) will be polar lobing (that manifests itself in frequency response measurements as comb filtering). 

This destructive behavior will be far more detrimental than simply using one pair of speakers to cover the entire region!

At the risk of sounding confusing if one is not already aware of this fact: one must evaluate the arrival of signals in the time domain with is causal. Address issues here and the derivative frequency response issues resulting from them will be resolved.


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## Owen Bartley

Well, at least that would mean I won't need 2 extra channels of amplification. I would try to optimize for the front row, while minimizing the negative effects on the back row... I wonder how bad it would be with the surrounds slightly in front of the seating position. 

Probably not too bad, but I wonder about the tradeoff. SAC, would the comb filtering and other negative effects of having 2 pairs of side surrounds really be that noticeable with the amount of content in those channels? There usually isn't that much in the surrounds, and it won't be full range. Would the amount of interference really outweigh the benefits of having the speakers in a more optimal position?

I guess the only way to know for sure is to plan for 2 pairs, and just disconnect one if they don't sound good.


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## SAC

Owen Bartley said:


> I wonder how bad it would be with the surrounds slightly in front of the seating position.
> 
> Probably not too bad, but I wonder about the tradeoff. SAC, would the comb filtering and other negative effects of having 2 pairs of side surrounds really be that noticeable with the amount of content in those channels? There usually isn't that much in the surrounds, and it won't be full range. Would the amount of interference really outweigh the benefits of having the speakers in a more optimal position?



First as to the destructive effects.

Yes, the resultant polar lobing consisting of lobes and nulls are noticeable!! And their gain is identical to the source signal, as that is exactly what is being attenuated!! It is not like some alien sound source is going to appear. Thus what you will have do to the 2 spaced sources is regions where energy at some frequencies will effectively be 'canceled' (gone), and other frequencies will be twice as loud, and others will be modulated somewhere in between at levels different than the source. Such is the fundamental of signal combination formally referred to as "superposition". The effect is not unlike someone having gone berserk with an EQ. And yes, the effects are not only noticeable, but destructive. (note, it is this destructive spatial lobing that appears as the pattern called 'comb filtering' in a frequency response.)

There is NO benefit to creating the exact type of effect that we are so busy trying to mitigate and avoid in other parts of the listening space! This basic bit of physics is not selectively good if intentionally created in one part of the room while being destructive in another! This behavior is to be eliminated to the greatest extent possible everywhere! And it makes absolutely no sense to intentionally introduce it!

As to how this can be minimized... On can position the surround speakers back sufficiently such that the greatest amount of seating is covered 'properly'. Then one will adjust the signal delay relative to the 'optimal seats'. The signal delay will be slightly less optimal for the other seats (depending of course upon their distance of separation, considering the rate of sound being ~1.13ms/foot. There is no reason to add additional speakers effectively as at best you can only optimize one precise point via signal alignment (delay) while creating substantial destructive interference over the rest of the speaker dispersion region!

If one wishes to pursue blame in this configuration, blame the surround format. (There are other 'surround' formats that avoid this (and no, in anticipation of the question, they are not possible to implement given the encoding in use), but they are far beyond the scope of this thread.) This is a fundamental limitation of using discrete separate sources for laterally arriving signals.


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## Owen Bartley

Thanks SAC. That is good to know. I'm just getting up to the reverberation chapter in the Master Handbook of Acoustics, but I won't lie, there's a good amount of the preceding content that confuses the out of me.

Do commercial theatres just accept that they will be introducing the above negative effects for some seats by using multiple surrounds, or do they use more advanced processing to get around it?


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## SAC

Theaters deal with problems associated with large acoustical spaces, whose acoustical rules differ from those of small acoustical spaces.

One of the most significant differences that define and distinguish the two spaces from one another is reverberation. Meaning, in one space it exists and is an issue, and in the other space it does NOT exist to any significant degree!

In a small acoustical space, you do NOT have a significant reverberant space. So, to make this issue simple for you, FORGET RTxx calculations in a small acoustical space! :unbelievable:

A small space does *NOT* have a significant statistically diffuse well mixed reverberant field.:huh: 
Instead a small acoustical space is dominated by room modes below the Schroeder frequency (a region whereby the wavelengths are larger than the room dimensions) and specular reflections that can be resolved into both direction, gain and arrival time - exactly the opposite of a reverberant sound field where said energy cannot be resolved into direction, as by definition, in a statistically diffuse reverberant soundfield the energy is arriving equally from all directions simultaneously! And all such RT calculations are predicated upon this definition.

(Also, to go just a bit more into this important subject, the only reverberant sound field that will exist in a small acoustical space will be at frequencies that are far to high in frequency for us to care about - meaning that the wavelengths are so short with such low energy content that they can be stopped by almost any object in the space. And a reverberant sound field is NOT like most imagine it to be! Most think of it as if it the electronic FX that we have all grown up with labeled "reverb" on an amplifier! This is NOT how reverberant soundfields behave! A reverberant soundfield in a large acoustical space manifests itself as the residual noise floor that may increase sufficiently such that it may destructively mask low level direct signals! Thus i hope this gives one a Quick idea of its difference.

For more information on this, PLEASE refer to _Sound System Engineering_ by Davis and Davis (2nd ed.) or Davis and Patronis (3rd. ed.) and review the chapters o Large Room Acoustics and Small Room Acoustics. This distinction is *critical* to understand, as far too many fall into the trap of using the larger more general rules that apply ti Large Acoustical Spaces to Small Acoustical Spaces. And to make it really simple, in a small acoustical space, forget RT calculations (as they are a derived calculation) and focus instead on the ETC response and on frequency specific resonances viewable via wither the waterfall of cumulative spectral decay.


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## Owen Bartley

Duly noted. That must be one of the biggest illusions of the whole home theatre culture... most of us at some point get it into our heads that the goal is to recreate the atmosphere and methods of a large movie theatre (whether knowingly or not). As it turns out, trying to miniaturize a commercial theatre and put it in your house doesn't really work, and some of the concepts and implementations of a commercial theatre will actually detract from the overall result when employed at home.

Sigh. Oh well, hopefully this is the kind of knowledge that can make our theatres just a little bit better and a little bit more special in the end.


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## mozez

Um ok I have a similar problem, my room will be 22.4x12.2x8.0' (LxWH) so if I decide 5.1 with 2 rows what's the best placement?


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## Owen Bartley

mozez, I think that with 5.1 you may be best just using one pair of surrounds, at least according to the discussion here so far. In a way, its actually easier with 5.1 because you can put the surrounds behind the back row, and still have them coming from the right direction in the front row. But it really depends on a few things. Room layout? Where do you have room to place them? ...Personal Preference? Do you like the surrounds to be behind or beside you? It kind of depends what you like and what you have available for placement. And speaker type as well, if you use bipole/dipole speakers you will probably want them positioned differently than monopole. If you have directional tweeters or mounts that allow you to aim the whole speaker, you may gain some additional flexibility in placement.


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## Todd Anderson

Have you thought about using bipole/dipole speakers for your side surrounds and then directional speakers for your rear surrounds?

Seems like the bipole/dipole as side surrounds would help you stretch the sound field on your side walls and help you spread out the side surround effect.

Anyhow, just my 2 cents!


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