Do you really need 2 back(middle)?

Discussion in 'AV Receivers & Amplifiers' started by Gadjet, Apr 21, 2002.

  1. Gadjet

    Gadjet
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    I am uncertain to which amp to buy, the yam az1, the pioneer VSA Ax10, or the new denon AVC- AISR. Obviously one of the main priorities being the middle back speakers for both ex and es. But with my room only being 13" by 12" will this make the sound similar to prologic with the speakers being so close, and surely with 2 middle even more so ( yam is only one middle speaker, important point?). Having the sound all accross your back might be great for some, but surely to others who might enjoy more the clarity of "5" speakers around your room. The ex & es might take that effect away. Does this happen? Each of the above amps has a quality the other does,nt. I just can,t decide. A final point being that P2 being another reason to upgrade as most vewing being P1, which one will have the best. I live in Wrexham, where is the nearest home cinema store to hear the above amps?
    I hope I get replies to help, and if you do thank you for your help and time,
    Phill:confused:
     
  2. buns

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    I cant answer much, but here is my bit!

    Quite what the official stance is i dont know, but thx ultra2 states to have the 2 centre rears side by side. I have to say that seems more than a bit pointless.....why cant we just use 1 speaker? Whether that is actually possible and still get the benefits of discrete dts and such i dont know, but im sure someone else will enlighten us!

    I dont understand why the size of room and number of speakers will make the sound pro logic like.

    You also imply you wish to be able to locate each speaker within the listening environment and do not wish a 'wall of sound' from behind. Maybe i interpret incorrectly, but you do want to be immersed from al directions, thats the whole point! You will still get the proper surround effect!

    im sure someone else will expand upon this!

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  3. Stuart M. Robinson

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    Gadjet,

    In Wrexham, try Acton Gate Audio (say “Hi” to Mike for me), they’ll have the Yamaha and offer great service in unusual surroundings.

    Two surround back loudspeakers are recommended for all systems, and they shouldn’t be side-by-side, as Buns says, that would be pointless.

    See: http://www.smr-group.co.uk/articles/loudspeakers1.html


    Stuart M. Robinson
    SMR Group – http://www.smr-group.co.uk/
     
  4. stranger

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    stuart, I would appreciate your comments on a reply from uncle eric to spectre regarding back surround speakers. according to this, thx ultra 2 stipulates 1 back centre or if using two, to put them side by side. the post was-speakers and subwoofers- 03/03/02. you seem to be completely at odds and I would like to have my setup correct. thanks.
     
  5. Stuart M. Robinson

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    Stranger,

    I can’t find the thread you mentioned (the title or a link would help) but regardless, there are countless acoustic studies that detail the unsuitability of a single, centrally mounted surround back loudspeaker. Without seeing the thread in question, I don’t know whether the use of two surround back loudspeakers side-by-side means actually adjacent to one another, or just on the same axis but separated by some distance.

    Although you don’t see it mentioned much, there is actually a widely accepted standard for multi-channel loudspeaker placement… and by that I mean accepted by industry professionals, acoustic engineers, scientists, broadcasting institutions, mixing stages, (most) multi-channel music producers etc. It’s called the ITU-R BS 775-1 standard and it’s based on years of research. Microphone placement is often governed by the same work.

    The ITU standard states that the front left and right channels should be ±30° from the axis of the centre channel (a slightly narrower angle also works well), and that the surrounds should be between ±110° and ±150°. Any further back – closer to 180°, which would be directly opposite the centre – and our perception of space and direction diminish greatly, we suffer front/back reversals (our brains are confused as to the direction of the sound) and envelopment is also compromised.

    In an EX system, where there are seven loudspeakers, the surround back pair are the true surrounds, so they should be no further back than ±150°. The regular surrounds are then positioned midway between the surround back pair and the front L/R pair. If they’re dipoles, then they should be directly adjacent to the listening position.

    I can find some AES pre-print numbers if you’re interested. Or try a Google search for ITU standards.


    Stuart M. Robinson
    SMR Group – http://www.smr-group.co.uk/
     
  6. Phil Hinton

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    I would also welcome YOUR thoughts stuart towards the latest line of thinking as I happen to agree with Uncle eric and THX regarding having both EX/ES speakers placed directly behind the listening position and next to eachother, Infact I have set up for this and it offers the most natural sounding placement. I really do respect your opinion so would welcome your thoughts, and have you tried it?
     
  7. Electric Mayhem

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    I have both my surround back speakers as close together as possible (as per THX Ultra 2 specs) and of ALL the combinations tried, this gives me the best results. I´ve tried one single rear centre, two rears, and various combinations of monopole, quadrapole.

    Finally settled on two monopole speakers, close together side by side, angled down towards the listening position. I have to admit that a single rear centre came a close runner up.

    Presumably, you´ll get different results for other combos in different rooms.

    As this setup seems to suit me & my system, I´m not likely to change it (knowing I´ve exhausted all the other posssibilities!!).
     
  8. stranger

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    stuart, sorry I forgot to type all the information in, look at subwoofers and speakers, thread is-rear speaker position question- posted by spectre on 03/03/02 page 8 of thread, thanks for your prompt reply.
     
  9. buns

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    If a single center were so ridiculous, why do we use single centers at the front?

    Logic would dictate that if we are to have a center rear channel or channels, they should in fact be behind as everyone is suggesting!

    The point I was actually making was that why have 2 speakers side by side? Can a single speaker not do the same job and potentially negate potential interference effects resulting from ovelap of the 2 side by side rear centers?

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  10. Pzycho

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    In a Dolby Digital EX or DTS ES system where two sets of surround 'speakers are needed, the second pair should be positioned about 150º from the axis of the front left/right loudspeakers (the centre is considered 0º while directly to the rear of the room is 180º). If dipoles are used their relative phase should be reversed and the null pointed directly forwards into the room.

    The use of a single, centrally positioned surround back loudspeaker is not recommended, psychoacoustic research shows that when sounds originate from more than 150º degrees (i.e. from directly behind you) that the brain confuses those sounds with those that originate from directly in front. As under no circumstances should the surround channels be distracting, this is the primary reason why the original THX specifications for Surround EX called for the use of two surround back loudspeakers. Even if the receiver, amplifier or processor you choose only has one surround back output, it's greatly beneficial to split that signal into two, either by way of a Y-splitter or by wiring two loudspeakers in parallel.

    Thanks to Stuart for the link.
     
  11. stranger

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    thx have apperently changed their minds and now recomend one back centre or two right next to each other. its not been made public because lots of people have bought amps with a7th chanel of amplification they dont need.
     
  12. Stuart M. Robinson

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    Guys,

    There are lots of interesting points here, so apologies in advance if I miss something.

    Buns asks: “If a single center were so ridiculous, why do we use single centers at the front?”

    The way we hear sounds from the front and rear differs. Our ear cavities point straight out to the sides of our heads, we judge direction according to the relative amplitude and arrival time at each ear (interaural time delay). What neither of these functions can do however, is differentiate between sounds directly in front and directly behind us. For that we need to judge the changes of dispersion brought about by the shape of our outer ear. What this means is that we hear sounds differently from the front compared to sounds from behind; i.e. our hearing is asymmetric.

    As has been demonstrated in countless studies, we can precisely attribute direction to sounds directly in front of us – not surprising given our forward-facing eyes and predatory nature – but not from directly behind us. As I mentioned before, the brain can actually confuse sounds coming from directly behind as coming from somewhere in front.

    Stranger writes: “thx have apperently changed their minds and now recomend one back centre or two right next to each other.”

    THX have a recent history of ‘dumbing down’ their product specifications, largely to increase licensing revenue. Dolby and THX spent considerable time developing EX and the original loudspeaker arrangement and I see no reason, technically speaking, for their recommended layout to change. In cinemas, the surround back loudspeaker array stretches from each rear corner of the auditorium inwards, so listeners never hear sounds from directly behind them.

    In the home, there are lots of reasons why a single, surround back channel is not the best idea, many of which I’ve already stated. One thing I forgot to mention however, was that a single centre is particularly unsuitable where listeners are seated in different locations within the room, in which case all those seated off-centre will hear the surround back channel over one shoulder. And let’s not forget that a single surround back channel only really came into being because manufacturers such as Yamaha were trying to avoid paying THX EX license fees.

    Phil wrote: “I would also welcome YOUR thoughts stuart towards the latest line of thinking as I happen to agree with Uncle eric and THX regarding having both EX/ES speakers placed directly behind the listening position and next to each other…”

    This is one reason why I’m not a fan of the THX Ultra2 specifications. There is proprietary processing added to each of those speakers in the Ultra2 modes, but THX appear to have overlooked all the issues involved when two loudspeakers producing the same or similar signals are placed alongside one another.

    Folks have to realise that even a pair of expensive loudspeakers – of the same model – can produce very different responses and that inexpensive loudspeakers (the type folks are likely to plonk in the rear of the room) often vary by three, four or even five dB in some areas of the spectrum. Even if the two loudspeakers are a perfect match, when in use they will interact with one another when placed side-by-side. Some frequencies will be boosted, others attenuated, so you end up with an uneven response that is listener-position dependent. The wider the variations between the two loudspeakers, the worse the problem will be. Low frequencies are not affected as much as mid- and high frequencies, but since most of us will cross over at 80Hz (especially in a THX system) the point is only mentioned for the sake of accuracy. All loudspeakers have a lobed dispersion pattern, and when two lobes interact you end up with comb filtering… which is not the same thing as in a TV, it’s where notches are created in the frequency response due to interactions with another similar or identical sound from an adjacent driver.

    So what I’m saying here, is that in my opinion two surround back loudspeakers side-by-side, even with the THX post-processing, is a worse idea than a single surround back loudspeaker. Tom Holman recently went on record as saying that a single, surround back dipole (null facing forward) was preferable.

    And yes, I’ve experimented a great deal with different EX loudspeaker set-ups. Folks may not remember but I was the first to write about EX in the American magazine ‘Audio’ and for that piece carried out extensive research and listening tests with the help of both Dolby and THX (which makes it even harder to reconcile the sudden change of position). I then tested the first EX consumer hardware and of course have been working with various loudspeaker arrays for hardware reviews ever since. My own room is set up according to the original THX layout and the ITU-R guidelines, although the front L/R pair is a tad closer together.

    Of course, what works for you is what matters, all of the above are just recommendations based upon known acoustic principals…


    Stuart M. Robinson
    SMR Group – http://www.smr-group.co.uk/
     
  13. Pzycho

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    Demoing a system today with a single rear center makes me doubt the 'old' reasoning for having two.

    Watched, amongst others, the THX intro on Toy Story (2?) and the soundfield was definitely a LOT better/bigger with the single rear - compared to none at all - and one could distinctly make out that the sound was coming from directly behind you rather than 'somewhere' in the room.

    Also watching clips from JP3 there were rattles from behind that made me turn around to see who was there.

    I'm going for a single center on my system - dont see the point of spending another £300 for something which _may_ improve something I am already more than happy with.
     
  14. stranger

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    thanks for that stuart, at the moment I have one bipole as back centre, a mission 77ds to match my rears. the null points centre to the room which is 12 x12 x9.5 feet, this seems to be a good set up asthe drivers both fire towards a rear speaker, any thoughts?
     
  15. Kane D Williams

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    Stuart

    I am dubious about paying for the upgrade to my Denon AVC A1SE as I worried that the THX Ultra 2 specs will force me to alter my speaker arrangement? I don't use THX speakers anyway, just 6 identical direct radiating floorstanders and matching centre (and THX Sub). I have no problem changing the speaker positions if it will improve the sound, but it seems like the new THX standards are contradicting their previous recommendations and could in fact be creating a lesser system? Also, I am worried that the new DACs will alter the sound of the A1Se to the worse also, what are your opinions?

    Cheers Kane
     
  16. Nic Rhodes

    Nic Rhodes
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    I'm a two speaker kind of guy.;) Doesn't work with one IMHO
     
  17. Stuart M. Robinson

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    Kane,

    “I am dubious about paying for the upgrade to my Denon AVC A1SE as I worried that the THX Ultra 2 specs will force me to alter my speaker arrangement?”

    You can ignore the suggested Ultra2 loudspeaker set-up if you like, Ultra2 hardware has controls that allow for more traditional, recognised rear loudspeaker arrangements, just as you can disable the surround back channels completely if you wish.

    “I have no problem changing the speaker positions if it will improve the sound, but it seems like the new THX standards are contradicting their previous recommendations and could in fact be creating a lesser system?”

    I don’t like the Ultra2 layout, so in my mind it’s a step back from what you already have, especially if what you’ve got now sounds good. If it ain’t broke, don’t fix it.

    Interestingly, I’ve just heard that Dr. David Griesinger, who developed the first consumer seven-channel formats and continues to refine them is “very perplexed” by the new Ultra2 loudspeaker placement recommendations. All of his research into our acoustic perception “…has concluded that this [side-by-side, in the rear of the room] is the worst place to put them.” Griesinger has a great many papers on the subject of surround reproduction on his web site: http://www.world.std.com/~griesngr/


    “Also, I am worried that the new DACs will alter the sound of the A1Se to the worse also, what are your opinions?”

    Unfortunately I can’t help you with that one, I’ve not heard an upgraded Denon. I’d guess that most folks will say their unit sounds much better after the upgrade, even if it doesn’t sound any different! Either way, the sound of the DACs themselves only has a small bearing on the actual sound of a unit, since creating a DAC with linear response is no longer rocket science. The analogue electronics after the DAC have more impact, and if I understand the upgrade correctly those will not be replaced.


    Stuart M. Robinson
    SMR Group – http://www.smr-group.co.uk/
     
  18. Kane D Williams

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    Stuart

    Thanks for the helpful reply.

    Are you then saying that DAC's aside, if I do upgrade my A1SE, I can leave my surrounds and back surrounds where they are (in standard config) and the system will sound the same with THX post processing engaged as it does now?

    What I am asking is, if Ultra2 has been designed to place 2 back surround speakers right next to each other, won't THX have had to implement additional processing to attempt to curb all those strange acoustic problems that this configuration creates, that were not part of the Ultra(1) processing? And if so, if I then leave my back surrounds where they are, the post processing will be all wrong for this positioning? THX's website has no info on Ultra2!

    I think that having two back speakers side by side is silly, but if I go for Ultra2 I may have to go this route just so it sounds "right"!? I too think this is a backward step and bet Ultra(1) is probably a better system? Unfortunately, all future THX amps/receivers will be Ultra2 (until Ultra3 of course)!

    Cheers Kane
     
  19. Stuart M. Robinson

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    Kane,

    “Are you then saying that DAC's aside, if I do upgrade my A1SE, I can leave my surrounds and back surrounds where they are (in standard config) and the system will sound the same with THX post processing engaged as it does now?”

    Yes. Leave your loudspeakers where they are, you can use the system in exactly the same way.

    “What I am asking is, if Ultra2 has been designed to place 2 back surround speakers right next to each other, won't THX have had to implement additional processing to attempt to curb all those strange acoustic problems that this configuration creates, that were not part of the Ultra(1) processing?”

    Ultra2 has some additional processing, mainly to synthesize two additional channels from regular (ie. no EX or ES) material. There is nothing they can do to compensate for the acoustic problems of adjacent loudspeaker placement.

    “I think that having two back speakers side by side is silly, but if I go for Ultra2 I may have to go this route just so it sounds "right"!?”

    Nope, just as you don’t have to use THX loudspeakers with a THX amplifier. The introduction of THX Ultra2 hasn’t suddenly re-written the laws of acoustics or movie reproduction. You’ll be absolutely fine with your loudspeakers exactly where they are.


    Stuart M. Robinson
    SMR Group – http://www.smr-group.co.uk/
     
  20. Couch Potato

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    Stuart,

    I posted a similar question a while ago after querying what WSR had done with their "Reference Holosonic Theatre" with an apparent unlimited budget and access to guys like Russ Herschelman, could you comment on the thinking highlighted by Gary Reber, Editor in Chief. I like many others are unsure whether to go for 1 or 2 rear channels and both arguements have their points, I dont know know about some of my fellow forum members but I would find it very dificult to play around in my own environment with 1 or 2 rears let alone the cost associated with it, I've heard both 1 and 2 rear channels in a variety of setups and both solutions do appear to work.

    With you depth of experience I would appreciate your comments on this thread

    Hope you can help.

    Steve
     
  21. buns

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    Coming from a more psychological point, I would say that given practice, the barain may well be able to learn what is coming from behind.....but then again its not a very logical way to deal with the problem!

    I think however, this point slightly loses view of the aim! Are we not trying to reproduce a surround environment, one that mimics the real world surround environment? I dont know of many pressure waves which redirect so that they arent coming from directly behind!

    Without analysing the problem properly, my gut feeling is that the difficulty we have is that the sound tracks and processing are not capable of dealing with a true surround arena, hence a single center rear plays havoc with our perceptions.

    I would say that sometimes a single rear may work better than 2, but it is probably very sensitive to room resonances, frequency responses and mainly where you sit! In my case, I dont sit in the center of my room anyhow, so any center rear will never be behind me!

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