[josh358]

Dipole line sources don't interact the same way with modes as monopoles. Some of the differences:

- They're directional, so if they're parallel to the front wall and ceiling they don't excite lateral and vertical modes. If they're toed in, they excite both, but partially. Parallel to the front wall, there is a significant measurable improvement over omnis, toed in, a moderate one.

- Full height line sources in conjunction with their reflection create a vertically shaded cylindrical wave of infinite extent.

- They excite the modes differently. A dipole at a node will behave like an omni at an anti-node, and vice-versa. This necessitates entirely different placement for a desire bass response.

- Below the lateral Nyquist frequency, in a rectangular room, and oriented parallel to the front wall, dipoles will cancel all remaining room modes when the listener is the same distance from the rear wall as the speakers are from the front. Omnis don't have this property, although a similar effect can be achieved with inversion and delay. The lateral Nyquist frequency depends on the greater of two distances, the distance between the acoustical centers of the two woofers and the distance between the acoustical centers of the woofers and that of their images in the lateral reflections, which is twice the distance from the lateral center of the woofer to the side wall, measured along a line perpendicular to the wall. The spatial Nyquist frequency is halved for a panned stereo signal, but this doesn't occur much with LP, since the bass is blended to prevent overcutting.

- Full-height line source dipoles don't suffer from cancellation due to floor and ceiling reflections, but shorter ones are similar to a point source in that regard.

- Dipoles, like omnis, suffer from comb filtering with the front wall reflection, and side wall reflections if they're toed in. But the frequencies at which it occurs are different because the back wave is inverted. I haven't checked but I suspect this effect will cancel as well for an equidistant listening position.

[josh358]

Well Josh, my setup partially meets more of that criteria. The riser has a bar-height rear wall which is precisely 9' behind my head. I assume the sofa acts as something of a bass trap, however (there are also bass traps in the front corners (been contemplating the addition of more)). I tried my 3.7s without any toe and prefer them as they are. The subs fire in three directions. I started with them on the side walls, but they ended up in the front corners. Thunderous depth isn't a problem, but more importantly, the lower mid-bass bloom issues have been practically ameliorated.

I have a sub that's not being used (well, it's a side table in my bedroom). I've been thinking about deploying it in the rear of the room, but there are a number of complications.

It would be interesting to see some measurements. None of this is all or nothing. Also, room dimensions don't translate into room modes in an obvious way. The frequencies tend to shift because of obstructions and varying construction materials. If that weren't the case, my own room, which is pretty close to square, would have awful bass, and it doesn't, it's actually quite good. I have a large opening to the side, and a mantle which interferes with imaging but also tends to break up the depth axial mode. The room is also quite transparent to bass, thanks to its rickety construction. This allows it to escape and reduces the magnitude of any modes. Of course, I haven't measured my room either.

[kevin360]

I guess the real question is, “Why don't I have a calibrated measurement system?” That's a good question, as there are some available for about the price of a set of front end NOS tubes, and I sure didn't mind paying for those. I suspect part of me is afraid of what I'll discover, but that borders on idiotic. After all, whatever I discover will arm me with a target for which to devise a specific remedy.

My real problem is the thought that $X can go towards actual system components (including room treatment), and I don't feel 'done' yet. There are areas in which I am aware that my setup is deficient. A problem there is that I can't stop buying media. We get 90% of the way 'there' and listening becomes really addictive – and it's always fun to hear something new. We're wired to get a kick out of novelty – a feature of neurology we all share (and it operates on the chemical axis (dopamine) that 'moves' (quite literally) us). Anyway, I do buy from other sources as well, but I am an Amazon junkie – placing orders on a weekly basis (often, multiples).

My other problem, and I think we all share this one to a point, is the limitations imposed by things that are 'planted' in my room. Perhaps, I just didn't plan very well. The thing is that I dumped six figures into the project, but still couldn't incorporate all of the construction elements I desired. I had to lower my sights a bit on the structure so that I could actually put a system in it. Of course, the video system, which wasn't part of my original plan, further ate into my budget – I highly recommend the Runco & Stewart combination, but it isn't cheap. My point is that certain things had to be mounted, so I had to design the layout of the room before actually experimenting with placement options. The distance between the projector and the screen is dictated by the screen size and the screen size was dictated by the calculated placement requirements for 3.7s in a 16' wide room. As the room is the gathering place for college football season (for the friends who prodded me into adding video), it had to afford a second row that is elevated for an unobstructed view – the location of which I had to determine before the carpet was installed (I had to build the riser somewhere). In fact, the tertiary row (bar/stools) gets occasional use. My calculations didn't predict where experimentation would lead me, but I'm stuck with a few results of those calculations. This, I think, is a ubiquitous problem – we must all deal with immovable objects.

Those immovable objects make it virtually impossible to locate a sub in the rear of my room – unless it hung from the ceiling.

I don't have a calibrated measurement system because I'm a dunce. Hand me my hat and I'll sit in the corner. Better yet, I'll get off my duff and buy one. I just bought a hoist system for my hardtop – that would have paid for a rather nice one. That's the thing about money – there are always so many places to spend it. Being semi-retired, I'm not putting in anywhere near what I'm taking out and I do have a finite supply! Okay, back to the corner.

[Davey]

Taking measurements can be a real double-edged sword. 

Identiying/performing a proper testing scheme, knowing how to interpret the results, etc, etc.
Many folks, when they see how non-flat their measurement results are, then turn their attention to figuring out how to "fix/repair" the design "mistakes" they (or their speaker designer) made. 

The best answer to the OP's original query is.....put 'em in a bigger room.    Everything becomes simpler/better when you do that.

Cheers,

Dave.

[*Scotty*]

Assuming you already have a sub in the front, a sub installed at the ceiling/wall junction would work just as well as if it was on the floor. If you use the CABS approach, the rear sub only has to cancel out the bass that remains after losses due to transmission through the walls and absorption. In fact you won't even be able to tell that it is functioning from the listening seat until you turn it off, its SPL output may be as much as 15db down when compared to the output from the sub in the front of the room. You only can hear the output when you are right on top of it.
 A screwy location I know, but it would work to even out the bass energy in the room. Bass impact in addition even energy distribution can be preserved if the output of the rear woofer is delayed via a Behringer DCX 2496 unit. All you have to do is let the bass wave front get past everyone's listening position before you cancel it out behind the risers location before it hits the rear wall.
Scotty

[josh358]

I guess the real question is, “Why don't I have a calibrated measurement system?” That's a good question, as there are some available for about the price of a set of front end NOS tubes, and I sure didn't mind paying for those. I suspect part of me is afraid of what I'll discover, but that borders on idiotic. After all, whatever I discover will arm me with a target for which to devise a specific remedy.

My real problem is the thought that $X can go towards actual system components (including room treatment), and I don't feel 'done' yet. There are areas in which I am aware that my setup is deficient. A problem there is that I can't stop buying media. We get 90% of the way 'there' and listening becomes really addictive – and it's always fun to hear something new. We're wired to get a kick out of novelty – a feature of neurology we all share (and it operates on the chemical axis (dopamine) that 'moves' (quite literally) us). Anyway, I do buy from other sources as well, but I am an Amazon junkie – placing orders on a weekly basis (often, multiples).

My other problem, and I think we all share this one to a point, is the limitations imposed by things that are 'planted' in my room. Perhaps, I just didn't plan very well. The thing is that I dumped six figures into the project, but still couldn't incorporate all of the construction elements I desired. I had to lower my sights a bit on the structure so that I could actually put a system in it. Of course, the video system, which wasn't part of my original plan, further ate into my budget – I highly recommend the Runco & Stewart combination, but it isn't cheap. My point is that certain things had to be mounted, so I had to design the layout of the room before actually experimenting with placement options. The distance between the projector and the screen is dictated by the screen size and the screen size was dictated by the calculated placement requirements for 3.7s in a 16' wide room. As the room is the gathering place for college football season (for the friends who prodded me into adding video), it had to afford a second row that is elevated for an unobstructed view – the location of which I had to determine before the carpet was installed (I had to build the riser somewhere). In fact, the tertiary row (bar/stools) gets occasional use. My calculations didn't predict where experimentation would lead me, but I'm stuck with a few results of those calculations. This, I think, is a ubiquitous problem – we must all deal with immovable objects.

Those immovable objects make it virtually impossible to locate a sub in the rear of my room – unless it hung from the ceiling.

I don't have a calibrated measurement system because I'm a dunce. Hand me my hat and I'll sit in the corner. Better yet, I'll get off my duff and buy one. I just bought a hoist system for my hardtop – that would have paid for a rather nice one. That's the thing about money – there are always so many places to spend it. Being semi-retired, I'm not putting in anywhere near what I'm taking out and I do have a finite supply! Okay, back to the corner.

The media really does come first. I subscribed to MOG a few months back, and I've become completely addicted to it. Listening to performances that I've never heard -- I've tried to gather the best over the years, but we need variety as well or as you point out things become stale. MOG is also a great way to make new finds. I wish they had an uncompressed option, though, I'd gladly pay more for it. 320 is OK on my computer and for historical recordings, but I can't see listening to it on my main system, except to check something out.

That being said, a calibrated Behringer is only something like 80 bucks. I have an uncalibrated one but I'm going to get the calibrated one, or spend a bit more and get a calibrated Earthworks.

I think we all live with real-world limitations in our listening rooms and it's important not to be too obsessive about it. Just as MOG has been great despite its sonic limitations I wouldn't want to give up my projector to get slightly better sound. And there are ways to improve the sound in just about any space. I can't do the plane wave thing in my current room but I could get damn good bass out of the MMG's, within their limitations. The midbass was more of a problem because of the reflected backwave and the Tympanis will fix that, as well as giving me more bass extension. Between dipole placement, trapping, EQ, and supplementation as necessary with subs and/or the Maggie woofers I think you can get great bass. Even in an ideal room with a plane wave setup, you may need bass EQ because as Wendell points out they can only design the woofers for an average room and placement, some installations are going to have too much bass and some too little.

Wendell is very enthusiastic now about using the DWM to supplement the Maggies in larger rooms. He says you can get Tympani-quality midbass with them, and they would fit nicely in your room. (He points out, and I concur, that midbass performance is more important than deep bass).

[josh358]

Taking measurements can be a real double-edged sword. 

Identiying/performing a proper testing scheme, knowing how to interpret the results, etc, etc.
Many folks, when they see how non-flat their measurement results are, then turn their attention to figuring out how to "fix/repair" the design "mistakes" they (or their speaker designer) made. 

The best answer to the OP's original query is.....put 'em in a bigger room.    Everything becomes simpler/better when you do that.

Cheers,

Dave.

I think most people will do well just to equalize the region below 200 Hz at their listening seat, taking care not to apply no more than a mild boost to any dips. That's pretty safe and almost guaranteed to improve the sound in that location. Things become more difficult further up, as you point out, and unless someone invests the time to learn something about room correction -- target curves, where and how to measure, on axis vs. power response, what is and isn't audible as a response variation -- he'll likely do more harm than good.

[kevin360]

Hmmm, you're giving me ideas Scotty – thanks. I'm still digesting that paper, but I get the gist of it and it makes a lot of sense. Heck, I've always wanted to try the swarm approach, but this might be easier. As it is now, I am a heretic – I have a large sub located in each front corner, perched atop ASC sub traps. I have them there because that is where they give me the meatiest bottom end, but there was apparently an issue north of 40Hz because it's pretty much gone now that my XO is set at that point – at least, it's not irritating.

It's hard to know precisely what 'solved' my bass issue, but I found a workable combination. I should think employing the CABS approach would definitely be a further improvement. If a pair of ceiling mounted subs could be effective, it improves my chances of doing it. Obviously, I can't hang a pair of 120lb subs from the ceiling though, so I think I need to know what my problem areas actually are. That way, I'll know how low the rear subs need to extend. It's great news that they could operate as much as 15db down.

My mind is reeling. Of course, I didn't plan for this, so it's going to be a challenge to do it neatly. Bass is the biggest challenge - period. Some choose to pretty much live without it. Others have to have it, warts and all. Still others, get it right. I'd love to be a member of that third camp. I'm not foolish enough to assume that I'm getting the smoothest bass possible for my room. I just know that it's very good. I'd like to think that a pair of 10” subs in the back of the room could do the trick – I may be able to accommodate them now that I don't have to magically create the floor space.

Josh, I agree that smooth midbass is the greatest challenge of all. I've had the depth, but have been plagued by some lower midbass issues. I wouldn't be surprised to find that some issues remain, but it's significantly improved since taking it to the 9s. The toe angle of my 3.7s is fairly slight – the inside edge of each speaker is but an inch and a half closer to the front wall than the outside edge.

The thought of adding a pair of DWMs is intriguing. If I sell some of the gear I have that's doing nothing but collecting dust, I could apply those dollars towards bass improvements.

As far as going nuts with measurement guided tweaking is concerned, well that's another reason I've been avoiding it. My greatest concern is that I would act on flawed data with a flawed fix that just ended up making things worse. That's the conundrum: it's practically impossible to tune a room by ear, but misused tools can lead things even further astray. So far, I've been rather conservative and 'general'.

Unfortunately, I agree with Davey regarding the OP's issue. If it's a personal space that needn't accommodate more than a single listening position, I'd rotate things to the long wall – and sit close. I'd also definitely give alternative layouts a try.

[Wayner]

Unless I don't understand your initial picture, may I ask why are you setting up on the short wall? I have my Martin Logans on the long wall, with about 12 feet between them. This lets them breath, puts you in a better sweet spot and gives you room to put a sub-woofer (on the outside) of the speakers....

Just a different approach.....

Wayner

[*Scotty*]

If there is room on the floor between the back of the riser and rear wall you could place one of your front subs on the floor back there centered on the wall to test out the concept.
If you take the bass below 100Hz out of the panels and let the subs do the work, the low frequency response and room behavior will become much more predictable. If the floor is concrete in your room go ahead and put the other woofer in the front on the floor as well so that both woofers see the a similar boundary loading condition.
 I would buy the cheapest, longest set of generic
Audio/Video ICs I could find to get the signal to the woofer in the rear for this test. I use the 25ft. long El Cheapos for this less critical job in my system and they work just fine.
You may not need two more woofers at the rear of the room to make the trick work.
I don't know what size your front subs are but a single 12in. sub for example placed in the center of the ceiling wall junction could work just fine. In fact merely relocating one of the front subs to the rear might be alright. You will get better bass slam at the listening position with two subs up front providing the sub 100Hz wave launch and one or two located in the rear and timed to cancel out the front wave after you get a chance to hear it. The need for headroom on movies will probably require the two subs remain in the front. If you check my gallery you can see that the main speakers in my system consist of two 12in sub-drivers per side which provides the impact in the system and I am able to cancel out the remaining bass energy at the rear of the room with one 10in. sub that has similar bass extension. Symmetrical placement of the subs in the room is critical to achieving optimal cancellation of the bass wave and smooth bass response in your room.
Scotty

[kevin360]

Okay, you talked me into it. I'll get a 10” sub in the back/center of the room and then tune the delay. This is tangible physics – I can get my head around the concept. Hell, I had an SAE 6000 something or other time delay & reverb box back in the 80s – actually added a great deal of spaciousness to recorded music. Anyway, I'm game.

I apologize to the OP for this diversion, but take solace in the fact that this is a general problem and a general solution – all of us could benefit from it. Having said that, I'm making it a little more specific by posting these photos of what is in the back of the room. Please pardon the crappy cell phone photos. Please pardon the mess as well. The time has come to part with the Pass X350. Some recent furniture changes in the house displaced my photos, which are now on the bar. There's more crap back there too - a couple of projects are unfinished (as always).

This is the bar (still in its temporary form – after nearly two years), which is the back of the riser – fully framed, with flooring nailed to both sides. It dissects about 2/3s of the floorspace at the rear of my room. I mention it because I assume it presents an issue for a sub placed on the floor behind it. The space to each side is already at the bare minimum, due to my ceiling shape.




At the very back is cabinetry, the entry door, and the door to the bathroom. Any ceiling space that doesn't block the doors is fair game. Could I construct a sub in the space above the cabinet? Its height is a full 12”, which would permit a 10” driver and plenty of width for tuning the cabinet dimensions. It could be built outward somewhat as well. That single light can go - won't miss it. What I like about using that space is that I could make it look very neat. I could even use what is now cabinet space. What are your thoughts?



I could locate the driver near, but not quite at the center of the back of the room.

[*Scotty*]

Close to the center would be good enough in the back, be sure to pull the front subs as far away from the corners as possible. The neat thing about this is that in the real world, as opposed to the context of a doctoral thesis, close can be good enough.
 The rear sub will need to have flat output and very close to the same bass extension as the front subs for the best results. Avoid like the plague any sub-woofer that has any type bass boost or nondefeatable equalization. If the front subs have a flat near-field response, than the end result when a sub with a similar flat response is used out of phase at the back should be a flat bass response curve for the room.
Scotty

[kevin360]

Well, the manufacturer of the subs recommends placing them in the corners. The way they are made, it makes sense. Their rated output is down to 11Hz, and it's not hyperbole. I started with them on the side walls, but they gravitated to the corners and stayed. They are limited to 40Hz and below, where they truly shine. The current placement of the 3.7s has them producing exceptionally flat and solid bass down to the 40Hz area.

I'm very happy with what I'm getting right now, but I'm aware that it isn't perfect. A little more smoothing would be welcome, but I'd like to leave the front layout as is - if possible. Here's the business end of the room.

[*Scotty*]

I probably wasn't as precise in my recommendations for the positioning of the front subs as I should have been. A better case position for the subs in the front would be to move them along the wall towards your rack. From what I can see from your photo of the front of the room, this would mean a lateral shift of the left channel sub towards the right until it is immediately next to your rack. The right channel does not appear to able to be placed any closer to the rack than it already is due the presence of the mysterious box with blue lights sitting on what looks like an extension of your rack towards the right side of the room. 

 The predictable results that CABS can produce are based on a very specific set of conditions using monopoles operated below 100Hz placed against the front and rear walls and the generation and cancellation of a planer wavefront at a specific longitudinal position dictated by a variable delay of the planer-wave output from the rear sub/subs.

 Using dipoles to cover the bass range down to 40Hz substantially deviates from the CABS approach and the conditions it creates which guarantee a predictable result. I have no way of knowing how effective a sub in the rear will be towards evening out the low frequency energy distribution in the room with dipoles covering the bass range down to 40Hz.
Please let us know how your experiments turn out.
Scotty

[kevin360]

Well, even the subs aren't monopoles. They do have a single 14", forward firing driver, but it's supplemented by a pair of side 'firing' tuned radiators. So, they load the room differently from simple monopole subs, but the arrangement does yield an effective driver size of 26" - with acceleration/deceleration characteristics unobtainable from a 26" driver.

The mysterious box of lights indicates that it was 11:03:47 when I took that picture.

Yeah, I actually read the PDF you linked in this thread and discovered how far from the conditions for the CABS approach my setup is. It may be a stillborn idea for me, but I find it intriguing.

Quite honestly, I'm very happy with the quality of bass I'm getting presently. I am, however, certain that it could be even better. I'd like to get it as good as I possibly can, so I'm open to ideas and experimentation. I'll file this one in the 'maybe' folder.

I honestly think it's time to take some measurements, but I've always viewed that as something of a Pandora's box - I may end up wishing I'd never opened it.

Thanks again for the suggestion. I hope some of this discussion can benefit the OP.

[kevin360]

Unless I don't understand your initial picture, may I ask why are you setting up on the short wall? I have my Martin Logans on the long wall, with about 12 feet between them. This lets them breath, puts you in a better sweet spot and gives you room to put a sub-woofer (on the outside) of the speakers....

Just a different approach.....

Wayner

I had the same question until I read the initial post again. There's a fireplace on the long wall.

---

There's just too much in too little space is my guess. If the traditional approach isn't working, why not bring the room back to life and try something extreme like the 'Rooze'? At this point, I think you have nothing to lose. Could you return the 3.7s in favor of something else?

Do you have subs? If you can get everything but the best bottom end out of your 3.7s in your space, could you squeeze in a sub or two? Personally, I wouldn't have my 3.7s without subs.

[MGbert]

Sorry I'm late to the discussion, but I posted this here a while ago:

http://www.audiocircle.com/index.php?topic=107064.0

and it might be of use to the OP, since controlling the first reflections could be a factor.

MG-bert

[SteveFord]

I'm too much of a clodhopper to build these without it turning into an ashtray but I really think Jazzman's Ripoles might be worth investigating if you're looking for subs.

http://jazzman-esl-page.blogspot.com/2011/01/ripole-subs-are-underway.html

[tberd]

 Ahhh, the never ending small room headache. 

 The room can either make or break a system. Move a poor sounding speaker/system in a larger room and look out......

tberd

[Red Dragon Audio]

Mr Little Jeans,

I would consider soffit bass traps around the perimeter of the room to help with bass anomalies - maybe invest in some HQ subs.
Everyone else has offered great suggestions and things to try.  Small rooms can be trick especially when they're a bit uneven in their shape.
With patience, persistence and testing - you will get it worked out.   


Kevin,

When I saw that space above your bar I just figured a Chunk style Bass Trap would fit there perfectly if the sub doesn't work out.