[musiclear]

I would like to take a look at what might work to help get those nasty bumps out of our lower frequencies.

The idea is to talk strictly about things we can design into our speakers.  NOT to talk about room treatment. 

To me room treatment will come after we do whatever can be done in the speaker design itself.

I've got a dual woofer system so for this example and because it opens up more possibilities, I would like to talk about dual woofer options.

I'll start with this commnet.

I've seen monitors from Sonus Faber put a woofer on the back to compensate for baffle diffraction.  Wouldn't that also reduce the problems with the wall reflection from the front woofer?

[JLM]

Visit Duke LeJeune's child board here at Audio Circle or better yet read Floyd E. Toole's "Sound Reproduction" regarding swarm theory.  In-room bass peaks and dips (+/- 20 dB) can only be resolved by multiple/remote bass sources.  Best design IMO would be 3 or 4 transmission line (natural roll off matched room gain without loss of efficiency from sealed designed) bass sources crossed over at the Schroeder frequency (where sound transmission changes from waves to rays, roughly 135 Hz in an 'average' room) with the mains. 

Duke (and others) promote constant directivity speaker designs, but to reduce room effects beaming of higher frequencies would be one approach.  Single driver designs do this (4 inch drivers start to beam at 8,000 Hz).  Another way to reduce room reflections at higher frequencies is to set up the speakers so that first reflections are at least 11 feet longer to the primary listening location than the direct sound wave (takes a minimum of 13 ft wide space with 8 foot ceilings assuming typical listening/driver heights). 


Or you could just invent headphone/earbuds.   

[jimdgoulding]

I think SF was going for a better in-room response rather than removal of baffle diffraction.  Wave guides can be of use for that, but you state you are concerned about bass response.  Try moving the speakers around and see if you can find a place for them that makes these frequencies more even sounding to you.  Tilt them back a shade, too, perhaps.

[PMAT]

How about a tall 3 way with a 10 or 12 woofer in front at the bottom 1 passive radiator on the left starting at half way up the front woofer, 1 passive on rt side starting half way up the other passive and another woofer in the back starting halfway up the right side passive. That would give you bass radiating at  4 levels in one cabinet. The other cabinet would NOT mirror but be the same. That would give about 8 woofer patterns and thus smooth out the whole room. Done! I just solved the room node dilemma in 2minutes.    please send me the royalties!

[JohnR]

I would like to take a look at what might work to help get those nasty bumps out of our lower frequencies.

The idea is to talk strictly about things we can design into our speakers.  NOT to talk about room treatment.

At lower frequencies, there's actually not a huge amount you can do with the speaker design itself, as the dispersion pattern "wants" to become omnidirectional. And you're in the modal region, so the primary factor is location, location, location. While not the only factor, it's a big one.

Here are a number of subwoofer locations, each moved only about 20 cm:



Here is another example of two different sub locations:



I am, frankly, unconvinced that there is any point in going beyond one or maybe two (monopole) subs (*) UNLESS you can get the one or two of them right first.

(*) With the particular exception of DBA, and assuming that you don't have an optimization algorithm to run on it like Welte at al.

With that said, here are some other points:

• Dipoles can give you controlled directivity down to low frequencies, unlike a horn/waveguide (for practical/size reasons). However it's not a panacea, location still makes a big difference...
• Line arrays can (in theory, I haven't tried this one myself) remove the floor-ceiling modes.
• A wide baffle will give you a "sort of" waveguide, and if it's big enough will be effective down into the modal region. Planning to try that - see http://www.troelsgravesen.dk/Acapella_WB.htm
• The DBA Double Bass Arrayarrangement is in theory perfect at low frequencies, if you have a perfectly rectangular room. Not tried that one and probably will never have a chance to.
• If we're talking about a separate low-frequency subsystem, then look into "nearfield" placement. Especially effective with dipoles, you'd be surprised.
• In the end, you may look into nice headphones...

Edit: I forgot to include cardioids anywhere above. They could be an interesting option to explore.

Hope that helps

[musiclear]

This is great guys.  Thanks.  PMAT, I like the idea of multiple bass radiating points in one cabinet.  The PR's, would be radiating below the room problems I am having so I think they can be near the floor with no problems.  The rear radiating woofer is interesting.  Are there any current designs out there that use  that?  Would the fact that the rear woofer would be out of time alignment make any difference at 100hz and below?

[musiclear]

JohnR, your graphs help quite a bit.  A huge difference in response from a small change in location. From what I am seeing with them, would I be able to leave my system the way it is with the bump at 90hz and dip at 70hz and simply place a sub that would subdue those peaks and dips?  Do your think moving the lower woofer in this system to the back on the floor tame those room interactions?  Would that be a good idea because it is in the same place on the wall but in different time domain compared to the front woofer?  What kind of problems would I potentially introduce by moving a woofer to the back of the cabinet?   

This is the system we are talking about.  Unfortunately, there isn't a lot of options for moving things around.  I am assuming the reflection is coming off the back wall, but I guess I really need to take some measurements and see where they are coming from.

[JohnR]

Hi, I would suggest drawing up a plan of your room, marking on it all remotely possible locations for the speaker(s), then moving the speaker(s) to each location and measuring and listening. Once you have that "map" you will have information that you can use to decide on the next step. You've made some lovely speakers, I'd hate to see you chuck something out for lack of a few hours of extra work

[musiclear]

I appreciate your concern and I would not be chucking anything out.  I've purposely not finished the veneer for this very reason.  It would b relatively easy to add a set of speaker holes for the back and carefully fill the existing holes in the front.  Then finish the veneering.

My question is will moving one of the 10in drivers to the back of the cabinet help activate the room modes more evenly?

I could easily drop the crossover point of the rear driver so it doesn't interfere with the crossover point of the mid.

[JohnR]

My question is will moving one of the 10in drivers to the back of the cabinet help activate the room modes more evenly?

If I had to take a guess, I would say probably not, the first set of graphs I posted above was a sub with two 10" drivers in it, and one of those graphs was with the sub pointed into the room/into the wall instead of along it. That wasn't the orientation/location I chose.

Perhaps you could measure the effect your proposal would have by putting the speakers right next to each other, one facing backwards, and disconnecting one bass driver in each.

[jimdgoulding]

Maestro, are there any advantages to having one's sub firing downward and out?

[JohnR]

Hi Jim, you are asking me? I don't conduct But in any case, I would think any observable effect would be related to different effective positioning and acoustic loading of the woofer. On the specifically negative side, most subs generate a vibrationary/reaction force and in my house anyway, downwards/upwards is the worst possible direction for it.

[jimdgoulding]

 

Musiclear, is there space behind your seat?   

[musiclear]

JohnR that is a great idea.   I will move these things around and do some measurements.

The seat is a couch and behind the couch is a walkway and behind the walk way is a bar seating area that goes into the kitchen. 

If I understand the direction jimdgoulding is going, I could potentially put a sub under the bar area pointing into the sound room.  It would be about 3' behind the couch.  I may also be able to fit a sub in the middle of the cabinet between the speakers. That would give me what some consider to be optimal sub placement.

I hope to find positive results from the experiment JohnR suggested.  I would love to be able to deal with these room artifacts by placing a rearward facing woofer in the cabinets I already have.  My hope is that since these are 2' deep boxes, the difference that offset makes will be enough to make a difference in frequency response.

I've got such pin point detail now, that I am concerned by adding more woofers, I may effect that negatively.

[Letitroll98]

I don't get it.  You're willing to cut up your beautiful speakers and/or install multiple subs, but are unwilling to try the cheapest, easiest, and most effective solution, a couple of bass traps in the corners.  It's your home and your system and I support whatever solution works for you, but could you explain the objection to room treatments?  I don't wish to be overbearing, but the problem just screams bass traps to me.

PS, the idea of moving the speakers around is of course the first possible solution to try.  However you may find it has a positive, but limited effect.  It might be enough, might not.  Forward and back to affect the low bass, wider and narrower to affect the midbass.  Odd fractions of the room dimensions to decrease response and even to re-enforce, i e 1/5, 1/3, conversely 1/8, 1/6, 1/4.

[musiclear]

There are two parts to this answer.

Part of my desire is an engineering exercise.  I would really like to know what can be done to design for room modes better?  This may help any future designs I may choose to do.

I have moved these speakers about as much as I can with very little change.  I really can not move the couch.

And, I'm not against room treatments, but from what I understand they only really improve things somewhat.  If it turns out that I can move a woofer and solve some issues and maybe use some traps after that, I think I may have a more balanced system and that system should work better in virtually any environment I may move to in the future. 

On the other hand, maybe bass traps work better than I think and that really is the way to go.

[Quiet Earth]

I like your initial idea of learning how to make the speakers work with the untreated room first, and then add a little treatment later (if necessary) as icing on the cake. I think bass traps and other treatments are highly over-rated, not to mention butt ugly. Yes they do work, but they are the icing, not the cake. (And that is some ugly ass icing on such a tasty cake.)

John had good suggestions. You can also study up on some of the legacy speaker companies who built speakers designed to go smack against the wall, near room boundaries, corners, etc. Try to learn a little about everything before adding treatment.

[jimdgoulding]

I can't imagine why you even need a sub, actually.  My suggestion was going to be depending on your reply that you move your speakers further into the room and further away from the back wall.  There is natural dimensionality to be gained inherent in good acoustically made recordings.  Bass might smooth out, too. 

[musiclear]

Jimgoulding, I have moved these speakers as much as I can with very little difference.  Unfortunately, this listening area is fairly sort in distance from the listening seating and speakers.  If that was an option, I would be happy to take it and move on.

I can understand your point about not needing a sub.  With all this woofer and amp that I have, there is a spectacular ability to produce SPL.  Unfortunately, with energising certain frequencies within the room, it is an imbalanced presentation.  I mean, with eq, I have modified the peaks down so it really sounds pretty good, I just would rather not have to do that.  I would rather have an naturally balanced sound stage with minimum EQing.

It seems the old tried and true meathod of adding more radiation points may be another option.

[musiclear]

Now this was a disappointment.  Prompted by one of JohnR's suggestions, I tried some measurements with a speaker pointed to the wall and then next to it, the other speaker pointed into the room as usual.  I expected some differences due to the fact that I have two large speakers side by side, but I also expected to see the room modes effected a lot more than I did. 

I guess this is why the manufactures don't bother with front and rear speakers.

Unless someone can see a major hole in my practice, I'll have to scrap the idea of moving one of these woofers to the back of the cabinet and look for better ways of minimizing room artifacts.