An introduction, and a few questions from the noob!!

[Brian C.]

Hello all!!

I've been lurking for the last couple weeks (since I recently found this fantastic forum!), and decided to hop on board and try to learn some more! It appears that there's a veritable fountain of knowledge here, and even though I get lost in most of the science and theory that goes into the design of loudspeakers, I figured at least I could avail myself of some of the goods! :W

A little background: I've been known to dabble in car audio and home theater. My system is almost always in flux, but I'm finishing a dual-purpose room, and I'm trying to come up with a set of speakers for the front. My previous system consisted of the (old) Fosgate Audionics speakers, three LCR-220's, two SD-180 dipoles, and a pair of the FA-400 passive subs. I've since given the LCR-220's to my dad, and acquired another pair of the SD-180's. As far as electronics, my system is always in flux. Currently, I have a Sunfire TGP-IV processor, and 3 Fosgate 4125 amps. I'd like to sell the Fosgate amps, as I have a stack (7) of the Soundstream DA-2 amplifiers, though they are the 240v versions, not 110v. No problem there... and old high school chum is a licensed electrician.

Anyhow... I can relate to one of the other members here (Kingpin) as far as wanting BIG sound, which may or may not relate to BIG speakers. I've always dreamed of having a front three consisting of Dunlavy SC-IVa's and an HRCC center channel, but hate to spend that much money. Then, when I see that Dunlavy's have been talked about, and that a similar or BETTER sounding speaker can be built for less money (and the fact that HRCC's are about as rare as hen's teeth), I'm up for building some.

I've poured over the designs in Missions Accomplished, and think that the Dayton RS 3-way MTMWW fits the bill for the L&R, and was hoping to see a tried and tested center channel design come out of that exercise.

(Man, am I long winded or what?? :P )

A few questions:

1. Dunlavy made use of a stepped baffle, and I'm assuming it was in an effort to time align the radiating surfaces of the drivers. Is there any real advantage to this, or is it a moot point with proper crossover design?

2. Would a center channel complimentary to the Dayton RS 3-way simply be a matter of placing the woofers on either side of the MTM, and for aesthetic reasons, is there a problem with aligning the MTM in the vertical plane (as opposed to having the tweeter offset)?

3. Lastly, much has been made about making the crossovers readily accessible for possible modification at a later date. Is there any particular reason why they could not be "divorced" from the cabinet, and in their own enclosure, or is there an ideal "maximum length" for the leads from the crossover to the drivers, and should the lengths for all drivers be equal?


I picked up a copy of Vance Dickason's LSDC (6th Edition) a couple years ago, and have only recently picked it back up and started getting back into it. Like I said earlier, I'll never get to the point of understanding the science and theory behind all of it, or the real "black magic" of crossover design. But, one thing I CAN do, is build the cabinets. And do some other trick things as far as the construction (I've got a CNC mill at my disposal... just as good as a CNC Router). Also, I've been investigating an alternative material for the cabinets, one that I've been intrigued by for a long time (since I've had opportunity and experience in machining it). It's a product called Richlite. It's a composite made of paper fiber and resin. Extremely dense and rigid, and looks like it could be the hot ticket for cabinets. It comes in 3/4", 1", and 1-1/2" thicknesses, and has a pretty pleasing appearance in it's raw form.

Whew!!! I'm done talking for now... I'll take a breather and start soaking in all the info I can!

Cheers!!

[ThomasW]

Dunlevy use a stepped baffle AND 1st order crossovers to keep the design phase coherent. .......Been there, done that, and to be honest no it's not worth the hassle with different XO topologies.

No you can't simply put the woofers on the side to make a center. The crossover design is based on the baffle width. Obviously that changes with the woofers on the sides.

The tweeter is off-set to keep the midwoofers close together.

There's no reason the XO's can't be outside the speaker.

[cjd]

1. Stepped baffle would be to physically time align the drivers. In this case, phase is built into the measurements and is taken into account. The merits can be discussed, but the more important item is that it's not something you want to change on a published design. For myself, I find the step below any driver and the diffraction this would introduce to be a potential disadvantage - a sloped or curved baffle would be a preference, but... well, yeah. Some day. Time aligning in the crossover just happens though, so it's not like it's an unusual complication to deal with.

2. In the smaller WTMW CC thread I published a crossover that should sonically blend with the larger towers well - Brian built this I believe, so he can comment. I would LIKE to do a larger CC but that requires building and measuring - you definitely can't just move the woofers over in this case as it will change diffraction and baffle step quite a bit - and I think absolutely would not want to go horizontal on the MTM if it can be avoided. In other words, it would be a new project. Kingpin may get there if he builds a SWMTMWS center-channel/tv-stand. Will need live measurements though, and it may have some interesting complications. The best results would be to go with a front projector and an acoustically transparent screen and a vertical center - you'd need plenty of room behind your screen though, probably 4-5 feet.

3. Crossovers can be outside the cabinet, absolutely. Woofer leads, there is advantage to as short as possible - longer = more impedance = higher overall Q = muddier bass. But it's relative, and subtle. Getting the woofers out isn't too hard, and had I been smart and used industrial strength velcro to hold them in place, almost as easy as outside. Less room for error I think. Plus easier to sell, which is something I wouldn't mind managing to do with mine.

C

[cjd]

The tweeter is off-set to keep the midwoofers close together.

Actually, it's offset to clean up diffraction. If you go felt treatment I bet it could be centered without a problem.

C

[Brian C.]

Re: ThomasW...

Ahhh... see, I knew in my subconscious that the size and shape of the baffle was at issue, and I just chose not to turn on the lights in that corner of my brain. What with all the talk of edge treatment of the baffle (chamfer, rounded corner, etc.), it only makes sense.

I do understand the issues with taking a MTM and positioning it horizontally, and as far as I'm concerned, sound quality takes precedence over aesthetics. I'd love to do identical tower for the front three, but even if I had a microperfed screen that was TALL enough to put a tower behind it, it just isn't practical in my application, because I've got an opening in the wall behind a powered screen with blackout backing. Maybe in another room, later on...

So... how did Dunlavy (and various others) get around the baffle step issues with a WMTMW center channel, with the MTM's arranged vertically, especially when it appears that the driver's were stepped (recessed) into the face of the cabinet? It seems as though this would only exacerbate the problem. Does that explain the liberal use of heavy felt? Is this an attempt to eliminate the baffle step issues, and render the cabinet inert?

On the spacing between the drivers... It seems as though the drivers could be arranged much closer than the plans show? And if need be, even tweeter flanges can be modified (machined) to allow closer driver placement?

Forgive me if I sound like a total neophyte here... I'm just trying to understand as much of this as I can!

[cjd]

Baffle step is a separate issue from diffraction which is a separate issue from time alignment.

A good crossover deals with them all as part of the crossover (though diffraction can be helped quite a bit through the use of felt).

Many commercial companies will say "ahh, who cares about the problems, it's what the customers are demanding" and built a horizontal MTM (or WMTMW) and hope everyone sits within the narrow area it works well.

I mean, people buy those Bose cubes a ton, and it's NOT because they sound so good.

C

[Brian C.]

Yes, I've had an ongoing debate with my father, because he took one of the Fosgate LCR-220's laid it on it's side for the center channel (behind a wall, no less...).

He's always had a problem with lack of dialogue and low SPL out of the center, and I told him to tell the in-home interior decorator to "deal with it", and pull the speakers out from behind the wall, and stand them all vertically.

I'm in the same camp as some of the other "size is no object" members... with a projection system, I don't care how big the center cabinet is, it just can't be more than about 30" tall.

[ThomasW]

As Chris noted Dunlevy and others with physically stepped front baffles (aka drivers recessed to time align the acoustic centers of the drivers), use tons of felt to kill diffraction problems created by those steps (notches).

Note this reference to 'steps' (notches) has nothing to do with baffle step compensation (BSC)

[cjd]

with a projection system, I don't care how big the center cabinet is, it just can't be more than about 30" tall.

That could work pretty well. With any horizontal center below the screen, you want to keep the tweeter as close to the screen as possible. It's still a compromise. It would be a new design effort though, and would require a prototype to be built and accurately measured. Which is the biggest reason it hasn't been done (yet). I would imagine the mid-woofers could both be placed below the tweeter snugged up nice and close to each-other. And the woofers might have to be tucked in close too, to keep their center to center relationship within range...

C