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Old 19th May 2004, 07:45 AM   #1
tcpip is offline tcpip  India
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Default Comments/guidance invited: TMMMM sealed box

I'm trying to build a good quality sealed-box speaker which should be full-range, go up to reasonably high SPLs, and will be used for 2-ch music listening. I intend to use active xo and LT, and relatively inexpensive drivers. I am using a certain 6.5" poly-cone midbass unit made in India, and a dome tweeter (as yet undecided), xo'd at 2.5KHz. I want to use a sealed enclosure plus LT to get decent bass extension, but with one midbass unit, I'll run out of excursion. Hence I want to use four midbass units (the drivers are about USD 10.00 each, so I can afford them).

I am therefore thinking in terms of a TMMMM design. Any tweeter will do, provided it has an fs of about 1K or so and is reasonably flat and well-behaved above that. I'm using LR4 (electrical) active xo.

The background for this xo and this midbass unit has been based on a simple sealed two-way built by my friend, who used the same midbass driver, the electrical LR4 and LT for his unit. I'm using a different tweeter, but I don't see much fundamental issues for the overall design if I replace the tweeter. My friend built the speaker (a small two-way TM configuration tower) and listened to it, measured its response etc, and found that it sounds very clean, has very good crisp mids, and measures flat to about 30Hz, provided one can keep the SPL low. I have heard his speakers, and they sound very clean, detailed and musical (as in "not analytical or hyper-detailed"). I am replicating his design idea, just adding more midbass drivers.

Any comments/guidance/suggestions?

In another thread, where I had broached this idea, I had got some very useful tips from Sreten and others. In particular, Sreten had said that I should split the four midbass drivers into two pairs, and use one pair "full-range" from the bottom till the tweeter xo, and the other pair from the bottom to the BSC knee, after which I should roll them off using a first-order slope. I think that makes sense, and I'll try that.

Any other comments? I was wondering what would happen regarding comb filtering when all these M's interact with the T. (If I use Sreten's BSC idea, two of the M's will be rolled off and cut down quite a bit by the time I reach 2.5KHz, but they'll still be audible.) One idea was to use an MTMMM, where the two top M's are used full-range till 2.5KHz, while the bottom two M's are used for BSC.

On the other hand, there are speakers like the Straight 8 which seem to have eight midbass units in a row, interacting with the tweeter. What about comb filtering there?

In case you want to know, I'll be using solid state amplification for these speakers. I intend to use one amp per driver (five amps per enclosure), and the first version will use non-inv LM3875 amps.

Any comments would be useful. This is my first major speaker building project, and while I'm building on the good groundwork of my friend's design, I'm still making changes big enough to need help.

Thanks for any inputs.
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Old 19th May 2004, 08:15 AM   #2
sreten is offline sreten  United Kingdom
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Well how active is active ?

The obvious choice is using one driver for mids only,
and applying the LT to to other three bass units only.


sreten.
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Old 19th May 2004, 08:53 AM   #3
tcpip is offline tcpip  India
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Quote:
Originally posted by sreten
The obvious choice is using one driver for mids only,
and applying the LT to to other three bass units only.
You're basically suggesting that I go three-way, not two-way at all, right? One advantage with that will be that the TM interference will be about as low as one can hope for, as clean as any two-way. The disadvantage will be that I'll lose out one one driver for driving the SPL at low freq. What do you think?
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Old 19th May 2004, 09:06 AM   #4
sreten is offline sreten  United Kingdom
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If your going active, BSC is electrically very easy, and there
is no need to maintain sensitivity matching of driver arrays.

Even five drivers is possible :

An MT mounted on the front, and 4 drivers in series/parallel
with two drivers mounted in each side in force cancellation
mode, i.e. the magnets physically linked and decoupled
mounting of the drivers.

For the above a full 3-way design is suggested, the default
response with 1st order bass / mid crossover will be very
near to including BSC.

sreten.
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Old 19th May 2004, 09:17 AM   #5
tcpip is offline tcpip  India
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Quote:
Originally posted by sreten
If your going active, BSC is electrically very easy, and there
is no need to maintain sensitivity matching of driver arrays.
Nice to know that something is very easy for me. Sigh...

Quote:
An MT mounted on the front, and 4 drivers in series/parallel
with two drivers mounted in each side in force cancellation
mode, i.e. the magnets physically linked and decoupled
mounting of the drivers.
I understood the bit about the front-facing MT... that part's just a simple two-way, probably reaching down till a few hundred Hz. But below that, the four side-firing drivers will be in push-push mode, if I've understood you right. Have I?

In that case, I'll have to bring those four drivers in at a frequency high enough to take care of BSC, don't I? This means they must reach upto at least about 400Hz or more. But at those frequencies, sound is very directional, and won't it be better ("crisper" sound and the like) if they fire forward? I'm no expert, just asking...

Quote:
For the above a full 3-way design is suggested, the default
response with 1st order bass / mid crossover will be very
near to including BSC.
I don't think I understood what you mean by 1st order xo for bass-to-mid. Why should I do a 1st order? If I do a first-order, won't the side-firing speakers reach quite high and won't this muddy the midrange or something? Basically, I'm quite unclear about what happens if different speakers are firing in different directions in the mid or high ranges.

Maybe one answer to my worry about high mid-to-bass xo is to make the front baffle wide ... maybe 18" wide. Then the BSC knee can be lower, and the muddying of the midrange, if such a concern is legit, will be less of an issue.

The more I think about it, however, the more I feel that I'd probably like to build a two-way, if it can be made to work. A three-way is not totally ruled out, but I'd just like the simpler active xo of a two-way if possible.

And if I do shift to a three-way, then left to myself I'll probably go for a TMWWW, where the M is pure midrange, and all drivers are front-firing, as you'd suggested earlier. In that case, should I xo the mid-to-woofer at the BSC frequency, or should I xo at any frequency (something lower like 200Hz) and do a shelving filter on the midrange to take care of the BSC? I hope my thoughts are decipherable... if not, I'll elaborate later if you want.
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Old 19th May 2004, 02:04 PM   #6
sreten is offline sreten  United Kingdom
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For an active 3-way its best to c/o where you'd ideally like
and add a BSC shelving circuit before the bass / mid filters.

For an active two way a MTMMM with a passive 0.5 way is best.

sreten.
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Old 19th May 2004, 03:14 PM   #7
tcpip is offline tcpip  India
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Quote:
Originally posted by sreten
For an active 3-way its best to c/o where you'd ideally like
and add a BSC shelving circuit before the bass / mid filters.

For an active two way a MTMMM with a passive 0.5 way is best.
Thanks for the patience. I'm getting the drift.

I was trying to understand why you suggest an MTMMM specifically for a two-way. I guess it's because if I do a three-way with the MTMMM, there will be only two drivers handling all the lower frequencies, which will be too little drive, and two whole drivers doing just the mids, which will be too much drive. Right? I was getting around to that view myself.

I'm now becoming clearer that for the first version of my speaker, I'll go with a two-way and keep it simple. Maybe I'll do an MTMMM, maybe a TMMMM, I don't yet know. I'd have done an MTM.. but I've read some negative remarks from others on this forum about how MTM didn't sound good and TM was much better. That's sort of put me on guard. Is there anything I should be careful about if I go MTM?

And for the BSC, I may put a passive line-level shelving xo in the path of the low-pass output of the xo. Or maybe even in the input to the xo... how does it matter? With a passive line-level xo, the circuit will be simple. Important issue for a beginner like me.

One more thought about the two-way MTMMM option. Suppose I went for this, and did a 0.5 at 500Hz, with the tweeter xo at 2.5KHz. In that case, how would it be if I did a first-order at 500Hz, and then a sixth-order or eighth-order Bessel at maybe 1.2KHz or so? That way, the two lower drivers would be all but inaudible at frequencies where the tweeter kicks in, thus totally cutting out chances of interference and combing etc. Is it worth the extra trouble? Any ideas?
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Old 19th May 2004, 05:32 PM   #8
sreten is offline sreten  United Kingdom
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Just use a LR2 or LR4 at normal crossover frequency,
as its active this will apply to all bass units.

Inteference from the passive 0.5 units would not be an issue.
Do not go for 4 speaker drive with active BSC, its pointless.

sreten.
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Old 20th May 2004, 04:19 AM   #9
tcpip is offline tcpip  India
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Quote:
Originally posted by sreten
Just use a LR2 or LR4 at normal crossover frequency,
as its active this will apply to all bass units.
Okay.

Quote:
Inteference from the passive 0.5 units would not be an issue.
Okay here too. This lays one of my questions to rest.
Quote:
Do not go for 4 speaker drive with active BSC, its pointless.
Sorry, couldn't understand this one. What is the connection between passive versus active BSC and the number of drivers? When does active BSC become important?

And thanks again for all the help.
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Old 20th May 2004, 04:27 AM   #10
Variac is offline Variac  United States
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Hey Mr. Pip,

I am intrigued by your proposal. It seems to combine a lot of good ideas
floating around the forum these days.
First of all, the idea of "one driver, one chip amp" needs to be used a LOT more, IMHO.

The "pros" seem to agree that a high power amp of over 100 watts/ch is a different beast
than a lower power one, with much more serious consequenses
when things go terribly terribly wrong
So, by using a bunch of chip amps you have the equivalent of a couple hundred watts/ch
with few of the risks AND great sound quality. Making them with active crossovers,increases the efficiency so you are ahead
there too. Stir in the multiple mid/basss drivers, and you are not going to be deficient in power!!!
I think Seigfreid Linkwitz and Nelson Pass would agree as both their speakers use multiple amps.

Now a tower with multiple drivers has a lot going for it too. If it is tall and narrow,
it is appealing in a SAF way, while if it is deep enough for the internal volume required,
people can't place it so the drivers are too close to the wall
(this is the only downside of the push/ push approach)

Thinking about the Straight 8, I think that that speaker is closer to a real line source in a
typical US 8' ceilinged room compared to a 4 mid/bass speaker as you propose, so I wouldn't count
on yours working the same.

I agree vigorously with sretin that making the lower 2 drivers pretty much deal
with the lower bass only and roll off at Baffle step, is a great way to go. That way they compensate for
the baffle step and also give you excursion where you need it- lower bass- especially when you EQ to
get more bass.

I agree with you that a 2 way is much more appealing- especially if you have one to copy and
haven't designed many speakers. Many times I have preferred well designed 2-ways over 3 ways (probably because
designing the 3 ways is so much harder!!

I have to say that then having 2 drivers handling midrange as well as lower worries me a bit.
I don't know, but somehow feel that it woiuld be much better to have only one driver doing the midrange
OR!!!!!: have a MTM setup on top which kinda is the same thing!!

So I'd think either have 3 drivers hooked up for bass and one full range and the tweeter
OR 2 bass and the MTM ie MTM+MM. check out the photo link below for a speaker like this.


My vote? MTM+MM 2-way. Start rolling off the lower 2 drivers at baffle step.
For awhile I was thinking that it might be good to roll off the bass a bit on the drivers that also handle midrange, but on further thought, I think all those drivers sharing the excursion for bass will
keep down IM/Doppler distortion- especially at normal volumes!

Yes, there is a backlash against MTM it seems, but many, many, very high end models use that configuration,
In fact I'd say it is about the most popular for very high end models from well known manufacturers. Not to mentiond The Ariel by Lynn Olsen. When I discuss this with people that have misgivings about MTM they generally have a theorectical concern that it should have some downside, but generally admit that the can sound very very good, in fact say they "think " they prefer a single driver.


Check out the "Reference Speaker" thread for drivers and speaker designs that seem to be relevent to your design
in some ways. People seem to like the Vifa d 27 tweet and various 5.5 to 6.5 mid/bass drivers.
Also good info there on dealing with baffle step.


Planet 10 Dave has posted a pic on this page that is similar to what you are proposinf I think, Post #129

diyAudio reference speaker project

This is an exciting project- especially since it appears that it is hard to find good stuff easily in India.
Perhaps one of us here can make a similar speaker with some of the drivers mentioned in the reference speaker thread
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