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Old 14th May 2007, 08:25 AM   #1
obiwan is offline obiwan  Australia
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Default mmt or mtm give less lobing?

I have a DEQX and am thinking of making just a "practice" pair of bookshelf speakers to go with my Velodyne DD15 sub.

As DEQX offers up to 300dB per octave crossovers, if I want to use 2 mid bass drivers, am I theoretically better off doing an MTM or an MMT. I'm thinking that in the MMT with the midbass drivers closer together you'd have less lobing. Would the benefits of this outweigh the benefits of the theoretical point source you get with an MTM arrangement?

I know you'll say why not just have one driver for the midbass duties, but that's what I have now with my Dynaudio Special 25's that I've biamped via DEQX. I just want something more articulate in the midrange and I feel the 8 inch Special 25 midbass driver just isn't refined enough in the upper midrange. But if I go down to a 5.5 or 6.5 inch say Scan Speak Revelator driver, I'm worried about the loss in sensitivity and output that a single smaller driver would deliver.

Any thoughts?

Thanks

Obiwan
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Old 14th May 2007, 08:41 AM   #2
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2 way TMM might reduce combing between woofers but the tweeter and outside woofer combing would get worse. This would cause more unpredictable lobing I would think than the MTM.
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Old 14th May 2007, 09:14 AM   #3
Dag is offline Dag  Sweden
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Why not try a MTB 2.5-way??
With M for both mid and bass and B only for bass. M&B can be the same woofer. => God bass and no strange loobing in the midrange.
Peerless HDS 5.5" or 6.5" could be a good woofer (and MUCH cheaper than the SS)
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Old 14th May 2007, 09:51 AM   #4
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Quote:
Originally posted by Dag
Why not try a MTB 2.5-way??
With M for both mid and bass and B only for bass. M&B can be the same woofer. => God bass and no strange loobing in the midrange.
Peerless HDS 5.5" or 6.5" could be a good woofer (and MUCH cheaper than the SS)

That reminds me of something I've wanted to try. It's an attempt to make a MTM without the combing problems. Use the 5.25" HDS Nomex woofers and a Vifa D26NC55 with the flat edge of the flanges touching. Push the XO as low as possible and you *may* not have the combing between the woofers that you normally would.
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Old 14th May 2007, 11:26 AM   #5
obiwan is offline obiwan  Australia
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Thanks for the replies guys,

The problem with going to MTB means that I'll use up my 3 frequency bands on the DEQX leaving me without the ability to add the sub with the DEQX. If I want DEQX to blend the sub, then the main speakers have to be 2 way.

Augerpro, I think with 300dB per octave crossover there would be virtually no lobing between the tweeter and the woofers as they are operating in totally different frequency ranges.

Obiwan
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Old 14th May 2007, 11:32 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

You could go active / passive 0.5 way TMM, using 8" unit for BSC.
That is use that driver with a series inductor, other full range.

/sreten.
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Old 14th May 2007, 01:45 PM   #7
Dag is offline Dag  Sweden
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If you have the amps you can use a very simple 1st order RC filter (at ~500Hz) for the amp to the 2nd woofer.
You can start with an MTM 2nd order and if you don't like the mid you can add another amp + filter later. This way you will avoid the crappy inductor in series with the woofer.

Streten:
If the two woofers are different will you not have problems with the phase between the two woofers?
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Old 14th May 2007, 04:22 PM   #8
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Dag

Streten:
If the two woofers are different will you not have problems with the phase between the two woofers?
Hi,

I'm not sure I understand the question, but to use different sized
woofers they have to match to a degree, moreso if they share the
same box volume, moreso if they also share a port.

Sensitivities need to be very similar, also cone mass per unit area,
or another way Vas x cone area similar etc ....

You are right about the input filter if a spare amplifier is available.

In a 0.5 way the phase digressions should be well above the 0.5 way point.

/sreten.
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Old 14th May 2007, 11:22 PM   #9
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Quote:
Originally posted by obiwan

Augerpro, I think with 300dB per octave crossover there would be virtually no lobing between the tweeter and the woofers as they are operating in totally different frequency ranges.

You don't necessarily want to cross over that sharply.

DEQX looks as if it uses FIR filters, so the sum on-axis should be unity. Off-axis could be a problem, especially if woofer and tweeter dispersions are different, because at the (smallish) overlap there will be a disparate contribution from the two drivers. Since FIRs of that steepness have fairly long impulse responses, the room contribution could well be ringing at a narrow band around the crossover frequency, so I'd advise using a shallower slope even with a linear-phase crossover.
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Old 16th May 2007, 05:06 AM   #10
obiwan is offline obiwan  Australia
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Thanks for responses guys.

DSP_Geek, I've heard people comment on ringing with steep crossovers, but I don't know what it is. I remember trying to search about it, but couldn't find much.

Are you able to explain it in layman's terms for me?

I think the shallowist DEQX will let me select is 48dB per octave.

Obiwan
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