Benefit of separate mid boxes? - diyAudio
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Old 10th June 2007, 05:47 AM   #1
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Default Benefit of separate mid boxes?

In a three way, other things being equal (yes, in some ways they will be not), if the bass is vented: how much SQ benefit is there putting the mid in a separate sealed box, within the bass box?

With drivers I have, a sealed 6 litre mid box within the bass box, results in a Q of 0.55; or the mids mid in the 38 litre vented bass box, results in a Q of 0.497. (The effect on bass response can easily be offset by varying port tuning).

Thanks
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Old 10th June 2007, 06:37 AM   #2
Hartono is offline Hartono  Indonesia
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Hi Rick57,

putting the mid in separate box have plenty of advantages:

1. The bass driver didn't affect mid operation too much (and vice versa).
2. Resulting sound is cleaner.
3. Since interaction between woofer and mid is minimal, calculation will be easier.
4. Mid and woofer can have different tuning (vented, sealed, free air, etc )

5. SQ benefit is big, and worth it IMO, practically all hi end gear do this at their upper class product for exactly the same reason.

disadvantages are:
1. more cost for commercial speaker is not good, not much added cost for DIY.
2. box is harder to construct and will be heavier , which is not a big problem for DIY.

You don't have to put the mid box inside the woofer box though.

How is it going in Melbourne ? are you using passive or active XO ?

for my lowly advice, please feel free to send some four and twenty meat pies

Hartono
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Old 10th June 2007, 08:10 AM   #3
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Hi Hartono,

~ I gather youíve had the pleasure of Melbourneís Four and Twenty pies?

Thanks for your advice, I suspected putting the mids in a separate box has advantages.

In this case I will put the mid box inside the woofer box, as I came across a very nicely made woofer box that I donít have the box skills or woods to match in a separate box. The outputs donít match and I have an ok 6 channel amp: Iíll go active XO.

Do the thinking/ philosophies about box building, eg mass & bracing, apply to mid boxes inside woofer boxes?

thanks
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Old 10th June 2007, 08:27 AM   #4
Hartono is offline Hartono  Indonesia
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Hi Rick57,

" ~ I gather youíve had the pleasure of Melbourneís Four and Twenty pies? "

Yummy..............

"Do the thinking/ philosophies about box building, eg mass & bracing, apply to mid boxes inside woofer boxes? "

Yes it would be the same, except you don't have to do any finishing . You might like to browse a bit on Diyaudio.com for building and bracing box, plenty of good information, as my experience in woodworking is limited I can not offer much handy information.

wish you good luck, with your project !!


Hartono
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Old 10th June 2007, 08:32 AM   #5
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A good approach to a mid enclosure is a terminated transmission line. Just build it long and tapered, as large as is reasonable, and stuff it full of polyfill. The idea is to kill the back wave.

If you were to have the mid in the same enclosure space as the woofer.... it's bad. As the woofer pressurizes the box, it pushes the mid outwards. This causes unwanted excursion which will cause harmonic and intermodulation distortion. Even if the box for the mid doesn't go to any heroic lengths to prevent internal reflection if the sound it radiates into its own box, it's still better than leaving it exposed to the effects of the woofer.
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Old 10th June 2007, 08:46 AM   #6
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Thanks Joe,

I donít have space here for a transmission line

Wrt mid boxes inside woofer boxes

> Yes it would be the same, except you don't have to do any finishing
Thatís a big advantage to me right now

I was thinking of low mass mid boxes; surrounded by high mass eg lead lining, between the mid boxes and the woofer boxes: would the high mass lead between the boxes be useful?
Cheers
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Old 10th June 2007, 09:39 AM   #7
terry j is offline terry j  Australia
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gee, I thought that all mids needed seperate boxes??? otherwise the pressure from the bass unit could-at worst- blow the mid.

Maybe I've misunderstood the question ar at least have been sold a whopper (not some sort of meat pie ).

Tweeters of course are (usually) always sealed so won't suffer from the bass pressure.

If I got that wrong I'd love someone to explain.
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Old 10th June 2007, 09:45 AM   #8
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Quote:
Originally posted by rick57
Thanks Joe,

I donít have space here for a transmission line

Wrt mid boxes inside woofer boxes

> Yes it would be the same, except you don't have to do any finishing
Thatís a big advantage to me right now

I was thinking of low mass mid boxes; surrounded by high mass eg lead lining, between the mid boxes and the woofer boxes: would the high mass lead between the boxes be useful?
Cheers
What do you hope to accomplish with this method? The first concern would be that the mid enclosure keep any air pressure from the woofers from modulating the mid. This would best be achieved with a really stiff box. Considering how small the mid box probably needs to be, single layer 0.75 inch MDF or plywood with a single brace across it would be more than adequate. Constrained layer damping, in my opinion, would be overkill for the task at hand.

The second concern with the mid enclosure is to minimize the ill effects of the rear wave firing into a box. The best method after a transmission line (you might be surprised how small these can be- check out B&W) would be a "golden ratio" shaped box.
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Old 10th June 2007, 12:26 PM   #9
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Having separate boxes is a bit like having double-glazed glass instead of a really thick single layer.

As long as you know what you're doing in terms of creating a sound barrier that prevents crosstalk between the bass and midrange, then the final design should be determined by other considerations like: baffle diffraction,...

What if you want to be able to upgrade the midrange without changing the woofer? Or adjust the time alignment to suit the furniture?
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Lech
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Old 10th June 2007, 06:39 PM   #10
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Rick the benefit is huge as long as the mid is not cramped into too small of an enclosure.

THe midrange is actually the most important driver in a three way system unless it is high passed at a very high frequency like some of the older very cheap commercial systems. But in those systems you really had the woofer doing most of the midrange activites so I will not consider them.

Also, if you want the best midrange sound in a sealed enclosure make sure it is critically damped. If you isolate the midrange in a high Q sub enclosure the sound will not be that good.

You should think of the midrange as a full range driver that needs the highest of fidelity and isolated as much as possible both acoustically and physically from the other drivers.

The midrange should ideally cover the 80-3000 Hz range with perfect fidelity which is where nearly all the fundemental tones in music and voice are.

If this area is not handled right the speaker can never be more than mediocre sounding no matter how good the bass and high end are.

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