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Old 1st November 2005, 11:08 AM   #1
tcpip is offline tcpip  India
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Default Any thoughts on midrange enclosures?

What are the Good Things about midrange enclosures one can keep in mind?

If I'm one of those poor souls not yet ready to build an OB mid section, then what kind of enclosures are good for midranges? Is a simple rectangular box the best that one can do? Will simple non-parallel sides be an audibly better alternative? What about something like this?
Click the image to open in full size.
In this design, the rear wave doesn't bounce back straight to the cone... it goes bouncing around first to the floor of the box, then to the top, and so on in a generally dissipated and somewhat more confused way. Is this audibly better than a rectangular box? Would Jim Griffin's JX92S-GS2i be audibly better if he'd used this box design compared to his rectangular box? How much difference, audibly, do non-parallel sides and sloping backs make?

What if I extend this design to make the rear wall slope at something like 15-deg to the horizontal, instead of the current 45-deg shown in this drawing? Will this approach the Nautilus tapered tube? Will this completely absorb the rear wave and make the reflected wave zero?

What if I make this sort of pointed and narrow tapering enclosure and then stuff just the pointed tip with some tight-packed fibres to absorb any waves which do reach it?

I even see ported midrange enclosures in three-way systems. I thought all this trouble with a ducted port was to extend the bottom end, and a sealed box would give better transients and so on, which would be much more desirable than brute low-end extension in the case of a midrange enclosure. So, why would anyone use a ported enclosure for a mid in a 3-way? I think either the Stradivari Homage or the Armati Homage uses such an enclosure, and they are no-expense-spared commercial designs.

I've read Dickason, but I can't seem to get any really clear treatment about midrange enclosures. I see only hints in all sorts of forums and articles ("Use non-parallel sides", or "Put a Blackhole panel just behind the cone" or "fill the midrange box with stuffing but lightly, lightly", and other similar words of wisdom), and I'm generally confused. If I'm going to go all out and use some fairly expensive drivers (the JX92S) as midranges in three-ways, I'd like to take that bit of extra trouble in designing the midrange enclosure to get the clearest mids. What should I do?

In general, can you share all your thoughts about midrange enclosure design? Me newbie, I'd learn a great deal.
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Old 1st November 2005, 12:33 PM   #2
Zaph is offline Zaph  United States
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How about the attached midrange enclosure below? Similar concept to one you mentioned above. I have not built this particular one, but have have built a couple simpler ones based on the same concept, and I suspect performance is improved particularly in the lower midrange and midbass. I'm encouraged to look for measurement techniques that "prove the improvement".

I'm working on a project right now that has a similar tapered midrange enclosure. In general however, experience has told me that oversize midrange enclosures work a lot better than undersize. Those notorious plastic midrange cups are the worst.

John
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Old 1st November 2005, 02:47 PM   #3
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Zaph, if you haven't seen it already, you should download B&W's white paper on the new 800D speaker system. They gave some very interesting insight (propaganda mostly) into the shape of their midrange enclosure. While you've got to take it with a grain of salt, they did have some very interesting comments about their sphere-with-a-cone shape.
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Old 1st November 2005, 03:43 PM   #4
JohnL is offline JohnL  United States
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Zaph, I understand there may be a benefit to placing the driver a bit down into the line as you have (I am no TL authority at all), but I would think it might be more beneficial to isolate the rear of the tweeter from the woofer backwave...

Just a thought
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Old 1st November 2005, 04:41 PM   #5
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If I'm one of those poor souls not yet ready to build an OB mid section
You can still get much of the benefit, i.e. the "boxless" sound, by simply leaving the back of the box open and stuffing it lightly. Internal box reflections are gone but the polar response is still that of a monopole in the midrange. Note, this is only for true midrange drivers in a 3-way. It won't work very well if you want the driver to play down into the bass region.
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Old 1st November 2005, 06:02 PM   #6
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Quote:
Originally posted by catapult
You can still get much of the benefit, i.e. the "boxless" sound, by simply leaving the back of the box open and stuffing it lightly.
An aperiodic TL (stuffing getting denser as you approach the terminus works well... similar concept to what Zaph posted but can be quarter wave instead of half-wave. Simplier build as well... a chunk of PVC pipe will do, or a rectangular wood tube (some taper would be even better)

Here is a realization of Zaph's concept (B&W Nautilus prototype)

Click the image to open in full size.

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Old 2nd November 2005, 01:00 AM   #7
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I think it's worth mentioning the idea that the two parallel sides in the coiled TL can cause problems if the distance between them approaches a certain fraction of a wavelength within the bandwidth of the driver, especially if this occurs much below the high 100s of hz, where stuffing is less effective.

For example, if the driver were a woofer operating up over 500 hz in a coiled line with 12" seperation, I would expect some problems at about 500 hz. It might be subtle, and would likely be more of a dip than a peak, but for so much effort in construction it seems worth considering such things.
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Old 2nd November 2005, 12:52 PM   #8
tcpip is offline tcpip  India
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Thanks guys. It's more interesting stuff than I'd thought I'd get.

It seems all of you are inclined towards long, tapered pipes. Are these things modelled using MK's models, just like the TLs we build for bass extension? Is such modelling even necessary for pure-mid enclosures where we know we'll roll off the mid driver electrically before its acoustic bottom rolloff is reached?

And catapult, your idea of an open-back box is interesting, though I'm not quite sure how it'll be a monopole, not a dipole, at the mid frequencies. Will the backwave absorption due to the stuffings be adequate to completely convert it to a dipole? And what about the pipe resonances (I think that's what it's called?) of the open-back box acting as a short pipe? Won't standing waves be built inside it? Will the stuffing be sufficient to kill these standing waves? And yes, I'm thinking pure midrange, ie not a midbass which reaches to the bottom of its acoustic range.
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Old 2nd November 2005, 03:41 PM   #9
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Quote:
Originally posted by tcpip
Thanks guys. It's more interesting stuff than I'd thought I'd get.

It seems all of you are inclined towards long, tapered pipes. Are these things modelled using MK's models, just like the TLs we build for bass extension? Is such modelling even necessary for pure-mid enclosures where we know we'll roll off the mid driver electrically before its acoustic bottom rolloff is reached?
The pipe doesn't have to be all that long, only long enough so the absorptive material can work on the backwave. Non-parallel sides increase the effective length of the material by causing reflections at an oblique angle (draw a wave bouncing inside a cone for a visual representation).

Too long can be bad for some applications. If you want to use the midrange rolloff as part of the crossover, then you want to make the largest dimension smaller than (resonance wavelength)/10, otherwise you're dealing less with a tuned cabinet and more with a transmission line. For example, say your mid cabinet has a Butterworth alignment at 100 Hz, perfect for a 4th order Linkwitz-Riley at 100 Hz, which means you only need a 2nd order B'wth at 100 Hz to finish the job. The wavelength at 100 Hz is 135 inches (about 460 cm), so the furthest point from the driver shouldn't be more than 13.5 inches (46 cm). That depth is still enough to to attenuate the backwave greatly with any competent absorptive material.


Cheers,
Francois.
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Old 2nd November 2005, 03:51 PM   #10
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Quote:
Originally posted by tcpip

Will the backwave absorption due to the stuffings be adequate to completely convert it to a dipole?

And what about the pipe resonances (I think that's what it's called?) of the open-back box acting as a short pipe? Won't standing waves be built inside it? Will the stuffing be sufficient to kill these standing waves?
1. You mean monopole?

2. The effect is negligible if the walls are short (and especially if they're wide apart), but the stuffing will attenuate the pipe resonances, if any.

If the high-pass of the mid is 300Hz, then the minimum depth required for a 1/4 lambda pipe resonance is 0.2867 meters, or approximately 11-1/4". If you make the box wider, rather than deeper, you can get the same internal volume without any worries of resonance.
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