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Old 13th October 2007, 05:18 PM   #1
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Question Midrange enclosure volume and EBP question

I keep returning to the same state of confusion on the matter of the midrange chamber in a 3-way.

The driver I'm working with has QTS .35, QES .42, Fs 65, and Vas .18 ft3. Its faceplace measures 5" in diameter. It will be a midrange crossed at 1k and 3.8k. Speaking in closed-box terms, I'd like to keep Qtc around .58.

All of the reading that I've done so far focuses on low frequency response - how to acheive better low frequency response from a closed box or how to do the same from a vented box. Here's the problem I keep running up against...

The EBP of this driver is way up at 155, which means that this driver is obviously suited for use in a vented enclosure, correct?

So let's say we go with a vented box with a 1 3/8" port. Vb Optimal = .133 ft3 or 229 in3. That's a pretty small box! The tuning freq really doesn't matter since it's handling much higher freqs. If the width (internal) is 6" - the height has to accomodate both the driver and the port and the depth has to accomodate the port length (~5") + at least 2 1/2" - then everything I do to roughly approximate the acoustic ratio pushes the volume way up.

Same thing with a closed box... Qtc of .58 = Vb .103 ft3 or 178 in3. That's even smaller! I don't have to accomodate the port here, but I wonder about the high EBP and using this driver in a closed box. The depth of this driver is 3 3/16". How close can you move the rear of the enclosure before causing serious problems?

All this talk about (sub)woofers, when perhaps the real mystery is in the midrange!
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Old 14th October 2007, 12:30 AM   #2
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Quote:
Originally posted by marscoast

All this talk about (sub)woofers, when perhaps the real mystery is in the midrange!
Greets!

Duh! Well, mid-bass, mid-range, and HF since the lower the fundamental the wider its BW. A resonant system wants to 'feel' a matching compliance, or ~311"^3 and since it's a point source it needs a 0.5 Qtc for transient 'perfect' response and combined with a ~371 Hz mass corner a 1 kHz XO point seems an odd choice to me. Anyway, closed back mid-range have a lot less rear chamber Vb and work quite well with just a little felt damping to attenuate reflections back through the diaphragm, so not sure what you're concerned about: http://www.partsexpress.com/pe/showd...number=280-020

WRT venting higher BW drivers, ~aperiodic is the norm, so stuffing the vent is desirable and using a TL normally yields a nice compromise to all the conflicting requirements, though at ~0.463 ft^3 it may be too big for your app, but maybe it can be shrunk a bit due to the 1 kHz XO point.

WRT EBP, it's just a gross first approximation guideline and pretty much worthless at the extremes, so I personally pay no attention to it. I mean my bass bin drivers have a 100 EBP, so being at the cusp of pure vented Vs suitable for sealed or vented they seem an obvious choice to use in a HIFI app, but with a 20 Hz Fs, 0.2 Qes, nothing could be further from the truth overall in today's 'small' world.

AFA putting a high EBP driver in a sealed cab, there's nothing wrong with it, just that it won't have a low F3, like you care with a 1 kHz XO point.

GM
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Old 14th October 2007, 02:54 AM   #3
rinx is offline rinx  Estonia
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Hej! Maybe its wrong question in this contects. But I am also in 3-way planning, so I was wondering how should I measure box for midrange. From what it depends.

Maybe some webpages for me

Thank You!
Ringo
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Old 14th October 2007, 06:36 PM   #4
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Greets!

Sorry, I don't understand the question. I mean ideally you should measure the driver's T/S specs, frequency, electrical and acoustic phase response, and measure it on the baffle size/shape you plan to use to see how it affects its response, so what else needs measuring for design purposes?

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Old 14th October 2007, 09:57 PM   #5
rinx is offline rinx  Estonia
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Hej GM!
What i wanted to know, is that if I already know T/S then how big should be box for driver. The thing is that I have heard-for midrange use aprox. 6-8L box and a lot dumping material. Then you get best freq. level.

So, what i should do-put just approx. (it dont sound like a good idea) or calculate with some audiosoftware (Bassbox) a box for midrange?!

Did I explain things correctly? Again my not so good english...
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Old 15th October 2007, 04:53 AM   #6
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Greets!

OK, since you have the specs and BassBox's ability to help you with designing a multi-way, let it help you design the appropriate Qtc sealed cab that works best with the XO.

GM
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Old 15th October 2007, 07:17 AM   #7
kstrain is offline kstrain  Scotland
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Default Re: Midrange enclosure volume and EBP question

Quote:
Originally posted by marscoast

The driver I'm working with has QTS .35, QES .42, Fs 65, and Vas .18 ft3. Its faceplace measures 5" in diameter. It will be a midrange crossed at 1k and 3.8k. Speaking in closed-box terms, I'd like to keep Qtc around .58.
<snip>
Same thing with a closed box... Qtc of .58 = Vb .103 ft3 or 178 in3. That's even smaller! I don't have to accomodate the port here, but I wonder about the high EBP and using this driver in a closed box. The depth of this driver is 3 3/16". How close can you move the rear of the enclosure before causing serious problems?
Does the driver have a vented magnet? If not the back of the box can come very close. Avoid narrow "ducts" where the air speed would become high as the cone moves a lot. Boxes do not have to be rectangular (many closed-back mids have cylindrical chambers).

When you've chosen the system Q you want you know the box size (with this driver, if the params are correct).
As another said, make this choice as part of crossover design.

If you can make the small box fit the driver with good internal air flow you've done the job. I see no point in vented for this application.

Small boxes have fewer modes in the band, those are mainly at higher frequencies and so they are more easily damped by your stuffing material.

Ken
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Old 15th October 2007, 07:36 AM   #8
Fosti is offline Fosti  Germany
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Default Re: Midrange enclosure volume and EBP question

Quote:
Originally posted by marscoast
...The driver I'm working with has QTS .35, QES .42, Fs 65, and Vas .18 ft3. Its faceplace measures 5" in diameter. It will be a midrange crossed at 1k and 3.8k. Speaking in closed-box terms, I'd like to keep Qtc around .58.
...
Hi,

you are aware of the fact that Qtc describes only the behaviour of the system around its build in resonant frequency fc, which is in your case around 100 Hz and which is far below the highpass XO to the midranke of 1000 Hz! So Qtc is meaningless for your midrange!

Cheers,
Christoph
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Old 15th October 2007, 08:19 AM   #9
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You're prolly gonna have to play around with an enclosure size and adjust by ear. I myself have done a 3-way (speaker) in which the mid is more of a wide band driver, x-overed at 400hz. I used three different rear baffles for the midrange before I found one that sounded right. Instead of trying for the minimum enclosure size, might I suggest an 'infinite' enclosure size (for the mid).
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Old 16th October 2007, 05:12 PM   #10
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Fosti,
"Qtc describes only the behaviour of the system around its build in resonant frequency"

Yes, that's one of the things that has made the midrange enclosure confusing to me. The equations we are given for box design are tools to help us achieve good response at low frequencies (and help us deal with the infinite number of compromises therein!).

So I do the math and come up with a pretty small box with an Fcb around 102 and F3 around 139... but none of these numbers seem to apply to this application and I'm left wondering if I've wandered off the path. BTW, this low-to-mid crossover is active, so I do plan on fiddling with my crossover point; but even if I came down to, let's say 650, I'm still far away from those numbers and wondering how it all applies.

I guess my point is that, in all the reading I've done thus far, no one mentions the matter of cabinet design for a speaker that's aimed specficially at... let's just arbitrarily say the three octaves between 512Hz and 4096Hz.

And my other point is that it sure is hard to accomodate BOTH a real small enclosure AND the acoustic or golden ratio.

So, Ken's point is well-taken: go with the small, sealed box and strive for good air flow. 3-LockBox's is too: experiment with it. I guess it's time to stop thinking so much, get to work, and let my ears decide!
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