funky T-S values; drivers any use?!? - diyAudio
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Old 13th August 2004, 11:23 AM   #1
sean99 is offline sean99  United States
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Default funky T-S values; drivers any use?!?

Hi Folks:

I found a pair of 2-way concentric in-wall speakers (labeled 'SpeakerCraft 6.5') in my neighborhood, and took them apart to get a pair of 6.5" mid/woofers and 1" fabric dome tweeters. I then measured the woofer T-S parameters using the added-mass method with SpeakerWorkshop. I intend to use these things to build my first DIY speakers, presumably sealed for ease of design.

The problem I have now is that the T-S, if accurate, seem to require a very large enclosure: for sealed with Qtc=0.71, the driver needs 157 liters, or 5.5 cu.ft., compared to just 11.2 liters, or 0.4 cu.ft., for the Vifa P17WJ.

Here's the measured T-S given by SW:
Fs=59.6 Hz
Vas=16.17 liters, or 0.571 cu.ft.
Qts=0.676
Qms=4.928
Qes=0.783

This is a 6.5 inch/13.6cm driver.

With a sealed box of 157 liters (!) F3 is 62.3; with a more reasonable (but still large-ish for a 6.5" driver?) 45.4 liters (1.6 cu.ft) F3 is 63 Hz and Qtc (Qb? that's what Brian Steele's "ported.xls" spreadsheet calls it) is 0.79

I realize that I don't *have* to have Qtc=0.71, but I assume it means *something* (and I further assume it's something bad...) that the things want so much more volume than the Vifa driver does for a given Qtc.

Are these drivers worth messing with? I have some decent-looking 3/4 inch plywood that I scrounged at about the same time, so it would only cost me some time and glue to create some enclosures and find out for sure, but I thought I'd ask for experienced opinions.

If I do plow ahead, am I correct in assuming I should stick with a sealed box design?

Using "Golden Ratio" internal dimensions of 7.75"x12.6"x20.16" I have just one shared standing wave frequency, 2.68 KHz for the 5th harmonic along depth, and 8th harmonic along height. Volume is 32.2 liters, or 1.14 cu.ft., and Qtc is about 0.83.

If I stretch the height to get tweeter closer to ear-height (w/o stand) I can avoid shared standing wave harmonics (w/in 50 Hz), using 7.75"x12.6"x28.35", giving 45.4 liters, or 1.6 cu.ft., and Qtc is about 0.79.

Are either of these designs, used with these drivers, something with a chance of sounding OK?!? Of course, proper bracing, etc., would be required as well.

OK, sock it to me! But really, thanks for any helpful thoughts at all.
-Sean
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Old 13th August 2004, 12:05 PM   #2
SY is offline SY  United States
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There are a few possibilities.

First, open baffle dipole use.

Second, put 'em in a closed box, accept a higher Q, then use electronics to drop the Q to something more like what you want. For example, if you use a box to get a Qtc of 1, then a simple RC filter ahead of the power amp with the same corner frequency as the driver-box resonance will give you something that looks like a third-order Butterworth. You'll lose some bass, but you'll have something that works very well with a sub (this is what I do with Dynaudio 17W75s).

As a side note, don't take that golden ratio stuff too seriously. Avoid integer multiples at relevant wavelengths and stuff your box; that's all you really need to do. Fibonnacci used Quad ESLs anyway.
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Old 13th August 2004, 01:03 PM   #3
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The specs of those woofers are exactly what one would expect for drivers intended for an in-wall infinete baffle alignment and that's probably the best way to utilize them.
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Old 13th August 2004, 02:19 PM   #4
Zaph is offline Zaph  United States
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Default Re: funky T-S values; drivers any use?!?

Quote:
Originally posted by sean99

With a sealed box of 157 liters (!) F3 is 62.3; with a more reasonable (but still large-ish for a 6.5" driver?) 45.4 liters (1.6 cu.ft) F3 is 63 Hz and Qtc (Qb? that's what Brian Steele's "ported.xls" spreadsheet calls it) is 0.79
A Qtc of .79 is completely acceptable.

Realize that many small commercial designs have a Qtc of over 1, which helps small woofers give the impression of more bass than there actually is.

Personally, I can comfortably go for a sealed Qtc up to about .85 before midbass boom starts to get annoying.

You could always use apperiodic damping if you wanted a smaller enclosure and don't mind giving up a little bass depth and power handling.
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Old 14th August 2004, 05:25 PM   #5
sean99 is offline sean99  United States
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Default Thanks to you all!

Just a quick "thank you" to the three of you for your guidance. OB looks interesting, but is not best for my living room, and in-wall isn't an option (I'm a renter), so I plan to go ahead with the 1.6 cu.ft. sealed box at Qtc=0.79. Thanks for your help, guys.
-Sean
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Old 14th August 2004, 07:02 PM   #6
markp is offline markp  United States
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You might want to leave them as concentric drivers to get a point source a la KEF. You can always try them that way and if you dont like it take the tweeter off and cut a hole for it.
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