Question: Very high Qes on vintage Alnico driver - diyAudio
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Old 11th July 2014, 04:29 AM   #1
EWong is offline EWong  United States
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Default Question: Very high Qes on vintage Alnico driver

I recently bought a compliment of vintage alnico drivers on ebay: a pair of (rebuilt) 6.5" woofers and little horn tweeters that were taken from an old speaker called the Coral BX-200. I wanted to design a new and better cabinet for them, so I measured the Thiele-Small parameters of the woofer using the procedure at Measuring Loudspeaker Driver Parameters.

The values I got were pretty reasonable (60Hz resonance, Qms of 3.5, Vas of 27 liters) EXCEPT I measured a Qes of 1.5, making the total Q of the speaker above 1, which is way higher than any other modern 6.5" driver I'm aware of. I re-measured several times because I thought this was a mistake. But I'm pretty sure it's right. This high value totally screws up the online ported enclosure calculator I was using, so I figure this driver can't work in a ported cabinet. I know that the original BX-200 speaker used a sealed acoustic suspension design, and I've learned that, for instance, the Eminence Alpha 15 has a similarly high Qes and Qts.

So, what kind of enclosure is best for this weird driver? I hear open baffles are good for high-Q speakers. If I decided on an acoustic suspension, how would I calculate the optimum volume? Eminence gives a recommended ported enclosure for the Alpha 15 -- how did they calculate this, given the super high Q of the speaker?

Thanks in advance.
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Old 11th July 2014, 04:48 AM   #2
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An Open Baffle.

If the magnet charge has decreased (likely) then Qt goes up amoungst other changes.

dave
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Old 11th July 2014, 05:35 PM   #3
GM is offline GM  United States
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Interesting! I don’t have any measured specs of the BX-200, but of the ones I do have, all the older models all fall between 0.4-0.5 Qts with Qms ranging from ~2.5-11!, so like Dave, I assume the magnets are drained for whatever reason.

Anyway, based on a ~1.05 Qts, OB as Dave noted or TL loading.

Acoustic suspension would have to tuned to > Qts, though even at a 1.051 Qtc, net Vb = ~14,168.253 L! Any smaller and it just becomes increasingly under-damped with an increasing Qtc with decreasing net Vb, though up to a point you could just cram more stuffing in it to control the worst of it. Better then to use the TL where its ¼ WL pipe action can help damp it.

WRT the Alpha 15, look at the huge mid-bass peaking that occurs with BR loading a high Qt driver. This cab ‘rings’ like a ‘ten penny nail struck with a ball-peen hammer’! Fine for PA apps where everything is EQ’d to a fare-thee-well, but for HIFI/HT? Not a good plan unless one needs the high power handling it yields and already has the necessary frequency shaping electronics.

If you still want to use a BR with a similar response to the Alpha 15, then ~28.35 L/21.43 Hz Fb. Note that the vent will be long though, so again, TL loading is the best overall compromise.

GM
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Old 11th July 2014, 07:09 PM   #4
EWong is offline EWong  United States
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Thanks for the replies! I also considered the possibility that the magnets in the drivers had lost strength and needed to be recharged, but I stumbled on this website for a speaker rebuilder (Vancouver Audio Speaker Clinic - Magnet recharging). They claim that coral drivers (among other brands) don't usually need to be recharged unless they've been damaged -- and my speakers don't seem damaged, and both measure about the same. So, I think that there's little I can do to alter the Qts of these drivers.

Speaking of transmission-line speakers, I'm very interested in something like Lynn Olson's Ariel speakers, but I'm not a woodworker and that seems like too much of a project for me right now. I'm also not sure about large baffles because I don't have much set-up space for speakers. So, I think I'll go with large closed boxes with some stuffing. Not ideal, but maybe in the future I'll remove the drivers and try something more exotic.
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Old 12th July 2014, 03:10 PM   #5
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You're welcome!

Hmm, slapping together four 1x12 x 5 ft long knot free boards using butt joints to create a tall, ~square column TL and adding end plates, driver mounting hole and ‘vent’ cutout at the bottom front requires no more tools, skill than constructing a large sealed cab. The only ‘challenge’, such as it is, is making a base sufficient to stabilize it from being easily knocked over.

From the top edge of the baffle board, the driver cutout centerline is down 20-7/16”. At the bottom, the terminus [port] is full width [11-1/4”] and 9-3/4” i.d. high to get the same open area as the TL’s internal area, so the baffle board is only 49-1/2” long.

Damp to ‘taste’ along its length with polyfil or other suitable fibrous or open cell foam material.

GM

Note the dashed blue line is the 14 k+ sealed cab, which doesn’t go audibly lower than this ~3.71 ft^3 TL nor does it play as loud down low as shown in the excursion plot plus you may not need any baffle step compensation [BSC] depending on how much you damp it.
Attached Images
File Type: gif Coral BX-200 57 Hz TL.gif (6.7 KB, 23 views)
File Type: gif Coral BX-200 57 Hz TL excursion.gif (6.9 KB, 24 views)
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