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.
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.
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
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
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.
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.
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.
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.
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