disadvantages of using second woofer for BSC... - diyAudio
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Old 1st October 2004, 12:39 PM   #1
navin is offline navin  India
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Default disadvantages of using second woofer for BSC...

what are the DISadvantages of using a 2nd woofer for BSC with fullrange/widerange drivers?

this is a 2 part question

i have been looking at some small drivers like the Jx92 and FX120 to be used in a fullrange. these drivers are not very sensitive and the FX120 has very limited power handling.

The JX92 is about 88db/1w and can absorb about 17dbW in power giving a midrange SPL of about 105db (although i dont think it can deliver this at 100Hz).

the FX120 however is 89db/1w and can only absorb 10dbW in power giving a midrange SPL of about 99db. this driver might be even more limited in the LF (below 200Hz) due to it's smaller Xmax.

Q1.
what if one were to mate drivers like these (some have done his with the TB 871) with a second driver that is not as expensive but has the same sonic signature. the 2nd driver need not have the HF response of the JX92/FX20 etc... as it would be rolled off at 6db/oct about 200-500hz (depending on baffle design and speaker location).

Q2.
if one were to use a second driver for BSC and the system were to be horizonally placed would the listener be subjected to polar interference? the XO freq in these cases is rather lower than most tweeter/woofer combos and hence the wavelengths are reasonably large so i think there might be no audible distortion on this account.

eg: a tweeter/woofer combo has a XO freq at about 2-5kHz while we are talkng at 200-500Hz an order of 10 lower.

BTW
can anyone recomend a woofer for this purpose (to mate with the FX120 or JX92)?
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Old 1st October 2004, 02:07 PM   #2
rabbitz is offline rabbitz  Australia
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The main disadvantage I can see by using a 0.5 driver for BSC is the cost, but doing it by an xo isn't cheap either and you do lose SPL. I've never done it with a full range driver, only an identical mid woofer and I can imagine it would be hard to find something that is a sonic match as well as SPL.

Regarding Q2, if I understand things properly, the box hides any possible interference.

I can see 3 options.

1. Use the speaker in 2Pi so it doesn't need BSC.
2. Add another driver (identical or one that is suitable) and roll off at BS F3.
3. Use an identical driver at the rear so it becomes a Bipole.

I'd go for No3 as it would increase the SPL and reduce the xmax by half in one swoop as well as getting rid of BS.

My thoughts anyway.

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Old 4th October 2004, 02:06 AM   #3
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Quote:
Originally posted by rabbitz
[B1. Use the speaker in 2Pi so it doesn't need BSC.
2. Add another driver (identical or one that is suitable) and roll off at BS F3.
3. Use an identical driver at the rear so it becomes a Bipole.

I'd go for No3 as it would increase the SPL and reduce the xmax by half in one swoop as well as getting rid of BS. [/B]
Dave, (planet 10) also proposes this. however i am looking to wall mount my speakers so that will not be possible.

my only real option is #2. and i have found that finding drivers that have simiar sonic signature to fullrange drivers is quite diffcult.

martin audio has done this. i recently heard a HT system that was doing a very nice job using something that looked like either Martin Audio WT15 or EM15 and 4 martin subs each using 2 6.5" woofers.
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Old 4th October 2004, 03:16 AM   #4
jdybnis is offline jdybnis  United States
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Quote:
I can see 3 options.

1. Use the speaker in 2Pi so it doesn't need BSC.
2. Add another driver (identical or one that is suitable) and roll off at BS F3.
3. Use an identical driver at the rear so it becomes a Bipole.

I'd go for No3 as it would increase the SPL and reduce the xmax by half in one swoop as well as getting rid of BS.
Quote:
Dave, (planet 10) also proposes this. however i am looking to wall mount my speakers so that will not be possible.
Wall mounting them is option 1. You won't have a problem with baffle step.

Just build and see.
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Old 4th October 2004, 04:43 AM   #5
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Quote:
Originally posted by jdybnis
Wall mounting them is option 1. You won't have a problem with baffle step.Just build and see.
i figure as long as i am putting the woofer in a box there will be a baffle and hence baffle step. only in wall mounting would negate the use for baffle step correction. right? after all whith the woofer in a box that si say 4" deep the woofer will be ON wall not IN wall. However I do agree that one has to try it and see.
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Old 4th October 2004, 05:39 AM   #6
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Actually I would have thought it should not be too difficult to find a driver that sounds similar if used up to 200-500Hz. Surely a driver of the same size, xmax and cone material would be easy to find, and I would expect that it could be made to sound the same within its passband.

I suspect that "sonic signature" becomes much easier to match as you go lower in frequency.

This could be a driver of the same brand or from another. It might be a cheaper fullrange driver. Suppose you had a Seas excel midbass. You could then add an aluminium midbass driver that is much cheaper, but has the same motor design (or so I have heard).
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Old 4th October 2004, 11:38 AM   #7
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Quote:
Originally posted by paulspencer
Actually I would have thought it should not be too difficult to find a driver that sounds similar if used up to 200-500Hz.....I suspect that "sonic signature" becomes much easier to match as you go lower in frequency.
the BSC woofer would be XOed at 6db/octave so if it is rolled of at say 300Hz it would be -3db down at 300Hz and -9db down at 600hz and only -15db down at 1200hz. so what we really need is a driver with a similar sonic signature upto 1200-1500Hz.

sonic signature i assume depends on motor design, cone materials, and chassis amongst other things.
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Old 4th October 2004, 11:54 AM   #8
rabbitz is offline rabbitz  Australia
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Quote:
Originally posted by navin


i figure as long as i am putting the woofer in a box there will be a baffle and hence baffle step. only in wall mounting would negate the use for baffle step correction. right? after all whith the woofer in a box that si say 4" deep the woofer will be ON wall not IN wall. However I do agree that one has to try it and see.
I was under the impression that if the driver was in a box and the box was against the wall you didn't have to worry about BS as there cannot be any loss from wrapping around the box as in effect it's still in 2Pi. To get BS I thought the box had to be in 4Pi so there is space between the box and the wall for the lower frequencies to escape to.

Anyone like to comment.

I'd still go for option 1, no BS if wall mounted.
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Old 4th October 2004, 12:15 PM   #9
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Quote:
Originally posted by navin


the BSC woofer would be XOed at 6db/octave so if it is rolled of at say 300Hz it would be -3db down at 300Hz and -9db down at 600hz and only -15db down at 1200hz. so what we really need is a driver with a similar sonic signature upto 1200-1500Hz.
I suspect it is less of a problem than you assume.

At 1.2k the output from the .5 woofer will be masked by the main driver and it won't contribute much. I think in fact the differences between drivers are often exaggerated as we tend to focus on the small differences. If you spend a lot of time comparing two thing that differ by 5%, you may over time become so focused on the difference that you forget they are, by and large, very similar.
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Old 4th October 2004, 01:17 PM   #10
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Quote:
Originally posted by rabbitz


I was under the impression that if the driver was in a box and the box was against the wall you didn't have to worry about BS as there cannot be any loss from wrapping around the box as in effect it's still in 2Pi. To get BS I thought the box had to be in 4Pi so there is space between the box and the wall for the lower frequencies to escape to.

Anyone like to comment.

I'd still go for option 1, no BS if wall mounted.
I tend to agree with this thought. I think a bigger issue may be the tendency of FR drivers to have a rising upper end response.
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