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Old 17th December 2007, 08:26 PM   #1
TWOJZ is offline TWOJZ  Australia
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Default crossover/attenuation guidance?

alrighty, I'll be making some speaker boxes for my brother for christmas, using these tweeters:

tweeter

and TWO of these woofers per box, in an isobaric setup:

woofer


I plan on crossing them over at 3500Hz, 12dB per octave, but how much will the tweeter need to be attenuated?

keeping in mind, that the two woofers will be wired in parallel, to give 4 ohms, however, they will be loaded isobarically...

does this mean the sensitivity of the woofers, with 2.83 volts will be the same as a single woofer?

thanks for your help people!
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Old 17th December 2007, 08:59 PM   #2
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Isobaric is not recommended:
http://www.diysubwoofers.org/talksho...ages/13010.htm

Using Synrta to plot the CW2135 FR, they're only good up to ~1600Hz (Don't have the plot handy, it's on my other computer).

In a 54L box tuned to the low 30's, a pair aren't bad as part of a 3 way system.....but it seems you're trying to make something smaller???
I tried then in a 2 way using a SEAS tweeter crossed at 1400, worked pretty well, but not as clean as my Peerless speakers.

If you don't have the drivers already, Altronics has a tweeter at a similar price that has a lower Fs than the Jaycar one. (& a 5" driver that may go higher yet give a similar low end response)
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Old 17th December 2007, 09:48 PM   #3
Thawach is offline Thawach  Thailand
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Hellow! Twojz


You have 6" two drivers. Why don't you design the Symmetry loudspeaker.? Isobaric,Two woofers as the same single driver.
To be sure,It need to be attenuated.
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Old 18th December 2007, 07:08 AM   #4
TWOJZ is offline TWOJZ  Australia
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ok, so, anyway, I'm making an isobaric setup using these speakers, I will be crossing over at around 3500Hz, but I'm not sure if the efficiency of the two woofers, wired in parallel, will be the same as one woofer...

thanks.
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Old 18th December 2007, 07:13 PM   #5
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how do multiples of the same driver in a cabinet affect sensitivity?
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Old 18th December 2007, 07:14 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

They will be same sensitivity but half the impedance.

Quite frankly you will waste a lot of box volume going isobaric,
(cannot use clamshell arrangement) and whatever the midrange
response is like likely it will get worse with the isobaric chamber.

Slapping unmeasured drivers in configurations not really suitable
with a "cookbook" crossover that will be certainly wrong is not
the way to do it. Unknown responses = unknown problems.

FWIW after implementing baffle step the allegedly 94dB tweeter
will need attenuating 12dB or so to match the BSC equalised
82dB or so sensitivity (from 87dB) of the isobaric array.

/sreten.
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Old 18th December 2007, 07:35 PM   #7
TWOJZ is offline TWOJZ  Australia
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Quote:
Originally posted by sreten
Hi,

They will be same sensitivity but half the impedance.

....

82dB or so sensitivity (from 87dB) of the isobaric array.


that doesn't make any sense.
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Old 18th December 2007, 08:03 PM   #8
TWOJZ is offline TWOJZ  Australia
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and thats just awesome how that thread has nothing to do with isobaric loading....

ok, so, if I have two woofers, in an isobaric setup, but wired in parallel, how is the overall sensitivity of the woofers effected,

I believe you lose 3dB by isobaric loading, correct?, but seeing as the woofers are now at 4 ohms, each woofer is getting the same power, so, at 2.83 volts, I have twice the power of a single driver, so, at 2.83 volts, wont the sensitivity be the same?

I need to know how much the tweeter needs attenuating....
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Old 19th December 2007, 09:25 AM   #9
Thawach is offline Thawach  Thailand
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Hi! Twojz

When two woofers will be wired in parallel or series the sensitivity be the same. But 4 ohm, The sound level is higher than 8 ohm. I don't know how much. I think of it's not over than 3 db.


Thanks
Thawach
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Old 19th December 2007, 09:32 AM   #10
TWOJZ is offline TWOJZ  Australia
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its twice the power, so, while the 8ohm tweeter is getting 1 watt, the 4 ohm woofer is getting 2 watts, and is therefore 3dB higher when designing crossovers,

so, Im working to the assumption, that the woofers, in an isobaric setup, loose 3dB 1w/1m, but at 2.83 volts (same as the tweeters 1 watt measure) they are back up to the original 1 watt efficiency (~87dB) so, I will be attenuating the tweeter by 7dBs
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