sensitivity calculations

michael · 12th December 2003, 05:00 AM

I am designing the XO for an MTM centre, the woofers have a sensitivity of 88db whilst the tweeter is 91db the woofers are 8ohm and have an Re of 5.7ohm, the tweeter is 6ohm and has an Re of 4.6ohm.
Im using XOsim to work out the figures, because the 2 woofers in parralel will be 91db this matches the tweeter, but the impedance will be lower,
do i need to compensate for this?

michael · 12th December 2003, 07:02 AM

bump

sreten · 12th December 2003, 08:17 AM

The two woofers is paralell raise sensistivity by 6dB.

However you need to take baffle step correction into
account, this will reduce midrange sensistivity by 6dB.

So your tweeter needs to be padded by ~ 3dB.

Tried checking out XOsim, can't find it,
many crossover sim's are far too simplistic.

sreten.

michael · 12th December 2003, 08:34 AM

so how do i make a baffle step circuit?
are there any decent calculators online?
and also how to i pad the tweeter?

sreten · 12th December 2003, 08:47 AM

Quote:

Originally posted by michael
so how do i make a baffle step circuit?
are there any decent calculators online?
and also how to i pad the tweeter?

a) Is complicated by the intended positioning of the centre
speaker, but basically a first order L or L//R in series
with the drivers followed by the crossover components.

b) I can't find a freeware one as yet, I've been looking.

c) R in series and parallel with the tweeter.

sreten.

michael · 12th December 2003, 10:31 AM

okay thats good info thanks sreten.

how do i workout what value that is of the L or L//R in the step. is there an equation to work it out, here is my XO so far.

michael · 12th December 2003, 10:32 AM

dang the pic didnt work, just go here

sreten · 12th December 2003, 10:58 AM

Its too simplistic.

The important thing is that the crossover is properly designed
to take into account the bass units inductance, the tweeters
fundamental resonance, any frequency response quirks of the
drivers, different sensitivities of the drivers, phase response at
crossover including the drivers phase response and offset, baffle
step compensation according the baffle width, acoustic summing
of the drivers at crossover, and off-axis dispersion issues.

You also have twin drivers - which needs to be accounted for.

Why I can't find a good freeware simulator.
They don't take acoustic issues into account.

sreten.

michael · 12th December 2003, 09:16 PM

here is the prgram i used so far http://www.geocities.com/f4ier/newxoversim.htm
there are some functions in it that i do not know how to use

sreten · 13th December 2003, 07:45 PM

I've seen this sim before, I'll check it out some more,
but it does not seem to be able to model the MTM
configuration acoustically.

sreteen.

12th December 2003, 05:00 AM	#1
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	sensitivity calculations I am designing the XO for an MTM centre, the woofers have a sensitivity of 88db whilst the tweeter is 91db the woofers are 8ohm and have an Re of 5.7ohm, the tweeter is 6ohm and has an Re of 4.6ohm. Im using XOsim to work out the figures, because the 2 woofers in parralel will be 91db this matches the tweeter, but the impedance will be lower, do i need to compensate for this?

12th December 2003, 07:02 AM	#2
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	some one please help.... bump

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12th December 2003, 08:17 AM	#3
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	The two woofers is paralell raise sensistivity by 6dB. However you need to take baffle step correction into account, this will reduce midrange sensistivity by 6dB. So your tweeter needs to be padded by ~ 3dB. Tried checking out XOsim, can't find it, many crossover sim's are far too simplistic. sreten.

12th December 2003, 08:34 AM	#4
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	so how do i make a baffle step circuit? are there any decent calculators online? and also how to i pad the tweeter?

12th December 2003, 10:31 AM	#6
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	okay thats good info thanks sreten. how do i workout what value that is of the L or L//R in the step. is there an equation to work it out, here is my XO so far.

12th December 2003, 10:32 AM	#7
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	dang the pic didnt work, just go here

12th December 2003, 10:58 AM	#8
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Its too simplistic. The important thing is that the crossover is properly designed to take into account the bass units inductance, the tweeters fundamental resonance, any frequency response quirks of the drivers, different sensitivities of the drivers, phase response at crossover including the drivers phase response and offset, baffle step compensation according the baffle width, acoustic summing of the drivers at crossover, and off-axis dispersion issues. You also have twin drivers - which needs to be accounted for. Why I can't find a good freeware simulator. They don't take acoustic issues into account. sreten.

12th December 2003, 09:16 PM	#9
michael diyAudio Member Join Date: Sep 2003 Location: Adelaide Hills	here is the prgram i used so far http://www.geocities.com/f4ier/newxoversim.htm there are some functions in it that i do not know how to use

13th December 2003, 07:45 PM	#10
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	I've seen this sim before, I'll check it out some more, but it does not seem to be able to model the MTM configuration acoustically. sreteen.

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