Path: utzoo!attcan!uunet!ogicse!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: jroth@allvax.enet.dec.com (Jim Roth)
Newsgroups: rec.audio.high-end
Subject: Re: crossover design
Message-ID: <9136@uwm.edu>
Date: 25 Jan 91 13:54:43 GMT
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Approved: tjk@csd4.csd.uwm.edu
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In article <9101@uwm.edu>, dlin@prodigal.psych.rochester.edu (Daniel Lin) writes...
 >                           Weems then goes on to present formulas to
 >determine the required components for that crossover frequency(i.e. both
 >drivers are 3 dB down at the desired crossover frequency). 
 >However, other articles concerning proper
 >crossover design have stated that both drivers should be 6 dB down at the
 >crossover frequency in order to achieve a flat summed response at that
 >frequency. Is there a contradiction here? If there is, which one is
 >correct, and how does one determine the proper components for the
 >crossover? Any tips concerning these questions would be greatly
 >appreciated. 
 
 This is a complicated issue that I can't really do justice to in a simple
 reply.  But basically it depends on what type of crossover filter and
 the relative phases of the signals sent to the drivers at the crossover
 point.

 For example, if you use 3'rd order Butterworth filters, then they should
 be 3 dB down at the crossover frequency so that the summed power response
 on axis will be flat - the signals will be 90 degrees out to the drivers
 at the crossover point.

 On the other hand, a Linkwitz-Riley (cascaded Butterworth filter) crossover
 keeps the signals sent to the drivers in phase at all frequencies, so the
 -6 dB figure is correct.  The advantage here is a better behaved polar
 response.

 Even this is oversimplifying things becasue you have to take into account
 the acoustic response of the drivers, as well as their electrical impedances
 (if a passive crossover is being developed) and modify the filter shapes
 and component values accordingly.

 It's not so simple when you "reduce it to practice".  If you use the
 network and equations in Weems' book, presumably they will be for a nominal
 crossover phased 90 degrees at the crossover point.

 A really good way to learn more is from the Speaker Builder magazine,
 as well as the loudspeaker article anthologies available from the Audio
 Engineering Society.

 - Jim