Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!sdd.hp.com!spool2.mu.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: jj@alice.att.com (jj, like it or not)
Newsgroups: rec.audio.high-end
Subject: Re: Crossover design
Message-ID: <9226@uwm.edu>
Date: 30 Jan 91 13:43:50 GMT
Sender: news@uwm.edu
Lines: 82
Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu

In article <9172@uwm.edu> dlin@prodigal.psych.rochester.edu (Daniel Lin) writes:
>
>	Engineering Society have focussed on high order crossovers, while
>	providing little or no information on the optimization of "simple"
>	first order designs. 
There are a few reasons for this.  First, since the excursion
of a driver falls off with the second derivitive, i.e. with
f^2, you will find, in general, that you get insufficient
excursion control with a first order high-pass filter.  This can
cause both bad sound (modulation by driver saturation or non-linear
excursion at lower frequencies than you expect) and driver damage.

>	Assuming that
>	one is interested in designing an all first order network for a
>	three-way speaker system utilizing "ideal" drivers- that is, the
>	drivers are well behaved beyond their pass bands, show a smooth
>	change in directivity across frequencies, and are free of
This means that all your drivers need to extend about 3 octaves
beyond the cutoff frequency you've chosen.  This is going to be
a tough problem.  What is the input characteristics of those
drivers across the same frequency range going to be like, too?

>	The
>	literature suggests that equations used to determine low and high
>	pass filter componenets cannot be applied to design the bandpass
>	filter due to interactions between components.
This is quite true, unless you figure the (complex) impedence of the
driver into your equation.  You will find out very quickly that
you will then have no first-order functions anyhow.  For instance,
let us assume you use a direct-series-capacitor for a high-pass
filter.  Assuming you do this, what is the response of the system
near the resonant point of the tweeter?  It's WELL above what you expect,
because of the order-of-magnitude peak in the magnitude of the
tweeter impedence.  Many, MANY first-order crossover designers haven't
taken this into account, and have had nasty nasty peaks just below
the crossover frequency.  Now, given some need for efficiency equalization,
this can be mitigated by absorbing most of the energy in the
resistive pad, BUT you have to think of that first, and have
appropriate efficiencies for your driver+box combo.

>	What kinds of
>	calculations are required to determine the necessary adjustments?
It's not easy to say. Each type of driver (sometimes with drivers with
bad QC, each DRIVER) has different characteristics.  Furthermore,
it's very hard to come up with an analytic model (pole/zero) for
most drivers, because of the many delay components and non-linearities
that are present, even if you have the appropriate equipment and
necessary math calculation abilities.  (And these are not easy
to come by, either, nor are they cheap. In addition, you will have
to decide level and frequency content for the model's applicibility,
because you will find that drivers are (*&(*& non-linear.)
>	Are any adjustments needed for woofer's low pass or the tweeter's
>	high pass crossover values? 
Typically, the inductor for the woofer will need to be slightly
larger thanyou expect, and the cap for the tweeter will be substantially
smaller.  These statements, however, are subject to much, MUCH
qualification!!!! You'll be better off measuring SPL individually 
at each driver and then in sum, and making a guess as to
what's happening, I suspect.

>		I suppose that once these questions are answered, I'll have
>	to return to real world conditions to begin to optimize around
>	driver limitations and cabinet effects. 
I think you'll find your life quite difficult.

The design of passive high-level crossovers is by no means
reduced to any reliable process.  The loads for the crossovers
are quite idiosyncratic, the parts accuracy is low, the
manifacture of accurate parts is tricky, and so on.

I've been predicting for years that people will start making
integrated amplifier/speakers, with all the filtering done passively.
So far, the integrated (circuit) amps aren't quite good enough
(although they COULD be, the manifacturers don't see their
market in quality), and the engineering (although well
known and understood) hasn't been present in a place
where it can be sold.
-- 
       -------->From the pyrolagnic keyboard of jj@alice.att.com<--------
Copyright alice!jj 1990,  all rights reserved,  except transmission  by USENET and
like free facilities granted.  Said permission is granted only for complete copies
that include this notice.    Use on pay-for-read services specifically disallowed.