Hi All,

Something that I've been thinking about for a while with the design of
speaker crossovers, but don't have the necessary knowledge to calculate
or figure out, is this:

If you were to take, for example, an 18dB/oct Butterworth high pass/low
pass crossover for a two way system, could you synthesize the same
summed electrical response (and therefore the on axis response) in
terms of amplitude and phase response if you were to lower the Q of the
high pass section, and raise the Q of the low pass section from their
standard 0.707, without changing anything else ?

(In practice you would do this by altering the L/C ratio of the
crossovers in opposite directions by a certain factor)

Why would anyone want to do this ? Recently I've come to the slow
realization of what may be one of the big downsides of having a
crossover in the first place, when doing listening/measurements between
a full range driver, and that same full range driver crossed over
normally with a tweeter.

Some classes of high pass and low pass filter have ringing at their
crossover frequency whose phase is opposite to their complementary
filter - so for example a symetrical high pass/low pass filter can have
quite bad ringing at the crossover frequency if you take only one
section on its own, but when the output of the two filters are summed
together (accoustically) the ringing is largely negated.

(My filter theory isn't strong enough here to know WHICH kinds of
filter this applies to so hopefully someone can butt in here and
correct me)

It occurs to me that this cancellation of ringing only occurs on axis,
and that even to the side on the horizontal axis this cancellation
effect will be incomplete due to the different horizontal dispersion of
the large driver vs the small one it is crossing over to.

The end result, particularly if there is a large difference in
dispersion between the two drivers as is often the case, is that the on
axis response may contain very little ringing at the crossover
frequency, and sound fine, but the ambient sound field in a typical
reflective/reverberant room due to the off axis power response of the
speaker near the crossover frequency will be predominately that of the
high pass section, as the smaller driver has the widest dispersion.

With the output from the low pass section unable to balance the output
from the high pass section in these off axis directions, the ringing
will not be canceled.

The ambient sound field may contain undesirable artificats of ringing
and in fact I think I can sometimes hear this characteristic in some of
the tests I've done.

Assuming that making the dispersion of the drivers more similar is not
an option, it occured to me why not make the Q of the high pass section
lower, so that the output of the high pass section which is dominating
the ambient sound field near the crossover frequency is free of
ringing, (or reduced) and then increase the Q (and/or do whatever other
correction is necessary) of the low pass filter to give the same or
nearly the same summed on axis response.

Because the driver connected to the high pass section is bound to have
much wider dispersion, you're not going to have a situation where the
higher Q low pass section is heard on its own - at any angle you can
hear that significantly, the output from the highpass will be audible
too.

Has anyone tried something like this ? Any gaping holes in my idea ?

Regards,
Simon