Crossover 102 - Electronic
Crossovers - Page 5
However, by definition, a Linkwitz/Riley filter circuit can
only be even order, where a Butterworth can be either even
or odd. There is also a very important difference in how the
turnover frequency or crossover point is defined. Butterworth
filter networks are defined by the half power or -3 dB down
points in frequency response. Where as the Linkwitz/Riley filters
are determined by the -6 dB down points. I'll repeat this for
you, Butterworth measure -3 dB down at the crossover frequency,
while Linkwitz/Riley are -6 dB at the turnover frequency. This
very important fact is often not understood by the typical
sound provider. Even though a Butterworth and a Linkwitz/Riley
filter set of the same order may share the same crossover point,
they are going to use different value components to arrive
at the common frequency.
I believe it is now time to discuss the summation of these
two filters at the crossover point. First we must define coincident
and non-coincident signals. If two different signals are at
the same level and are not coincident (or starting from the
same exact moment in time), then the most that they can add
when summed together is +3 dB. This is also true if they are
the same common frequency but exhibit a significant difference
in degree of phase angle. If however you have two absolutely
coincident signals in both frequency and level, then they will
sum to +6 dB when added or mixed together.
Butterworth filters when combined are said to have a smooth
power response through the crossover region. Since the crossover
frequency is defined as the point at which the spectrum is
attenuated or down -3 dB, the actual summation of the transducers
is essentially flat when the power is averaged. There is still
a little dip around the crossover frequency because the filters
are not in phase with each other at this frequency. Now there
are some analog electronic crossovers that introduce some signal
delay in one or more outputs. However the steps can be quite
broad depending on the chosen crossover frequency.
Okay, how do I set up a variable electronic crossover?
First of all there is no magic crossover frequency point. The
raw frequency response of each transducer or driver must first
be examined, to ensure that the intended drivers can indeed
effectively reproduce the chosen range of frequencies. The
second and most important consideration is to know the actual
sensitivities of each of the component drivers in the system.
The sensitivity of a loudspeaker is the internationally accepted
standard of 1 Watt @ 1 Meter. With one Watt of power sent to
the driver, a measurement is made on axis at a distance of
one Meter to determine how loud is the sound pressure level
(SPL). Once you know the sensitivity of the drivers you can
then set the gains of the crossover properly.
Let's say that we have a three-way system, and the low frequency
device can handle 500 watts continuous and produce an SPL of
100 dB (1W, 1M) with a frequency response of 45 Hz to 2 kHz
(+/- 3 dB). The mid frequency driver can also handle 500 Watts,
but it has a sensitivity of 103 dB (1W, 1M) from 70 Hz to 2.5
kHz. The compression driver on a constant directivity high
frequency horn can handle 80 Watts continuous, and has a mid-band
efficiency of 112 dB from 800 Hz to 3.5 kHz, with -6 dB per
octave roll off above 3.5 kHz.
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