Crossover 101
by Marty McCann
© 2001
What is a Crossover?
A crossover is a filtering device that separates the audio
frequency spectrum and directs portions of that spectrum to
a specific audio device. As an example, all sound reinforcement
quality two-way loudspeaker enclosures have a passive crossover
as part of the design that separates the audio spectrum into
two bands of frequencies. This passive crossover is a bit like
a traffic cop that directs the lows to the woofer and the highs
to the horn. All of the frequencies below the designed crossover
point (Low Pass) go to the woofer, and all of the frequencies
above this crossover point or frequency (High Pass), are directed
to the high frequency compression driver and the horn. This
is accomplished via a network of inductors, capacitors, and
resistors that direct or pass the proper information to the
proper transducer.
How does the crossover work?
Inductors pass low frequencies and capacitors block low frequencies.
Capacitors pass high frequencies and block low frequencies.
You can have both a very high quality woofer and horn/driver
combination, but it takes an equally high quality crossover
circuit to make the units perform well as a system. The more
complex the crossover network, the greater number of poles
or orders you will find in the design. The more poles or the
higher the order of crossover the more components you will
have in the low and high pass filter stages of the design.
The more filter stages, poles, or orders incorporated in the
design, the more phase shift you will introduce to the audio
signal. The phase of the output signal of the crossover can
either lead or lag that of the original incoming signal. When
signal passes through an inductor, the current flow is said
to lag or be behind the voltage in regards to time. When an
electrical signal passes through a capacitor, the current flow
is said to lead or be ahead of the voltage in time. So in a
simple single pole or first order crossover network, the voltage
output of the high pass port will be shifted -90 degrees in
phase, while the low pass will exhibit +90 degrees of phase
shift, with the maximum of 90 degrees being at the extremities
or farthest away from the crossover point.
Each crossover filter network stage (order or pole) will also
introduce a specific rate of roll-off or attenuation of the
signal, above and below the crossover frequency point. The
rate of attenuation or roll-off is measured on the decibel
scale in the form of so many dB per octave of attenuation (signal
reduction) above and below the crossover frequency. For instance
a 1st order filter network will attenuate at a rate of -6 dB
per octave. Let's use the example of a 1600 Hz crossover design.
The crossover frequency is designated by the -3 dB down point
from flat, so in a two-way 1600 Hz crossover design, the Low
pass output would be down -3 dB at 1600 Hz, as would the High
pass. Above and below this crossover frequency the system would
be flat (with the exception of CD horn attenuation and EQ,
which will be addressed later).
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