Crossover 102 - Electronic
Crossovers - Page 7
The Low Pass maximum SPL would be 133 dB (10 log 2000 + 100),
Mid Pass maximum SPL would be 133.8 dB (10 log 1200 + 103),
the High Pass would be based on the 80 Watt continuous rating
of the driver, plus the sensitivity, for a max SPL of 131 dB
(10 log 80 = 19 + 112).
Generally this would be a good starting for a Three-way or
Tri-amped system. The actual optimum crossover frequencies
would depend on the composition of the music being reinforced.
Most of the energy in contemporary music is below 250 Hz, which
is the most demanding bandpass. The second most demanding band
of frequencies is between 250 Hz and 1200 Hz. Middle 'C' is
261.63 Hz and the 'D' above the melody staff is 1174.66 Hz.
A Rock band with three electric guitar players may be more
demanding on this system than another band with only one guitar
player. It is now up to the system operator to determine which
of these three bandpasses (low, mid, or high) are working hardest.
This is easy of course if the amplifiers have an LED metering
system. If the amplifiers do not have metering arrays, you
would have to push this system until one of the three bandpasses'
power amplifiers began showing limiting (or even clipping).
Let's say the low pass amp is indicating that it is working
the hardest. You could then lower the low/mid crossover frequency
point, to allow more headroom in the lowpass amp, but now of
course the midpass amp would be working harder. If you then
got to a point where the mid pass amp began to show that it
was now working harder, you may try lowering the mid/high crossover
point a little. The idea here is to try to spread out the demand
more evenly. A general rule of thumb, for fourth order and
less analog crossovers, is that the crossover points should
at the very least be more than 2 1/3 octaves apart. This minimizes
something called out-of-bandpass by-product distortion, due
to the crossover points overlapping too much.
Many uninformed system operators keep the crossover at the
FOH mixer position, so that they can twiddle with it when they
deem necessary. This can be a dangerous practice, particularly
if you have no way of seeing how hard any of the amplifiers
are working. A particularly bad practice is to raise or lower
the level of an entire bandpass during a performance, as this
actually changes the crossover point of the system. A low cut
crossover frequency will move down when raising the gain, and
a high cut will move up in frequency. In the example 3-way
system below, crossed over at 200 Hz and 2 kHz, note how raising
the level of the midpass changes the effective crossover points:
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| Raising the output drive
level changes the crossover frequency |
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