Crossover 101 - Page 4
All high frequency compression drivers perform more efficiently
or play louder than their paper cone loudspeaker counterparts.
The efficiency of a loudspeaker is measured by driving the
loudspeaker with one watt of input power while measuring how
loud in sound pressure level (SPL) it will be at a distance
of one meter from the loudspeaker enclosure. This is called
the One Watt, One Meter Sensitivity rating of the loudspeaker.
A typical compression driver may have a one watt at one meter
sensitivity rating of 112 dB of SPL, while a typical paper
cone loudspeaker used for sound reinforcement may exhibit a
one watt at one meter sensitivity of 100 dB of SPL. In order
for the two transducers to produce the same acoustic level
from a loudspeaker enclosure, the crossover must provide for
-12 dB of Attenuation or reduction (Pad) in the signal level
of the high frequencies going to the compression driver.
Every two-way loudspeaker system provides this high frequency
pad or attenuation in the system's passive crossover, and is
standard throughout the industry. However, in addition to this
standard pad or attenuation that is designed into the loudspeaker's
passive crossover, it is also necessary to provide a special
high frequency equalization when the compression driver is
used on a constant directivity horn.
In today's professional audio, the constant directivity high
frequency horn allows us to obtain uniform high frequency response
with dispersion or angle of coverage. Before the introduction
of the constant directivity horn in the mid seventies, all
high frequency horns exhibited the same common problem, i.e.,
the horn may have measured very flat on axis or directly in
front of the horn, but as you moved off-axis of the horn itself,
the higher frequencies would not be equal in level to the mid
range of frequencies that the horn produced. This narrowing
of the beamwidth at high frequencies was due to the very rapid
flare rates associated with these earlier exponential horns.
An exponential high frequency horn is one whose flare rates
or taper increases proportionally to the square of the distance
away from the throat entry to the horn.
The very small wavelengths of the higher frequencies could
not cling to the rapidly expanding side walls of the exponential
horn to be directed off axis; therefore the high frequency
energy radiated directly down the center of the horn and exited
in a pattern about equal to the diameter of the entry to the
horn throat. Constant directivity (CD) high frequency horns
were first introduced in the late nineteen seventies. Using
computer assisted design (CAD) the internal parameters of the
side walls of the horn were manipulated resulting in flare
rates that were more gradual allowing the smaller high frequencies
wavelengths to be directed off axis. Today many manufacturers
make these constant directivity (CD) horns that offer uniform
frequency response with dispersion.
Since the CD horns are now able to direct the high frequencies
off axis, the amount of high frequency energy formally available
directly on axis is less. Therefore the CD horn no longer measures
flat directly on axis without its needed signal processing
in the form of a special high frequency equalization that is
designed to be the reciprocal or mirror image of the horn/driver
high frequency roll off response. This is what is meant by
constant directivity horn equalization (CD EQ). All CD horns
roll off the higher frequencies at about -6 dB per octave,
and the CD horn EQ is usually in the form of a +6 dB per octave
boost beginning at about 3 or 4 kHz.
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