CONSTANT DIRECTIVITY HORN
EQUALIZATION
By Marty McCann
Copyright 1995
Every so often I encounter floor salespersons and sound people
that do not have an understanding of why compression drivers
that are mounted on Constant Directivity high frequency horns
require a special form of equalization that permits them to
exhibit proper high frequency response. In this article we
will explain the need for the high frequency compensation know
as CD EQ.
The Peavey 22, 22A, 22T, 22TI, 22XT, RX-22, 44T, and 44XT compression
drivers require this form of high frequency equalization when
used on our constant directivity high frequency horns, or for
that matter, any manufacturers constant directivity high frequency
horn.
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.
At Peavey we measure the constant directivity of the horns
Beamwidth, using a TEF analyzer. (TEF stands for Time, Energy,
and Frequency). A typical compression driver may have a one
watt at one meter sensitivity rating of 110 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
-10 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 rate 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.
The high pass section of a passive crossover in a loudspeaker
system that employs a constant directivity high frequency horn
provides two functions. The first function is the necessary
pad or attenuation to match the sensitivity of the paper cone
loudspeaker.
Function two is the equalization necessary to allow the driver
to have a flat response in the last two octaves. How is the
equalization or boost of high frequencies accomplished in a "passive" crossover?
Essentially there is a primary crossover circuit that crosses
over the audio signal at the designated crossover frequency
and provides the proper amount of attenuation. Then a secondary
crossover circuit uses the voltage window of the unattenuated
high pass signal to provide an additional signal path for the
high frequencies with less and less attenuation as the frequency
rises.
Most electronic crossovers today have a circuit that provides
this very necessary high frequency equalization. There is usually
a switch labeled CD or Horn EQ. However it is imperative that
the sound engineer or installer know the sensitivity (one watt
at one meter) specifications of the systems individual transducer
components. The CD equalization only occurs as a function of
the Pad or attenuation that is dialed in via of the high pass
gain control. If no Pad is introduced, then there is no equalization
present. This is often overlooked by some individuals.
There are also electronic crossovers available that do not
provide for the CD horn EQ. If this is the case, the needed
equalization can still be obtained by plugging the high pass
powered signal into the full range input of the loudspeaker
instead of the bi-amp high input. In this case you would not
pad the high pass signal down because the passive crossover
will do it for you.
Everyone who works with these horns should be familiar with
the sound of an unequalized constant directivity horn, so they
may be able to recognize situations when the necessary CD equalization
has not been provided.
Without the proper Pad and CD EQ the loudspeaker system is
very honky or mid range sounding due the mid-band efficiency;
and the highest frequencies, such as those produced by a high
hat or cymbals, are buried due to the roll off characteristic
of the driver/horn.
I hope this article has helped you to understand the reasons
for this required signal processing for Constant Directivity
horns.
Yellow = Response of system with
no pad or CD Horn EQ
Red = Low Pass
Purple = High Pass w/ Pad & EQ |
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