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