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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