ABSTRACT
Drive unit equalisation usually involves the correction of minor humps and dips in the frequency response. For example, a consistent 2 dB dip in drive unit response might be cancelled out by a 2 dB peak introduced by an equalisation circuit. Exact cancellation will not be possible, either because the shape of the dip is too complex to be mimicked by practical amounts of circuitry, or because of variation in the shape of the dip due to driver tolerances or aging. This kind of correction is commonly performed by peak/dip equalisers.
A classic example of drive unit equalisation is Constant Directivity Horn Equalisation. Following the work of Don Keele at ElectroVoice [1], constant directivity (CD) highfrequency horn loudspeakers appeared in the late 1970s. Basically, an initial exponential section was combined with a final conical flare, causing the shorter high-frequency wavelengths to be dispersed more effectively off axis; purely conical horns, as seen on old gramophones and phonographs, are not satisfactory because of their poor low-frequency response. Because CD horns direct more high-frequency energy off axis, the amount of high-frequency energy available directly on axis is reduced. Therefore, the CD horn no longer measures flat directly on axis unless it is given equalisation that is the inverse of the horn high-frequency roll off response. This is called Constant Directivity Horn Equalisation or CDEQ.
