Equalisation 387
Having reassured ourselves on this point, things are generally very much otherwise in crossover equalisation. If a drive unit has, say, a 2 dB peak or hump in its amplitude/frequency response, this will probably be due to some underdamped mechanical resonance or other electro-acoustic phenomenon, and while it is, in principle at least, straightforward to cancel this out, it is very unlikely that the associated phase-shifts will also cancel, because the physics behind the drive unit peak and the equaliser dip are completely different.
Loudspeaker Equalisation
Active crossovers may consist simply of frequency-dividing filters but very often also include equaliser circuits with a carefully tailored frequency response which are used to counteract irregularities in the overall response of the system. Since the active crossover and its power supply, etc, is already present, relatively little extra circuitry is needed, and the cost is low. A large number of different equaliser circuits are described later in this chapter.
There are several reasons why equalisation might be necessary:
1.Correcting irregularities in the frequency response of the drive units themselves.
2.Correcting frequency response features inherent in the driver/enclosure system, for example 6 dB/ octave equalisation of dipole LF loudspeaker units
3.Using equalisation to evade the physical limits of driver/enclosure system; most commonly extending the bass response of any kind of loudspeaker.
4.Compensation for other unwanted interactions between the loudspeaker and its enclosure. The most common application is compensation for enclosure front panel diffraction effects.
5.Compensation to deal with interactions between the loudspeaker and the listening space, such as when a loudspeaker is mounted against a wall.
1 Drive Unit Equalisation
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 Electro-Voice, [1] constant directivity (CD) high-frequency 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.
388 Equalisation
Constant directivity horns roll off the higher frequencies at about −6 dB per octave from around 2 to 4 kHz, continuing to 20 kHz and beyond, so the equalisation therefore takes the form of a +6 dB per octave boost starting at the appropriate frequency. The equalisation curve is arranged to shelve to prevent excessive boost at frequencies above 20 kHz, which could lead to inappropriate amplification of ultrasonic signals and imperil the sound system stability. The frequency at which the equalisation
begins may be derived from measurements or from the−3 dB point of the CD horn frequency response, as provided by the manufacturer. The equalisation is typically performed by a HF-boost equaliser, as described later in this chapter.
2 6 dB/octave Dipole Equalisation
The operation of dipole loudspeakers was described in Chapter 2. To summarise it quickly, a dipole loudspeaker has a drive unit mounted on a flat panel, usually called a baffle, which prevents sound from simply sliding straight round from the front to the back of the drive unit and cancelling out. The larger the baffle, the lower the frequency down to which this is effective. The panel may be folded in various ways to save space, so it remains large acoustically but is physically smaller. The name
“dipole” is derived from the way that the polar response consists of two lobes, which have equal radiation forwards and backwards (in opposite polarities) and none at right angles to the front-back axis of the drive unit. Because the baffle is of finite size, there is a frequency at which the response begins to fall off as sound goes around the edge of the baffle (loudspeakers in sealed boxes are sometimes referred to as being mounted in an “infinite baffle”, but that does not of course mean that their response goes down to infinitely low frequencies).
Dipole speakers are therefore commonly regarded as needing equalisation in the form of a 6 dB/octave boost as frequency falls, starting from an appropriate frequency. This has to be done with great caution, as even apparently modest amounts of equalisation greatly increase both cone excursion and the amplifier power required; a dipole loudspeaker is particularly vulnerable to this because, unlike a drive unit with
a sealed box, there is no loading on the cone at low frequencies. It is essential that the bass boost ceases at a safe frequency, and it is wise to arrange for an effective subsonic roll-off if drive unit damage is to be avoided. There is often also a need to equalise away a peak in the loudspeaker response where the LF roll-off begins. The biquad equaliser, described later, can do the task effectively and economically but may need to be supplemented with further subsonic filtering to make sure that no accidents occur.
3 Bass Response Extension
As described in Chapter 2, the bass response of a loudspeaker is essentially that of a highpass filter whose characteristics are determined by the type of enclosure and the parameters of the LF drive unit. A sealed-box loudspeaker consists of a mass-compliance-damping system that gives a classical
2nd-order highpass response and can for some purposes be simulated by a highpass active filter of the Sallen & Key or other configuration.
Much thought has been given to extending the low-frequency response of such systems by adding bass boost in a controlled fashion. The presence of an active crossover makes adding this facility straightforward, though its design is not so simple. The basic principle is to counteract the downward