What is a Crossover and Why is it Important?
The basic function of any crossover, be it passive or active, analogue or digital, is to take the audio spectrum that stretches roughly from 20 Hz to 20 kHz and split it into two, three, or sometimes more bands so they can be applied to loudspeaker drive units adapted for those frequencies. In hi-fi use the crossover frequencies are usually fixed and intended for work with one particular loudspeaker design, but for sound reinforcement applications the crossover frequencies are normally variable by front panel controls.
There are also other functions that are sometimes but not always performed by crossovers, and we may list them all as follows, roughly in order of importance:
- Splitting the audio spectrum into two, three, or occasionally more bands
- Equalisation to correct drive unit frequency responses
- Correction for unmatched drive unit sensitivities
- The introduction of time delays into the crossover outputs to correct for the physical alignment of drive units
- Equalisation to correct for interactions between drive units and the enclosure, for example, diffraction compensation
- Equalisation to correct for loudspeaker-room interactions, such as operating in half-space as opposed to quarter-space
- Enhancement of the natural LF response of the bass drive unit and enclosure by applying controlled bass boost
Most of these functions are equally applicable to both passive and active crossovers, but the time-delay function is rarely implemented in passive crossovers because it requires a lot of expensive components and involves significant power losses.
The need for any crossover at all is rooted in the impracticability of making a drive unit that can handle the whole ten-octave audio spectrum satisfactorily. This is not merely because the technology of loudspeaker construction is inadequate, but is also based on some basic physics. Ideally the acoustic output of a loudspeaker would come from a single point; with such a source the sound field is uniform, because there can be no interference effects that result from multiple sources, or from a source of finite size.
A tweeter has a small physical size, with a dome usually around an inch (2 or 3 cm) in diameter, and approximates fairly well to a point source. This technology works very well for high frequencies, say down to 1 kHz, but is hopelessly inadequate for bass reproduction because such a small area cannot move much air, and to reproduce bass frequencies you need to move a lot of it. Low-frequency drive units are therefore of much greater diameter, up to 12 inches for domestic hi-fi and up to 18 inches or more for sound-reinforcement applications. As cone area is proportional to the square of diameter, a 12-inch drive unit has 144 times the area of a typical tweeter, and an 18-inch unit has 324 times the area.
It is not at present technically possible to make a big low-frequency drive unit that works accurately up to 20 kHz, because as the frequency increases the cone ceases to move as a unit-it is not one of those most desirable “infinitely rigid pistons” that are always cropping up in loudspeaker theory but never in manufacturer’s catalogues. This effect is often called “cone break-up” not because the cone physically falls apart but because, due to its finite stiffness, with rising frequency its surface divides up into separate areas of vibration. This unhappy state of affairs is put to advantage in so-called “full-range” loudspeakers which have a “parasitic tweeter” in the form of a small cone, attached to the voice coil. The idea is that at higher frequencies the main cone does its own thing and is effectively decoupled from the voice coil and the tweeter cone, allowing the latter to radiate high frequencies without being restrained by the much greater mass of the main cone; what you might call a mechanical crossover. As you might imagine, there are many compromises involved in such a simple arrangement and the response is generally much inferior to a good two-way loudspeaker with separate bass unit and tweeter.
Nonetheless, the field of audio being what it is, there are a certain number of hi-fi enthusiasts who advocate full-range speakers for various reasons. Eliminating a passive crossover naturally increases power efficiency (as none is lost in the crossover components) and reduces cost.
Excerpt from The Design of Active Crossovers by Douglas Self © 2011 Taylor & Francis Group. All Rights Reserved.