CROSSING OVER

Dividing the frequency spectrum between loudspeaker components has never been easier. For many sound reinforcement system techs, the crossover is simply buried in the loudspeaker processor, 'nuff said. For those of us that need more control over the loudspeaker system configuration, there has never been so much choice in selecting a malleable crossover filter. The ubiquitous DSP-based system processor will typically have all three of the common crossover filter topologies and the ability to create even highly non-symmetrical crossover configurations. The system processor will also provide the parametric equalization necessary to achieve smooth response within the driver passband and in the sensitive area where multiple drivers are operating.

Why do you want to set up your own crossover values instead of simply taking the driver manufacturer's recommendations? Customizing the crossover filters to suit the acoustical output from the loudspeaker array will result in better performance in the listening area. Unless you are simply using one full-range loudspeaker to cover the potential audience, the variables encountered in tweaking an array of loudspeakers will require a great deal of flexibility from the crossover filters.

I find it is best to start with the raw response of each device type in the array. Using a time-domain analyzer (such as the Gold Line TEF, JBL-Smaart, etc.), I set the equalization for a smooth response across the passband of the device and at least one octave into the intended crossover range. This will ensure that the crossover filter you set will actually reduce the output of the device at the intended slope rate (such as 24 dB/octave). For example, crossing from a 12" driver into a compression driver at 1.2 kHz demands that those (all too common) peaks in the 12" driver output at 2 kHz will not destructively interfere with the compression driver output at that frequency.

Once the individual drivers are suitably equalized, the crossover filters can be set. The selection of crossover frequency must be determined based on a number of criteria: power handling; frequency-dependent distortion; frequency response; and dispersion. The last criterion, dispersion, is easily overlooked when selecting a crossover frequency. As an example, the tendency for the coverage pattern of a front-loaded 12" driver to collapse above 1 kHz can cause the overall coverage pattern of the system to vary excessively if the crossover frequency is set too high. Setting the crossover frequency too low will reduce the pattern control of the high-frequency horn, even if it doesn't excessively stress the diaphragm at full power.

If the devices have been sufficiently smoothed using the parametric equalization, then setting the appropriate drive level for each band is fairly obvious. You know that further equalization is required if the muting of one band causes significant changes in the frequency response in adjacent bands. While noting the phase response of the loudspeaker at the crossover frequency, it is worthwhile (at least the first time with a particular component selection) to examine the behavior of the system using different filter topologies. There are situations that are better suited to the hard line 24 dB/octave Linkwitz-Riley filter type and others where a more subtle 12 dB/octave Bessel filter will work. The phase response will often indicate the need for component alignment delay and asymmetrical filter slopes to achieve the best transient response in the acoustical output from the loudspeaker system. The crossover must provide audible results, not just electrical response. This requires the inclusion of the natural filter effects of the driver itself and combinations of multiple drivers with overlapping coverage (such as bass and sub-bass drivers) in your view of the final result.

We have never before had such great crossover technology to use. The ease with which you can select filters, frequencies and signal delays make the delicate job of configuring loudspeaker arrays so much easier. Now if we can just convince the manufacturer's to stop putting the filters in so many different places within the software, we will be able to set up even very complex loudspeaker arrays in record time. With the right selection of loudspeaker components, time-domain measurement gear and a little time on site, the sound quality envisioned at the design stage of the array may actually be realized!

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