Is The Crossover Leaky?

A lot of low-end can still get into your mains.

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Every so often, I get to chew on a question that a reader asks me directly. I kinda wish that would happen more often (hint, hint, hint…). Anyway, I was sent a message on The Small Venue Survivalist’s Facebook page, asking if I could render an opinion on why a bass guitar seemed to have a surprising amount of LF information in the main speakers. The mains were being used alongside a subwoofer, with the sub providing a crossover filter at 100 Hz. What could the issue be?

There are a few explanations that would seem reasonable, if one discounts “catastrophic” issues like the crossover filter simply failing to operate as advertised.

1. Crossover filters, especially those implemented in active electronics, have a tendency towards a relatively steep slope. Even so, they usually aren’t brick-wall implementations. Everything below the cutoff doesn’t simply disappear – rather, it’s attenuated at a certain rate. With a filter set to roll off everything “below 100 Hz,” the mid-highs are still being asked to do a fair bit of work at the crossover frequency. The general vicinity of 100 Hz is actually quite bassy (depending on who you ask, of course), so the mains might be perceived as doing more than they should when everything is quite normal.

2. If a sizeable pile of low-frequency energy has been dialed into the bass-guitar channel, or the bass-guitar’s pre-console tone, that big hill-o-bass won’t be tamped down by the crossover. It will be split up proportionately, but following on from the first point, the mid-highs will still be tasked with reproducing their allotted piece of that big LF mound. Consequently, a surprising amount of energy may be present in the tops.

3. I have a suspicion that plenty of modern, two-way boxes receive some degree of “hyping” of their low-end at the factory. This makes them sound more impressive, and the manufacturer can get away with it because of safety limiters placed post-EQ. (The limiter prevents the low-frequency amplifier from supplying more voltage than the woofer can handle, and there may even be a level-dependent high-pass filter in play.) A low-frequency boost that occurs after the crossover reduces the crossover’s apparent effectiveness. Sure, the signal leaving the crossover might be down 12 dB at 75 Hz, but a +6 dB shelving filter put in place by the manufacturer at 100 Hz “undoes” that filtering to only lose 6 dB. Once again, a potential situation develops where the mid-highs are being asked to reproduce more “boom” than you expected.

It is entirely possible that an apparent problem isn’t covered by the three possibilities above, but they should catch quite a few scenarios where everything is hooked up properly and configured correctly.