Crossover Concepts

What exactly is a crossover and what does it do?

All loudspeakers are not created equal, some like to woof, and some like to tweet. The woofers (or bass drivers) are large with heavy paper cones and HUGE magnets. Their cones (diaphragms) can move in and out the better part of an inch. Because of their massive construction, they are hard to destroy, but cannot handle high frequencies. The tweeters (or horns) are just the opposite. They consist of a few hundred windings of hair thin wire, and a diaphragm little heavier than aluminum foil. The result: They can reproduce highs very well, but can be destroyed if any bass notes get through. Mid-range speakers are well, in the middle. They can do some bass and highs, but are optimized for sounds in the middle of the audio spectrum. The crossover’s job is to route the signals so that the bass speakers see only the bass notes, and the horns see only the high ones. There are two ways of doing this: Passive and active crossovers.

1) Passive Crossovers. They consist of a few capacitors and inductors mounted in the speaker cabinet. Their advantages are:

  1. Cheap to manufacture.
  2. Easy to make foolproof.
  3. Can be tailored for a specific speaker. They are non-adjustable, and always use up some amp power.

2) Active crossovers. (Also known as electronic crossovers). They hook up before your power amps and route the signal while it’s still at a very low level. Their advantages are:

  1. Easy to adjust frequency and loudness level in the field.
  2. Cheap to control banks of speakers (one crossover can feed literally hundreds of amps).
  3. More exact control (they can be made to provide almost any crossover slope desired from 6db/octave through 24dB/octave. Some incorporate limiters and low/high pass filters).
  4. Hi efficiency. Unlike the passive crossovers which waste some of your hard earned watts, an active crossover lets you extract all the power your amps can deliver. But it’s possible to switch hookups accidentally and send bass notes to the horns and burn them out, so be careful.

The two main ratings for both types are frequency and slope:

The crossover frequency is the point where signals are routed to either high, mid, or bass speakers. It is rated in Hertz (Hz) a.k.a. cycles per second. A bass guitar puts out around 30 Hz, Voice is centered around 1000 Hz, while guitars put out a lot in the 3000 Hz range. Cymbals and synths produce up to 20,000 Hz.

Slope is rated in dBs (decibels) per octave. A 6 dB slope may let too much bass into the horns, and too many highs into the bass bins. A 12 dB slope is standard and a good compromise between cost and performance. An 18 dB slope is excellent for most speakers, and has the advantage of no phase inversion at the crossover point. Units with slopes of up to 24 dB per octave are becoming more common, with Linkwitz-Riley now the gold standard for pro-sound systems.

Other considerations:

Professional sound systems hook up with XLR connectors, so this may be important to your application. You may also require a front panel cover to keep fingers away. Few things can do more damage to a sound system than the unknowledgeable tweaking of an active crossover. Make sure they look but don’t touch.

Copyright 1992 Mike Sokol _ All Rights Reserved


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