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What is a Crossover?

Crossovers are an essential part of an audio system, often ignored.   A crossover splits frequencies so that each speaker receives a certain range of frequencies.  This is done for two reasons:

  1. Avoid speaker damage: Speakers are designed to play only a certain range of frequencies. If other frequencies are played, then the speaker will produce distortion, which eventually will destroy it.
  2. Overall balance: If a system with subwoofers, and full range speakers doesn't have a crossover, then the subs will be playing, for example, from 20 to 1000 Hz, while the full-range speakers will be playing from 60 Hz, all the way up to 20,000 Hz. As it can easily be seen, there is an "overlap" of frequencies between 50 and 1000 Hz. In this overlap region, the levels are higher than levels below 50 and above 1000Hz, yielding to a non-balanced system.

There are 3 types of crossovers:  High-pass, low-pass and band-pass.  As it can be deducted from the names, a high-pass crossover will block low frequencies, a low-pass will block high frequencies, and band-pass will block low and high frequencies below and above crossover points.

From the description above, crossover operation sounds very simple, but it is a bit more complicated.  Crossovers do not block undesired frequencies completely (unless you are using digital crossovers).  Crossovers cut frequencies progressively.  A crossover "slope" describes how effective a crossover is in blocking frequencies.  The minimum slope is 6dB/octave.  For example, a high pass crossover at 1000 Hz, will let anything above 1000Hz pass.  The farther lower frequencies are from 1000Hz the lower levels will be.  At 500 Hz (1 octave), the level at the speaker would be 6dB less.  A steeper slope (i.e. 24dB/octave) will block undesired frequencies more effectively, but will cost more than a lower slope crossover.

If a speaker will be played near it's frequency range limit, then you need a high slope.  For example, a midbass rated at 50Hz on the lower range could be crossed over at 55 or 60HZ with a 24dB/Oct crossover.  If you want to use a lower slope crossover, then the frequency would need to be higher (i.e. 100 Hz at 6dB/Oct).

So what are good crossover frequencies? It largely depends on the car, speakers, and speaker location. Typical crossover frequencies are 100Hz (bass), 350Hz (midbass), 3500 - 5000Hz (highs). For more details of what frequencies to choose, see the speakers section.
  

Active Crossovers

Active crossovers (and equalizers) need external power to operate and work at low signal voltage levels (RCAs). Signal from the head unit's RCA's is split it into low-frequencies (bass), mid-frequencies (mids), and high frequencies (tweeters), to go to different amplifiers.

What are the advantages of active crossovers? Signal is not affected as much as with passive crossovers, since everything is done at low voltages. There is much more flexibility, since all that is needed to adjust crossover frequencies is to turn a knob, while on passive crossovers, the components have to be replaced. The problem is that more amplifier channels are needed to go to all the speakers.
   

Passive Crossovers

Passive crossovers work after the amplifiers, receiving high signal levels. Since all the frequency splitting is done after the amplifiers, more speakers can run off an amplifier channel, obtaining maximum power by playing with the resistances "seen" by the amplifiers.

Passive Crossovers are capacitors and inductors either in parallel or series, or combinations that are added to cut off highs and/or lows. A capacitor stores voltage, acting as an open circuit (blocks off signal) at lower frequencies, and acts as a short at higher frequencies (lets signal pass). An inductor, on the other hand, stores current, acting in exactly the opposite way of a capacitor. Inductors act as shorts at lower frequencies, and open circuits at high frequencies.

If a capacitor is hooked up in series with a speaker, it will be a high-pass crossover (signals at lower frequencies will be blocked, and higher frequency components of the signal will be allowed to pass). An inductor in series with a speaker will be a low-pass filter. Subwoofers need inductors in series (low-pass), while midranges will need both a high-pass (cut bass off) and a low-pass (cut higher frequencies that tweeters will be taking care of). Tweeters need also high-pass filters, to block lower frequencies.

An octave is double the frequency: i.e: 20Hz --> 40Hz --> 80Hz --> 160Hz, etc. The more components are added, the more effectively the filter will be, so if both an inductor and capacitor are used, the cutoff slope will be 12 dB-per-octave, then 18 dB-per-octave, then 24 and so on.  There are many types of passive crossovers that can be implemented, starting with the capacitor or inductor in series, and getting complex as more capacitors and inductors are added.

Capacitors and inductors also dissipate power, wasting energy that speakers could be using. Low order passive crossovers are not very expensive: Capacitors run $1 - $5, inductors run $10 - $20. Higher order (i.e. 24dB/Oct) crossovers can get really expensive, especially at low frequencies/high power applications.  Passive crossovers have another drawback, that is ignored most of the time for practical purposes: They introduce phase shifts, which put voltage and current out of phase with respect to each other, affecting delivered power to the speakers and affecting overall speaker "timing".  A 6dB/Oct crossover has a phase shift of 90 degrees, 12dB/Oct = 180, 18dB/Oct = 270, 24dB/Oct = 0.  Try to stay with even-order crossovers.  If you have a 180 degree shift (2nd order crossover = 12dB/Oct, hook up the speakers out of phase (+ to - and - to +).  On 4th order crossovers (24dB/Oct), there is no phase shift.

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