How to set crossovers for your head unit or amplifiers
by GlassWolf and Brad Warren

If you are using a head unit or amplifiers with built-in crossovers with multiple speakers and respective frequency ranges, you may be wondering how to set those crossovers for the best results. There are generally three settings to each crossover point, which are:
1. Frequency: Measured in Hertz (Hz) this is the frequency (range from treble to bass, or 20KHz to 20Hz) at which the crossover will begin to function.
2. High-pass, Low-pass, or Band-pass: This setting determines if frequencies above, below, or within the above crossover frequency or frequencies will be allowed to pass through the filter, or be blocked by the filter.
3. Slope or Q-Factor: Measured in decibels per octave (dB/octave,) this determines the rate at which the amplitude, or volume of the music will diminish as the frequency extends beyond the crossover frequency point.

I generally recommend that subwoofers with a low-pass filter be set to a steeper slope, such as 18 to 24dB/octave, while your midbass, midrange, and tweeters be set to a shallower 12dB/octace in most cases. Every system is different however, so fine-tuning is always up to you in the end.

What is a good idea however, is to leave a gap of approximately one octave between each crossover point, so that if your subwoofer for example, is set to 50Hz low-pass, then your midrange should be set to 100Hz, or double the low-pass crossover point frequency as the midrange crossover point. This gap is left intentionally because due to the gradual decline in volume beyond the crossover points of both the subwoofers and the midrange speakers, both the sub and the midrange will be playing the frequencies in that 50 to 100Hz gap at a decreased volume, but combined, the total volume will be level with the volume of the frequencies played by only the subs below 50Hz, and the midrange speakers above 100Hz. This provides a smooth transition between teh mids and subs. If both ranges of speakers were set to the same frequency, then both your subs and your midrange would be at full volume at the matching crossover point of, let's say as an example, 80Hz. This will cause a warm spot, or a spike in output at the frequency range of about 70Hz to 90Hz, making the system sound overly warm and boomy.


I thought I would post a couple charts to help explain what I am trying to say. The left chart shows the frequency responses of a subwoofer, a mid-bass driver and a high frequency tweeter. Note the "skirting", or overlaps in frequency output, where each driver meets the next at it's rolloff. The right chart shows the "summed" output response taking into account the fact that whenever two drivers overlap each other in frequency, the result will be an increase in amplitude in that range. This is why the dips that were present at max amplitude where the drivers meet in the left chart, are flat on the right chart. This is an extremely important theory to learn and if you can wrap your head around what is happening here, it will help you greatly in your future audio adventures.

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