You can also try reversing the polarity of the subwoofer(s) and see if the frequency response in the crossover range improves or degrades. Quite often, because of the acoustical nature of where we must mount speakers, and the physical characteristics inside of the vehicle, the sound quality of the system can be significantly changed by reversing the polarity of different speakers. Remember that even though the crossover might be set at 80Hz, the subwoofer will still be reproducing frequencies above 80Hz and the midrange speakers will be reproducing frequencies below 80Hz, and hence the sound from each will interact and may combine constructively or destructively at the listening position. If you hear a significant dip or peak in the crossover range, try reversing the subwoofer polarity and see whether it sounds better. Keep the setting that sounds best.
Lastly, remember to consider setting the crossover point higher if it doesn't negatively impact how the system sounds. This will help keep the lower frequencies out of the midrange speakers and let the subs do more of the high-power work in the lower frequency range, which can improve overall system power handling and reduce distortion at higher output levels.
More Advanced System Tuning
Everything we've discussed so far also applies to setting higher frequency crossovers and more advanced systems. Even if you are using all active crossovers, multiple subwoofers, dedicated midbass speakers, etc., the aforementioned techniques still apply. Let's walk through a few additional examples and details to better understand what we're talking about.
When setting crossovers between midranges and tweeters, always remember to set the highpass crossover on the tweeter no lower than the manufacturer's recommended crossover frequency. Set it any lower and turn the volume up too high and you may see "magic smoke" come out of the tweeter.
When working with higher frequency speakers, I recommend tuning only one channel at a time because we want to be able to hear how the sound from the midrange and tweeter from each side combine. Turn on only the left or right channel and listen to those speakers. A track with only spoken voice can be very revealing here because we're all familiar with how people sound when they talk and it's very easy to hear problems in this mid to upper frequency range with spoken voice recordings.
If you're using passive crossovers, try the different switch or jumper settings available. Listen for a coherent voice, smooth frequency response, etc., in the crossover range. Try all of the available setting combinations. Also try reversing the tweeter polarity. This might not make a significant difference, but occasionally it will improve the sound, depending on where the tweeters are mounted relative to the midrange.
With active crossovers on the midrange/tweeter crossover, you have more control and can try varying the crossover point, splitting the high and lowpass crossover points, vary the slopes, etc. Again, tweak only one channel at a time.
When you're happy with the sound for one channel, match the settings and polarities on the other channel and listen to the entire system. You may want to experiment further at this point, but make sure to document what you get at every step so you can get back to this point if future changes don't work out.
Using an RTA for Crossover Setting
An RTA can really help with initial crossover setting, if you have access to one. A 1/3-octave RTA will work fine for crossover setting, but a higher-resolution version, like a 1/12- or 1/24-octave RTA will help for critical equalization.
When using an RTA for crossover setting, start with the base settings we discussed earlier, use a pink noise disc, set the RTA response to a slow setting for a long average of the readings and remember to sweep the microphone slowly around where the listener's head will be as we discussed in the last article.