It sounded odd to me as well the first time I heard it, but it makes some sense when you think about what being at -3dB means. A 3dB drop represents a halving of the volume level, so it makes sense to pick a crossover point that moves at least some distance away from that point. I typically haven't stayed completely true to the doubling guideline, but I've had good luck using a point not too far removed from it. In your example, speakers with a -3dB point of 92Hz would probably benefit from a crossover of at least 120Hz, but it might take some experimenting to find what point works best in your case.

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