Originally Posted by
David Analog
This is not a step by step professional tuning prescription like you would get from Odin at Earmark Marine. Odin is a masterful acoustic engineer and walks his customers through a real pro tuning process. He can also dumb it down if you don't have the tolerance for the technical, however, he will take everything into consideration on his own as he instructs you. The following is just a few of the guidelines a pro might consider in making your particular crossover selections.
Alpine says that your woofer in your box will have a gradual deep bass roll-off starting at about 60 Hz, be at half power at 45 Hz, and roll-off rapidly below 45 Hz. Not bad for that small of a box. A typical active crossover is at half power at the crossover frequency so it also begins to roll-off gradually outside/lower-than the crossover region. The 'meat' of the bass is in the 50 to 80 Hz region ('meat' denoting the range that is perceived to be the loudest). So you are beginning to roll off on the bottom end below 60 Hz by nature of the enclosure/driver. If you select a lowpass crossover of 90 Hz for example you are beginning to gradually roll-off the top end of the sub below 90 Hz. That doesn't leave much bandwidth. Again, this would have you at half power at 45 Hz on the low end and at half power at 90 Hz on the top end. That is less than one full octave. That is also beginning to infringe on the power bandwidth of the bass (in other words the 'meat'). A 'Qtc' of 0.81 in your case will also generate a small peak in the response about an octave above the system (woofer in box) resonance. That becomes an accentuation of what could be an already pronounced peak by nature of the lower end and upper end roll-offs. So with what I am sharing with you, your conclusion should be that you do not want to crossover your sub below 90 Hz.... period. It's also to your advantage to have the lowpass of the subwoofer and highpass of the coaxials as close together as possible. Otherwise, as you spread the crossover points apart you get farther and farther out of phase with one another and any degree of coherency becomes impossible.
So why is coherency important and why do I place so much emphasis on having the best possible phase relationship? Because you are very dependent on your coaxials and the relationship between the sub and coaxials for bass tonal construction and pitch accuracy. Want proof? Easy. Just run your subwoofer in isolation in the lowpass mode and without the benefit of the coaxials or other fullrange speakers. No matter which sub you have it will sound drunk, soggy and muddy. Add the upper harmonics back in from the coaxials and you will find that structured bass tones actually are formed again.
If we change equipment or even change the sub enclosure we might be changing the crossover selection. It's a sequential process, a step by step prescription, and not a simple set of numbers that are commonly used and that would apply to all systems. And I've only scratched the surface. For a music lover it's a crime to simplify it.
But when tuned correctly the identical equipment can perform at twice the perceived level.