Highest high pass for FR125s with sub

[Foxx510]

Just after some opinions on how high I can crossover my FR125s with a 12inch sealed sub on each side without degrading imaging etc. I was thinking around 120hz 24db/oct. The subs aren't super high quality, some JBL car subs that will be in small sealed boxes with EQ, below and slightly offset the FR125s boxes on stands.
Thanks for any advice.

[richie00boy]

It really depends a lot on the subs. What you propose sounds like a good starting point, but what are you doing about the filtering of the FR125?

What you might find easy to do is use the natural roll-off of the FR125 in a sealed box as one part of the crossover and match the subs up to that with an appropriate 2nd-order low-pass filter.

[Foxx510]

I was proposing to roll off the FR125s at 120hz 24dB/oct, and the subs similarly. If I could go higher I will, this is what I'm unsure of. I'll experiment with sealing the top boxes, or aperiodic, the main aim is raising the power handling/spl of the FR125s. All done actively, of course.

[richie00boy]

My concern with hitting the FR125s with a textbook 4th-order 120 Hz filter is that the response would probably be impacted by the roll-off of the speaker/box combination.

[Foxx510]

Possibly, I'm happy to tweak the sub response to get things right though. I'll aim to make the FR125s box as flat as possible before I add the sub in.

[richie00boy]

What I would do.

Find the resonance frequency and Q of the FR125 in the sealed box and use that to design a complementary 2nd-order electrical filter to achieve overall 4th-order Linkwitz-Riley response. Put a complementary 4th-order electrical Linkwitz-Riley filter on the sub.

OR

Find the resonance frequency of the FR125 in the sealed box and do a textbook Linkwitz-Riley crossover at 2x the resonance frequency (thus making any effects of it engligible).

The first method might be best to avoid any nasties from the sub trying to reach up to maybe 300 Hz.

[Foxx510]

Thanks for the suggestions. I'm pretty new to this, and have a lot to learn. My thought was that the 4th order on the FR125s would be better for power handling, but I'm probably looking at it too simplistically. I think I see what you are saying, that the natural roll off of the FR125s below the crossover point combined with the 4th order highpass will mean that the sub would need something like a 48dB/octave low pass to match up. Is this right?

Thanks
Adam

[richie00boy]

No. It's that the response of the FR125 in the box will combine with the response of the filter, to end up with a response that is nothing like you thought you were getting from the textbook active filter.

I think even with my method #1 you will have plenty of volume, unless you want it really loud.

[Foxx510]

I want it as loud as possible, without it sounding terrible. What I am aiming for is not a super "audiophile" high resolution system, I just want something that sounds musical, and good with most sources, even mp3s. I think the FR125s are part of the way there, but just need some help down low. I'll build a mike this week and measure instead of guessing. I'm still a little confused as to what sort of response you expect I would end up with using the 4th order filter, but hopefully some measurements will make it clearer. So much to learn!

[richie00boy]

If you want to simulate what you would likely see, do a 4th-order filter as you propose and look at the response. Now cascade a 2nd-order filter with F the same as the box Fc and Q the same as the box Qtc. See how adding that filter affects the response to what you had before.