Ported mains and x-over frequency

[toenail]

I'm looking for feedback about situation I have with my HT set-up but posted it here as it applies to any ported speaker in general.

What effect is there when electronically crossing a speaker above the tuning frequency of it's port? I've always used sealed speakers in the past, and when listening in two channel stereo I don't bother with a subwoofer. For HT use, I get pretty muddy response if I combine the ported mains playing full range with a sub crossed at pretty much anything, even as low as 40hz. I've tried crossing the mains at the same frequency as the sub, but seem to get a midbass gap. I know that a port's contribution to overall sound increases as you approach it's tuning frequency. Does this mean that the frequencies leading down to that tuning frequency are also affected by the x-over point? Could this be causing my midbass gap when using same x-over point for sub and ported mains?

[N. Abstentia]

Just general answers here...let's just say your mains are tuned at 35hz which is probably a good ballpark for most ported mains. If you cross them over at 80hz or above at 12db/octave then it would mean nothing at all. It's already 12db down at 40hz so it wouldn't matter.

But if you cross them over at 40hz at 12db/octave then you'd have a bump at 35hz.

I'd say your midbass gap is more due to the sub, what kind of sub is it?

[toenail]

The sub is a DIY project from parts I had around. It uses two JL Audio 12W-0 woofers in a clamshell isobaric cofiguration with a ported cabinet. I modeled with WinISD and came up with tuning frequency of 22.5hz (if memory serves). When playing 20hz to 20khz sweeps to just the sub it has pretty flat in room response until upper roll-off, at least by ear. There are some variances for room modes/nodes etc but many are accounted for by correct placement and listening position.

[N. Abstentia]

Yeah, personally I think you have two things working against you...one being isobaric and the other is using car subs in the house. Those subs are designed to max out in the small confines of the cabin of your car so they will have trouble filling up a room properly. Isobaric will also tend to cut the upper bass range. I also remember from my car audio days that JL were boom subs and didn't have much on the upper end due to their heavy plastic cones. You might want to try running a single sub in a standard sealed box just to see (if you have the time/materials). Experimenting is half the fun!

[toenail]

Believe it or not I have experimented with several versions of the DIY JL Audio subs in my home. I've tried single sealed, double sealed, double ported and double isobaric ported. Without question the double isobaric ported went deeper, played flatter (smoother frequency response), had less distortion and was more efficient than any other version I tried. Of course all were done based on modeling with WinISD program before I built and demo'd them, and to look at the projected curves you'd think they would play ruler flat. I realize that in-room response can vary significantly. The car vs room issue seems like it shouldn't be a factor. I was under the impression that the models were based on theoretical anechoic response. In that case flat should be flat, regardless of home or car use. As far as upper frequencies, I would also think that the 80hz range shouldn't be affected by the cone mass or the clamshell configuration.

I'm starting to think that some of the difficulty I'm having is related to my lack of understanding the HT receiver that I'm using. I was just re-reading the manual and experimenting with some of the settings. In one section it tells me to set the main speaker to LARGE if I have large speakers. Then in another it states that I can select Sub, Main or Both for directing LFE. It appears that setting the main speakers on LARGE eliminates LFE from being sent to the sub, regardless of whether the sub output is set on Sub or not. Now that I've re-set the Mains to SMALL, my bass issue seems largely resolved. Wish that had been a little more clear in the manual. I don't see where it indicates one would superceed the other.

I may yet experiment with a home audio driver to further satisfy my curiosity. For the time being my non exsistent bass has returned and I don't have the mudiness of full range mains competing with subwoofer output. The result is reasonable so far. I'll have to experiment for a few days with some reference material to see if this really did clear up all of my issues.

[kexodusc]

The only way I could ever tame the muddy bass response in my system was to set all my speakers to "small" and let the sub handle 60Hz or 40Hz and below. This holds true especially for exaggerated LFE tracks in typical home theater use. Even then, the room would tend to enhance certain frequencies while diminishing others...When I used speakers to handle the bass, the response was a bit more even, but it could never reproduce the fun impact a good sub provides in an HT.
My own experience with ported vs. sealed speakers in this situation is that a sealed speaker generally behaves a bit more ideally as far as blending with a sub, but it requires the sub to be crossed over at a higher frequency, which too often leads to unpredictable responses. Doh!

The conclusion I came to was that room acoustics have way too much impact on frequencies below 80Hz. I plan on eventually buying a quality Parametric EQ to calibrate my sub...I don't think the YPAO on my Yamaha receiver has much impact below 80 Hz.

I'm still very torn on the choice to use sealed or ported speakers. There's certain attributes I love and hate about both. Think my next speaker project will be a closed box though...just for the sake of doing it...then a T-line.