Actually, what I said in a nutshell is that the footbox where I have the sub installed is so far from being sealed that my sub is more or less in a freeair state. As Razz says, there is a big hole on the end of it next to the ski locker plus there is a hole on top where all the cables run through plus there is a hole that leads to a chaise under the flooor of the boat. I know my Polk MOMO sub is not a free air sub and again, I plan to seal the enclosure or try to make a ported enclosure out of it and see how that sounds.
Originally Posted by Razzman
So I wasn't asking any questions about a free-air sub, I just mentioned that my sub is basically operating in a free air state.
While we're on the subject, what characteristics makes a sub a good "freeair" sub? That is, how is it different than a non-freeair sub?
The crossover frequency selection will be unique to each system. Usually you go a little higher with a small woofer, free-air woofer or woofer that is in a less than optimum application. You'll get an increase in overall output by widening the bandwidth which causes you to gain down the sub. And that results in improved tonal construction, particularly at higher volumes.
Here's one way to proceed. Set your high and lowpass crossovers at 120 Hz. With your tower speakers off, turn up your coaming coaxials to just under clipping (no harshness or significant distortion). With your sub down, incrementally increase the sub's gain until it is a linear extension of the fullrange coaxials. If it sounds boomy, remote or detached then back it down some until the sub sounds like its seemlessly spliced with the mids rather than sounding like an independent source. At this point you'll have good tonal construction with balanced output. If you want more bass emphasis then use your bass tone control. At least it presents a subtle octave-to-octave rise versus the ubrupt edge of a steep crossover slope. Listen to it for awhile this way and get used to it. It may not be for everbody but it is a good reference point.
From this point if you want to increase the sub's gain, the elevated output will raise the frequency of the lowpass -3dB intersection relative to the highpass. So as you increase the sub's gain you'll also simultaneously lower the lowpass frequency to eliminate the broad overlap of the high and lowpass crossover filters. Hopefully your amplifier has independent high and lowpass controls that you're able to stagger the filters. This will allow you to maintain a more uniform phase and amplitude response through the crossover region.
In an open-field environment , even with larger and more powerful subs, we rarely use a lowpass point below 90 Hz. Its very different from a vehicle.
For tuning purposes, use only good recordings without heavy bass emphasis. You don't necessarily want to use your favorite material. Use a variety for contrast. Stay away from downloads, burnt CDs or anything that may have built-in EQ. Have all tone controls flat with the bass EQ on the amplifier defeated.
I know its a protracted answer but there's no perfect frequency. This is a proven technique that has converted many nasty systems to pretty good sounding systems. Plus its a very subjective perception ranging from purists to bassheads.
I hope this helps. Its only a snapshot in the system tuning process.
A free-air sub is self-sufficiently damped so it has different electrical and mechanical parameters. It will also have a higher resonance. A 10-inch free-air would optimumly operate in an infinite baffle of 3 cubic feet or larger. Just like any other sub a free-air requires complete front-to-rear acoustic isolation and a very rigid mounting surface.
In contrast, acoustic suspension subs are very dependent on a precise enclosure (sealed or bass-reflex) for damping and control.
Actually free-air subs can sound pretty good if they're properly executed. Unfortunately you rarely see the correct woofer put to task in the right manner. JL Audio makes the best free-air subs by a considerable margin.
Now that I better understand your sub mounting, here's what I would suggest:
Get a true free-air sub. If a sub manufacturer claims their product is a jack of all trades then you can assume it does all applications poorly. Seal up the floor hump the best you can. You're only concerned with openings that are in close proximity to the sub. The distant openings represent too long of a pathlength to be a problem. You want the sub to realize as much of the bilge cavity as possible.
Reinforce the thin fiberglass hump mounting surface with a carpeted thick KingStarboard overlay.
Properly tuned it will be a day and night difference from what you have now.
Another option is to close off the floor hump opening and install a sealed box over the top of the hump that extends toward the bow mold. There's plenty of room up there to fit a side-firing sealed box with .675 gross (before driver) internal displacement. Its a bit tighter up there in the last several year models of Moomba LSVs but it still easily works with 5/8" material thickness. This would be an ideal displacement for a variety of subs whether JL, Alpine, Polk, Kicker, etc.
And Sandm is right, a bass-reflex enclosure doesn't fit well with your scenario.
David thanks for the explain. I run the Kicker ZX amps and they have plenty of adjustments easy to get to. I usually end up with my subs right around 80 hz and that seems the best.
Also, i have my sub on top of the footwell hump and it works real well up there, nice surround effect without being boomy.
Originally Posted by EarmarkMarine
Thanks for the info above. This will come in handy since I am about done with my amp upgrade.
One question: I have the ability to set the crossover frequencies either from my Kenwood head unit or from my amps. Is there an advantage to using one or the other? The head unit is a Kenwood eXcelon KDC-X692 and the amps are a JL Audio M6600 and a Kicker 08ZX700.5.