Subwoofer Enclosure Boxes

[txav8r]

That recess that Yamaha designed into the floor of the starboard compartment, to hold the ice chest has to be filled to give a base for the aft corner of the box. Or the appendage bracket could rest down in the bottom of it on the aft starboard corner of the box. But I kind of envisioned covering it with carpet and making that recessed hole disappear. The two perpendicular walls That form the bow and isle side walls are irregular because the compartment floor is recessed below the cockpit floor. And the compartment floor is not online with the wall. So the angles here are wacky. To fit it into the absolute corner is tough. Of course the attempt is to free up as much of the compartment as possible but I can see why you would just mount a square box on the wall directly opposing the hole for the woofer. I'm not throwing in the towel on this. The port wall is worse as the seat box pedestal has a tray lowering into it for a copilot a chair support. So any box has to mount much farther aft on a 240/242.

That helm compartment is inviting. But mounting a box in there with a face flush to the helm wall for the driver to mount to is a bigger challenge.

 

[txav8r]

Ok, one more question on this @David Analog . The silicone gasket. With the carpet in the compartment, cutting it back to the edge of the box will let it come loose according to Odin. He suggested leaving the carpet and just sandwiching it with a silicone seal. I may have misunderstood this and he may have intended that the carpet edge was inside the box edge but cut back enough for a good bead of silicone to seal the box to the wall. In any case, that wall is pretty irregular on the inside, so the carpet may not be of any issue. But I think unless you have better guidance, I will cut the carpet back and expose a fiberglass edge to silicone the box to.

 

[David Analog]

The 10-inch woofer hole is 8-7/8". The 1 cu.ft. enclosure baffle is likely to be 13.5"X13.5". That leaves about a 2-1/4" perimeter radius to seal. I would want to trim back that carpet to see how smooth or irregular that interior fiberglass surface is. Maybe I would sand it a bit and clean it up with mineral spirits. Maybe I would float it with Kitty Hair (a small strand fiberglass compound). But I probably wouldn't try to silicon seal over the carpet fibers and without knowing exactly what the surface is behind the carpet.
Mel, definitely get with me BEFORE you begin the enclosure construction.

 

[txav8r]

Will do...I am just trying to build a cardboard model to check fit and see exactly what I have to do to it. I have almost eliminated the isle facing spot, due to space and irregular surface. They bonded on a 1/4" piece of plywood as a stiffener right in the middle of where I need to put the woofer. But it extends to the bottom of the wall, so I can't cut it out completely. That makes that spot worse in some degrees than the radius'ed wall on the bow side of the compartment. Either way, it isn't flat, although at least the outside speaker mounting surface is flat on the isle side. On the bow side, I have to bevel the sides of a starboard ring to seat the speaker, and I probably have to bevel a birch ring to act as filler in the compartment, to seal against the box. Don't you just love a challenge?

 

[David Analog]

Sub inside the helm cavity??? There certainly are trade-offs. Here are a few acoustic factors that you just can't get around.
Placing a woofer/enclosure inside an expansive compartment with a rather small vent into the cockpit is like placing a woofer/box inside a giant bottle. The acoustic losses are significant so the woofer and amplifier have to work much harder to produce the same output = not as clean. The huge helm cavity becomes a non-linear resonant chamber which means whatever energy escapes does so with an entirely different character = quick midbass transients and bass articulation are just gone. The time delay is REALLY protracted which means the woofer sounds remote, detached and will never coherently sum with the cockpit coaxials. So the subwoofer becomes very tactile and a great boat shaker but not much of a musical bass maker.

I've always maintained that a good objective for the compartment vent is to be acoustically transparent and equal in open surface area to the sub driver. That's only a practical objective because the ideal scenario is not possible. In reality the size of the vent should be determined by the compartment displacement. In order to avoid steep acoustic loses, radical phase delays, and significant changes in the tonal construction, I would want a 3 sq. ft. vent as a minimal ratio to a 30 cu.ft. compartment.

 

[Evil Sports]

Can someone tell me why you would want to make a box instead of mounting it below the seat like most of us

 

[harsh]

I have ballast sacks on both sides of my boat and did not want to give up any storage space so I opted to use dead space for my box build. Storage space on a 21' boat is priceless to me.

I agree with txav8r. Its like a sub fully enclosed in the trunk of a car. It you port it into the living area the sound SQ goes up dramatically. With respect to the screws pulling out, you can find a number of threaded inserts to put into the MDF. They have a nice course thread outside that you can screw and glue into the MDF with a machine screw thread inside. They might work. Cam.

im

Can someone tell me why you would want to make a box instead of mounting it below the seat like most of us

 

[Evil Sports]

Ohh that ballast thing. Non surfer mistake

 

[harsh]

Ohh that ballast thing. Non surfer mistake

My previous jet boat was a 2008 212ss and I had 1 jl 12" under each seat with a 1.75 ' box which was definitely easier to construct with the sub on the outside and fiberglass constructed box on inside.

 

[Evil Sports]

I dont have a box around mine, am I missing something here??

 

[harsh]

I dont have a box around mine, am I missing something here??

probably just a matter of opinion but yes a correctly built box tuned to the right frequency will outperform any infinite baffle subwofer. That is my opinion and im not an expert, just from trial and error

 

[txav8r]

@Evil Sports , Bob, if you have an iB sub, infinite baffle or free air, it is designed to perform with a larger volume of air that doesn't have resistance. The acoustic suspension sub is designed to have limited space and at higher volumes, can get more extension and faster than an iB sub. I ran an iB sub and was quite happy with it. I am going the enclosure route to see for myself the difference. At low to medium volumes, I am told not to expect any difference. I assumed, maybe a mistake, that at high power settings on both the boat and the volume, that I would indeed notice a difference. David tells me that the wind and engine noise would certainly drown any advantage one sub setup has over the other...but at anchor and high volumes, you may indeed hear for fluid bass provided by a confined box that produces a tighter response. As they say 6 of 1, half dozen of the other. David also shared with me that surface area is the major contributor to bass response, followed by power. So a 12" sub, needing probably 4 times the power of my single 10", and a correctly tuned box and positioning would probably blow away two 10" subs. But subs, amps, and speakers are not all created equal, no matter what the spec's say, they just aren't.

 

[David Analog]

I dont have a box around mine, am I missing something here??

It absolutely depends on the particular subwoofer.
An infinite baffle subwoofer has a 'Qts' (open air accumulated damping factor) that is very high. So you could say that the woofer control is more self-sufficient. It still needs an enclosure in the form of front to rear acoustic isolation but it wants a compliant air mass large enough that it won't alter the woofer 'Qts'. In the example of a JL Audio IB 10" that air mass would be from 2.25 cu.ft. to infinite.
An air suspension subwoofer has a much lower 'Qts' but when placed in the correctly sized enclosure the 'Qtc' (sub loaded into box or 'system Q') is generally identical to the open air 'Qts' of an infinite baffle sub. So you could say that the air suspension woofer is very dependent on the trapped air mass within a sealed box to act as the primary means of control.
The final 'loaded' parameters (resonance, - 3dB down point) of the two versions are generally about the same. So one might ask why bother with an air suspension sub? Because air suspension when executed correctly is a more linear form of suspension and control....meaning, it maintains better control of the woofer at greater excursions (which translates to greater amplitude). Keep in mind that a hard driven woofer may incur 90 G's in transition. At 50 to 100 Hz (violent cycles) the large woofer cone, voice coil, and suspension parts are a lot of mass and inertia to control.
Every woofer needs to be 'tuned' to an ideal enclosure based on its individual Thiele/Small parameters. Over-damped means less low frequency extension (sounds very live but singular in its tonal construction). Under-damped means poor transients and control (goes deeper but sounds a bit lifeless).

 

[David Analog]

To place a finer point on Mel's power analogy....
The easiest and most efficient method to produce more output is with more cone surface area. If all other factors remain equal and you double the surface area you get very close to double the output (+ 3dB), "double the output" in acoustic terms and not as perceived.
The second and far LESS efficient method to produce more output is with greater excursion. It's a distant second because you will be way off a 1 to 1 ratio. To double the excursion it requires at minimum of double the power. Past a certain threshold, dynamic factors come into play, and it could require four times the power to double the excursion and double the output. Eventually, as you reach the speaker's dynamic limitations, you won't continue to increase output as you increase power. You will hit the wall. You can however over-drive the amplifier and speaker into distortion. A distorted woofer tends to protract the signal (BOOOOOM instead of BOOM) which many falsely interpret as additional output.

 

[txav8r]

@David Analog So what about recommended acoustic isolation? If a manufacturer "recommends" from 1/2 cu ft to 1 cu ft, are they saying optimum quality and performance will occur within that boundary or are you limiting those on one end of that range vs the other? If I don't need to build to 1 cu ft, and 1/2 cu ft gets me the same quality and performance, that would be better. Or is the recommended really .75 cu ft and they are giving you some leeway?

 

[David Analog]

Mel,
I used the term 'acoustic isolation' in describing that an IB woofer, even though it is often called 'free air', needs the front radiation completely isolated from the rear radiation for a minimum length no less than one quarter wavelength of the lowest frequency the subwoofer is expected to play.
In the air suspension version, you usually target a system 'Qtc' (driver in sealed box) of .80. An enclosure too large has a lower 'Q' and is under-damped while an enclosure too small has a higher 'Q' and is over-damped. Having heard this woofer in several applications I believe the best results are with a gross internal size of 1.0 cu.ft. that will decrease with the woofer displacement. You'll still net above 0.75 cu.ft. with a couple of enclosure build specifics yet to come. So stay with the 1.0 internal gross.

 

[txav8r]

As mentioned, I have to clip off the lower baffle end of the box to make it fit flush. And I have to have 12 inches of mountable surface above that clip. So the interior dimensions come out 14" tall, 12" wide, and 10.75" deep. The clipped lower front edge loses 94 sq in, so that brings me in at 1712 sq in vs 1728 sq in in a cu ft. Pretty close! The box overall will take up 15.5"Hx13.5"Wx12.25"D. Now that addition of a 1/4" "C" shaped ring to take up the slack in the interior mounting surface, adds a few square inches to the equation too, as does the compartment wall thickness. But the box itself comes in at 1712. Provided my build quality can be that accurate. The only point in question is the cutout at the bottom front wall. I can get the angles pretty precise on the sidewalls as well as the bottom and front edge, but trying to get the angles of the piece that fills it to match won't be perfect. I may get voids in cutting that will need filling, silicone or epoxy putty?

 

[latitudeadjuster]

Trying to find the thread, pretty recent, of a sub added on wall just below the throttles? Anyone remember or be the one who posted??