Fortunately, there was a rather large access panel on the port side wall in the salon. This became the perfect location for a subwoofer and it's enclosure. Now, if you have read any of my projects on speakers, you'll know that there are two ways to mount a loudspeaker; "infinite baffle" (sometimes called free air), or an enclosure. If not, I am providing a link here:
Its my preference to use an enclosure whenever possible, and the room behind this access panel allowed that. If you read my primer, there are two methods of determining the correct enclosure size; either calculation of the Thiele data, or the manufacturer's recommendation. Fortunately, for the DB104 subwoofer I am using, Polk indeed recommends an encosure; and the volume it specifies is 0.66CuFt. I have just enough room to build this sized box.
It should be noted that Polk makes several sizes of subwoofers, and it might surprise you that often, the best performing speaker may not be the largest. Here is why... For a 12" dia subwoofer, Polk recommends a 0.88CuFt box. However, suppose you may not be able to fit that sized box into your boat. If you have to use a smaller box than recommended, the loudspeaker will not perform the best. It is better to go with a slightly smaller sized speaker (which means you can use a smaller box), and use the box that will fit. In other words, a speaker will work the best if its enclosure is matched to the speaker.
Why this is so is beyond the scope of this project; however, searching the internet for "Thiele-Small Modelling", "optimum speaker enclosures", or similar keywords should provide more than enough information.
So here is the back of the access panel that I removed from the boat. This panel is located behind the sofa in the boat, which will keep the subwoofer hidden. I will mount an enclosure directly to the back of the panel.
The panel is covered with phenolic on both sides; with the front side painted white. As I am going to glue an enclosure to the back, I need to remove the phenolic to expose the plywood core. Othewise, I will not be able to obtain a good glue joint.
Removing the phenolic is easily accomplished with a router, a pattern bit, and a straight edge. Set the depth of the cut just enough to shave the phenolic off without cutting into the plywood. A keen eye might notice the width of the cut is wider than the thickness of the enclosure. I also plan to epoxy fiberglass cloth reinforce the box-to-panel joint, and again, the phenolic must be removed to do this. The box is going to be suspended by the panel, so its not overkill to make a strong bond.
I used 1/2" particle board for the enclosure. As it turns out, particle board is quite acceptable for speaker enclosures, as it is very dense, and suppresses vibrations. The ideal size of the box uses the "Golden Ratio" of 0.6:1:1.6. In other words, if a box had a height of 6", a width of 10", and a length of 16", it would fit the Golden Ratio. While the Golden Ratio is preferred, you must however, make sure the box will have sufficient clearance for the subwoofer.
To assemble the box, a waterproof glue is preferred, such as Gorrila Glue or epoxy. However, I plan to epoxy-over-fiberglass all of the joints, and paint the entire exterior with epoxy, so almost any glue will work for assembling the box itself.
The assembled box is now glued to the access panel. Here epoxy is a must as this must be a stong bond; otherwise, of course, the box might fall off. The use of clamps will help with the gluing process. Yes, epoxy is messy/gooey stuff, but get used to it, it is the boater's best friend. My epoxy of choice is West System; however, any good marine-grade epoxy will work just fine.
After the box was glued, I epoxied fiberglass tape to every joint, then "painted" the entire box with epoxy glue. After that cured, I painted the box a dark grey. Sure, it will be on the backside, but I like to be thourough.
The next step then would be to cut out for the subwoofer. I cannot emphasize here enough to follow the "measure twice - cut once" procedure, but you knew that. The size of the hole must be proper for the subwoofer to mount on the surface, yet ideally, be centered over the enclosure on the back side.
A Dremel Spiral Cutter with a circle cutting template works well here. Those "As Seen On TV" Spiral Cutters were all the rage a few years ago, and if you can still find one, here is finally one practical application for it.
After cutting out the hole, line the inside with Dacron. Only one of side of each opposing surface needs to be treated (one end panel, one side panel, and the rear panel); however, you can do all of the surfaces if you wish as Dacron is quite inexpensive, and you will surely have plenty of material. The easiest method of gluing the Dacron is perhaps to use 3M 77 Spray Adhesive.
The purpose of the Dacron is to dampen the sound reflections in the box that may get bounced around. The combination of using the Golden Ratio and Dacron should minimize them.
You will also want to consider how to get the speaker wiring in the box, and now is the time to do so, as the next step is to mount the subwoofer. Almost any method can be used (i.e. connector, or simply feeding the wire through a hole), as long as you seal the entry point so the box remains sealed.
Next the subwoofer gets surface mounted. I like to use a bit of silicone to the backside of the subwoofer to form an air tight gasket. Some people do not like to do this, but I have found that if you use silicone SEALANT, and not MARINE ADHESIVE (such as 5200), the speaker can easily be removed if needed as the silicone's adhesive properties are rather weak. A silicone seal not only ensures an air-tight fit, but also cuts down on vibrations between the speaker and the panel it is mounted on.
I also added rack handles, as well - I like rack handles. But more than that, this assembly weighs in at around 20lbs, so the handles aid in installing the panel. I should have indicated that the panel covers an access hatch on the boat, and must be removed occasionally to perform certain maintenance on the systems.
Finally, I used a Neutrik SpeakOn quick connector for the speaker leads. You can see that here, along with the fiberglass reinforcement along the seams.
I am about to replace the access panel on the boat. The keen eye will notice the rear of the enclosure is painted grey, and can also see the fiberglass reinforcing tape I mentioned. All that is left is to wire the speaker into the system, then mount the panel to the wall.
Although the speaker is located behind a sofa, I decided to install a set of "nurf bars" to protect the speaker.