A lot of boaters would like to improve their stereo system on board. But one of the issues we have is where to mount speakers? Often, there is simply not much room on a boat for a set of speakers, and as well, some factory speaker installations leave a lot to be desired.
The most important characteristic that determines the low frequency capability of a speaker is the volume of the enclosure it is mounted in. The volume of the enclosure is determined by the construction of the speaker itself, and the type of enclosure it is mounted in. For simplicity sake, we will be using a Sealed, or Acoustic Suspension enclosure.
Each speaker has its own volume requirements, therefore, the enclosure must be designed for the specific speaker in mind. Calculating the volume can be a complicated process and is beyond the scope of this article. Fortunately, many speaker manufacturers provide this information for the asking. I am using Polk DB525 speakers, and Polk says that the ideal volume should be from 0.15 to 0.25 cubic ft. That was simple.
The enclosures will be constructed in the shape of a ball using a "lost Styrofoam" casting technique described by West System Epoxy. Lost Styrofoam means that the Styrofoam will be destroyed, or lost in the process, so you must make one pattern for each speaker enclosure you make.
Step 1. For the enclosures, I used 8" Styrofoam balls, which turns out to be about 0.15 Cu ft, right at the low end of the recommended volume. If using round balls, you will have to "flat spot" the ball on one surface to mount the speaker. Unfortunately this reduces the volume further, but we can compensate for that later.
Other shapes can be made by gluing up blocks of Styrofoam, and using a Stanley Surform tool, round the block to the desired shape.
This process has only been tested with Styrofoam. Use of another brand foam may not work.
Step 2. Using 3M Super 77 spray adhesive, I applied a layer of 2" wide 9oz fiberglass tape to the balls. Be careful to use only enough spray adhesive for the tape to stick. If you use too much, the resin may not flow into all of the fibers in the cloth.
Step 3. Using West System 105 Epoxy resin and 205 fast hardener, mix a batch of epoxy, and put a coat around the ball. Make sure you wet out the fiberglass tape well. Depending on the temperature, the cure time may vary from one hour to 8 hours.
Do not attempt to use polyester resin as commonly found in auto departments; it will eat the Styrofoam.
Step 4. When the epoxy is almost cured, but has a slight sticky property, add a second layer of fiberglass tape to the balls. The stickiness of the epoxy will help hold the fiberglass tape down. After the tape is applied, put a second coat of epoxy on the balls.
Two layers of fiberglass should be considered minimum. If you have the time and materials, adding a 3rd layer won't hurt.
Step 5. After the epoxy is thoroughly cured for 24 hours sanding of the surface to remove imperfections is now required. Sanding will also remove the Amine Blush (a wax-like substance on the surface), which may prevent proper adhesion of new epoxy. You only want to sand enough to knock down any of the high points and remove the glossy Amine Blush, but not enough that you will sand into the fiberglass tape.
Step 6. After sanding, make two batches of epoxy. To one batch add West System 410 Fairing filler to create a paste, with a consistency of peanut butter. The paste will be thick enough that when applied to the balls, it will stay put.
Using the batch of epoxy without the filler, apply a thin coat to the balls as a wetting coat, which helps adhere the filler. Then immediately apply the epoxy with the filler to the balls. Here is where you will fill in all of the valleys and imperfections in the surface. If the consistency is right, you may have some sagging and dripping, but the filler should pretty much stay put.
Step 7. After the filler has completely cured, sand the ball smooth - again, knocking the high spots down, but not sanding into the fiberglass cloth. This is the most arduous part of the project, and may require several coats of filler to get a perfect surface. You may use a paint primer after the last filler coat to help determine if there are any remaining imperfections.
Step 8. Now it is time to remove the Styrofoam. Begin by gouging out the center of the Styrofoam to form a pocket. Then, in a well ventilated area (outdoors), pour Lacquer Thinner into the Styrofoam pocket. The Lacquer Thinner will rapidly eat the Styrofoam away, turning it into a goo. Rinse the balls out a couple times with Lacquer Thinner to remove all of the goo possible.
Be sure to properly handle, use, and dispose of the Lacquer Thinner and goo left by this process.
Step 9. At this point, there will be some goo left on the inside of the balls. If left for a few hours, this will harden and won't hurt anything. If you used two layers of fiberglass cloth, you should have a fiberglass ball about 1/8" thick.
Step 10. Using a Dremel tool, cut the ball opening to the proper size for the speaker. Then cut out a wood backing ring and mounting pad out of Maple and epoxy them to the inside. If you cut the ring as shown, it will easily slip into the inside by using a technique similar to putting a key on a spit-ring keychain.
The wood parts were glued into the ball using West System 403 gap-filling adhesive filler added to the epoxy mixture, again mixing to a peanut butter consistency. Since the inside of the ball will be rough due to the Styrofoam's texture, the filler will help create a good bond with the speaker backing ring. Clamp until cured.
Attach the mounting pad in a similar manner. You may also wish to run a fillet around the wood parts to provide additional strength.
Step 11. The finishing steps include priming then painting the balls with your favorite enamel, mounting the speakers, and attaching a mount. To mount the balls, I used the 109H swing arm RAM Mount system by National Products, Inc.
Ram mounts include a wide variety of mounting configurations, one of which should be perfect for your mounting needs, whether they be on a radar or wakeboard tower, or in the salon as I have done. The RAM mount is attached to the ball with hardware through the backing plate you installed in Step 10.
Step 12. As it turned out, the internal volume of the balls were just a bit on the small-side. For this reason, and to reduce any internal reflections, I loosely filled each ball with Dacron filling. Dacron filling has the effect of making the enclosure volume seem slightly larger to the loudspeaker.
When mounting the speakers to the balls, be sure to use a bead of silicone sealant to make them air-tight, especially if you desire to mount the systems on deck. According to Polk, the DB series speakers are marine certified, so they should do fine out in the weather.
Wiring the speakers is done simply by drilling a hole into the speaker where the wood backing pad is located, running the wire, then sealing the hole with a grommet and/or sealant.