Mobile Electronics Australia

Welcome to part three of our marine audio build, and if you haven’t seen Part One and Part Two by all means check them out prior to reading this stage of the build. It was time to design and fabricate the subwoofer enclosure for the Scarab, but with this project neither Craig nor I had any experience in what would be required to produce serious SPL in an open top marine environment. When motoring through Sydney Harbour the Scarab had an extremely high noise floor thanks to the wind, the waves smashing against the hull, as well as those dual 350 Chev engines roaring away at 4000RPM.

Adding complexity to the issue was the need for low current consumption from the amplifier array, since much of the listening would be done while anchored when the engines are off leaving the entire sound system and the boat’s other ancillaries like lighting and refrigerator all running off the battery bank.

Fortunately, Clarion’s marine range gave us a clear forward path thanks to the inherent design of their gear. The largest and most potent marine subwoofers in their range are the CM-Q250W 10-inch models, and throwing their electro-mechanical specs into BassBox Pro revealed them to be a very user friendly design. Each 10-inch would work extremely well in a very compact ported enclosure, with tuning at around the 35Hz mark offering nice linear response which would be ideal for open air applications where cabin loading doesn’t afford any low end gains.

To combat the efficiency issue we decided on a triple array of three CM-Q250W subwoofers in a shared chamber measuring approximately 3.25 cubic feet in internal volume. These would each receive a modest 90WRMS from one of two APX490M 4-channel amplifiers that would eventually be used in the system. The combination of large driver area and efficient ported box design should overcome the lower power input and, therefore, meet our requirements for both SPL and low power consumption. The location for the enclosure was a vacant space in the left side of the hull where all the passengers would sit.

Stage 1: Creating the front panel

The trim panel that covers the left side of the boat’s interior was removed and taken back to Craig’s workshop nearby where we would do the basic fabrication work prior to final fitment back on the boat the next day. This panel was used as a template to cut out a new panel that would become the cover panel that would sit over the new subwoofer enclosure.

Stage 2: Subwoofer & port recesses

The cover panel features an oval opening that the subwoofers would be flush fitted into, while the ports for the enclosure would fire downwards to avoid water getting into the enclosure. These opening were cut into the front panel and then this panel was used as a template for the actual front baffle of the subwoofer enclosure itself. This way the trimmed cover panel could be removed while the enclosure would be permanently fibre glassed to the boat’s hull.

Stage 3: Cutting the subwoofer baffle

The subwoofer baffle is thicker than the trim panel for added rigidity, and after careful measurement to ensure they lined up perfectly we cut out the holes for each Clarion CM-Q250W subwoofer. Since these feature neat plastic clip-on grille covers we need to allow for their width when measuring. When using a jigsaw on plywood there is almost no sawdust compared to the fine particles you get with MDF, so this is a much cleaner build than usual – for now…

Stage 4: New versus old panels

Here you can see that the new front panel is taller than the original panel, which was something we decided to do when conducting the initial measurements on the boat. The second image shows the subwoofer baffle as it will sit behind the new cover panel.

Stage 5: Building the sides

Due to the curved shape of the hull, and given the fact that we didn’t have the boat on hand for trial fitting during fabrication, we decided to build the top, bottom and side walls of the enclosure deeper than required and then simply trim them down once we got onto the boat. The rear wall of the enclosure would actually be the outer hull of the boat itself. You can see the dual 4-inch port openings that are in the bottom wall of the box. Once the correct PVC port tubes are inserted the ports should ‘load’ off the floor of the boat and hopefully gain a few precious decibels for free. The panels of the enclosure are both glued and screwed for a solid construction. Given its strength, plywood is surprisingly light compared to similarly built MDF enclosures, with marine ply given obvious advantage for marine use.

Stage 6: Trimming the cover panel

Where plywood is a little more difficult is during trimming in vinyl, since the physically small but large number of imperfections in the more natural surface (compared to MDF) all become visible under the vinyl very easily. Trimming the large panel takes a few goes and you really need to make sure you’ve sanded and cleaned the surface from dust and other imperfections to get a good finish.

Stage 7: Transfer to the boat

With the bulk of fabrication done it was time to load everything into the car and transfer over to the boat to continue the build. This was a time consuming process since you had to bring not only the parts you’ve fabricated by all the tools you’d need as well. Simply forgetting one important tool back at the workshop could be a major headache and waste hours of time. The Scarab is anchored about 200 meters from shore so we ferried to the boat each time via private tender. Ahhh, the glamour of marine life!

Stage 8: Positioning the enclosure

Here is the side wall of the Scarab where the enclosure will be situated, which sits neatly between the captain’s chair and the rear lounge. A host of control cables for the engine run down this side of the hull which needed to be accounted for during planning, as these need to run over the enclosure and remain fully functional. Craig sat the enclosure into rough position prior to marking where it needed to be trimmed down to fit perfectly.

Stage 9: Trimming the enclosure

Trimming the enclosure to fit snuggly into its new home took a number of test fittings, measurements and cuts with a hand held jigsaw. It was for this reason that final port dimensions were not calculated until we were done so that we had a fixed enclosure volume to calculate from. While the diameter would stay the same for each port their lengths would vary slightly to ensure the correct tuning frequency we were aiming for.

Stage 10: Securing the enclosure to the hull

With all the trimming completed the enclosure sat neatly against the hull and was ready for fibre glassing. The outer hull of the boat is a combination of wood and fibre glass that was the ideal surface to attach the new enclosure walls, and obviously you can’t simply use wood screws through the hull to secure the enclosure!

Stage 11: Trimming the subwoofer baffle

To match what we had done with the main speaker array in the previous stage Craig used stick on fake carbon fibre material as a contrast to the white vinyl. This was placed over the subwoofer baffle prior to the installation of the subwoofers and ads a nice modern touch to the interior of the boat. It’s also cheap and easy to apply given the large, flat surfaces.

Stage 12: Installing the subwoofers

Clarion’s marine subwoofers differ in construction to normal automotive subwoofers in a number of ways. The frames are actually cast plastic to avoid corrosion, while the cones are polypropylene and feature rubbed edge surrounds. This way moisture can’t get access to the copper voice coils easily. The frames are also pre-fitted with nice thick rubberized foam gaskets to ensure a perfect seal into the enclosure. Prior to screwing all three CM-Q250W subwoofers into place Craig soldered on some nice thick speaker wires.

Stage 13: Subwoofer enclosure finished

The clip-on covers that come with the CM-Q250W subwoofers are normally silver in finish, but since we wanted a matching look throughout the installation we decided to give them a coat of white paint instead. With the grille covers in place the speakers for this installation are all buttoned up and ready to go. In the next stage we’ll tackle the amplifier installation. See you then.