Hi-fi building

A couple of weeks ago I took Jim to Brownchurch in leyton to have a roofrack made. Whilst the truck was away, I took the opportunity to start on a project which for which I didn't need the truck next to me.

A year or two ago, I bought a cheap pair of speakers which has been removed from an install in a church. The cabinets were useless, being badly tuned chipboard monstrosities, but the drivers were a pair of 8" coaxial units made by Volt Loudspeakers, a well regarded UK manufacturer popular in studio and high-end hifi circles. Whilst the truck was away I started building a pair of boxes to rehouse the Volt FR220.1 drivers.

All loudspeaker driver are characterised by a set of variables called the TS Parameters; these define various characteristics such as the resonant frequency of the cone assembly, the motor strength, the compliance of the suspension, etc. On their own they don't mean much to me, but by using suitable software, they enable you to model a driver in a theoretical enclosure, and predict the response of the speaker. Most modelling software can predict the response in term of frequency, impedance, phase, excursion, etc although for a simple box like I needed, I was only interested in the frequency response.

I used the free, easy to use, and well regarded WinISD software. The software models various types of enclosures, but for a simple (mostly) full-range speaker, most people will choose a ported enclosure, which is essentially a box of determined volume with a port tuned to alter the frequency response. If I was going to build a more complicated enclosure such as a rear loaded horn, there are a number of more advanced programmes I could use, but such an enclosure would be complete overkill for a motorhome and would go down very badly with Naomi.

The drivers I was using are optimised for very small enclosures. Using WinISD I worked out that a box with internal volume of about 9.5 litres was ideal. Using a larger box would extend the frequency response a little lower, but the gains were small, even when doubling the volume.  Tuning the box to 60Hz gives me a frequency response which is flat down to 80Hz and usable to around 60Hz. This is not as low as many good hifi speakers but is a good deal better than most car, and truck audio systems and most people would not notice the notes missing at the bottom. Cutting these speakers off at 60Hz or so also conserves precious battery power, as it takes considerably more energy to reproduce bass than the higher frequencies.

At any rate, with the music I love and the way I tend to listen to it, people would be taking me to the doctors if I didn't also fit a sub in the truck.

Having settled on an enclosure volume and tuning, WinISD enables you to calculate the area and length of parts you would require. Based on two 30mm sections of waste pipe, the software told me I would need to cut each to a length of around 9cm. This area of the software is probably the least reliable as small changes in the cabinet, such as proximity of the ports to a wall, will affect the tuning. I will use the recommended length as a guide but will cut the pipes long and remove sections until I am happy with the sound.

Using the volume and port size calculated, I designed a simple enclosure to suit and started building. I wanted the speakers to swivel between the cab of the truck and the box on the back so that I could listen to them whilst driving or whilst parked; with this in mind, I designed the speakers with a curved back to allow me to fit them closer to the wall on their brackets.

I had a load of 6.5mm plywood left over from another job so these boxes are built from that. All the pieces are double layered (13mm) except the baffle which is 3 layers thick.

To do the curve I kerfed the ply at 8mm centres, leaving just over one laminate. I didn't realise the importance of bending along the grain, and so on the first cabinet, the outer curved piece was cut the wrong way. Consequently the panel cracked along several of the kerfs and so I had to use lots of filler to get it smooth. This speaker has not come out as well as the second but I can live with it. I had planned on bonding the curved sections with thickened epoxy resin but after experimenting with PVA I realised it would be more than strong enough.

The curve radius is 120mm, I would say that this is pretty close to the minimum bending radius you can achieve using this technique, you might be able to get it 10-20mm smaller. I had a load of 6.5mm birch to use up, so this technique worked well for me, but given the choice I would rather layer up 3 or 4mm sheets. These cabinets are small (266x266x273) and I still had to cut 100 kerfs per box, for a large box this would be a painfully slow process.

 

After building  the boxes, I filled the screw holes and joints with polyester filler, and started to put on some paint. I have now put on a coat of primer, 3 coats of undercoat, and a couple of coats of white satin. When I get the time I will give the boxes a quick sand and a final coat of satin before fitting the t-nuts for the hardware and drivers, and wiring the connector in.

Next job is to weld up a couple of brackets to fit the speakers to the walls in the truck. Brownchurch kindly gave me a length of 40x8 flat bar which should do the job.