The Andromeda, or building a reference speaker
By Tony Gee
02 Apr 2000
A loudspeakerbox is more than just a box
Once you have got these very nice drivers, what do you put them in? It would be a total waste just to build a simple rectangular MDF box of the correct volume and mount them in it. In the past I have done several experiments on cabinet constructions and have come to the conclusion that the quality of sound produced by a loudspeaker can simply be divided into three equal parts:
- 1. The quality of the drivers.
- 2. The quality of the crossover.
- 3. The quality of the cabinet.
It is amazing how much of what is heard from a loudspeaker is produced by the cabinet. Vibrating cabinet walls, standing waves inside the cabinet, diffraction interference due to sharp edges on the outside of the cabinet, the baffle transferring vibrations from one driver to the other, and so on.
As you might have guessed I wanted to build a cabinet of which you could only hear the positive things. So that vibrations of the woofer could not interfere with the other drivers, the woofer would be mounted in a separate cabinet.
The woofer cabinet.
Fig. 15. The CAD drawing The basic construction for the cabinet walls is a sandwich of two layers of MDF of different thicknesses with a layer of lead-bitumen in between. Fig. 4. A close-up of the sandwich construction Lead-bitumen comes in flexible sheets of 1x1 meters, 4mm thick and weighs 8 kg/m2. From the outside in you have 22mm of MDF, 4mm of lead-bitumen and 18mm of MDF. All three are glued together using a paste made for glueing parquet floors. No screws are used otherwise vibrations from the inner panel may be transferred to the outer panel.The cabinet is internally strengthened with a sort of matrix made of 18mm MDF with circular cutouts to let the air flow freely.Fig. 5. The bass matrix To minimise vibrations of the bass-driver, its magnet is supported by a cutout in the internal bracing. Fig. 6. The woofer support
All internal walls are covered with Pritex foam to minimise standing waves. The front baffle has two outer layers of MDF and therefore has a total thickness of 62mm. The front edges are angled just for looks. The total weight of the woofer + cabinet is 95 kg (209 pounds).To stand this firmly and level on the ground you need spikes. A very cheap and easy way to make spikes for something this heavy is to use four lengths of M10 bolt-thread mounted in the bottom of the cabinet. Place a large washer over the thread and then fix it with a M10 nut. There should be about 30mm of thread left sticking out onto which you screw a M10 capped-nut (a nut with one end closed by a round shaped cap) - being a nut it is very easy to adjust the height using a spanner. The cabinet will now have minimal contact with the floor as it is resting on four little metal ball-shaped "spikes". The internal volume is about 100 litres and the bass-reflex ports are tuned to a low 25Hz. I had originally designed it as a closed box with a Qtc of about 0,5 but I wanted a little more low bass so I added the ports. This makes it sound more exciting when listening to low synthesizers or Jurassic Park, but for normal listening I just fill the ports with old socks to get a nice tight sounding bass. Do not misunderstand me, even with the ports open it produces about the tightest bass I have ever heard, but with them closed it is simply incredible.
The other cabinet.
The cabinet for the mid-woofer and the tweeter is a closed cabinet built using the same principles as the woofer cabinet except for the damping material, which is a 100% fill of glass wool and bonded acetate fibre. Fig. 7. Damping
The tweeter is housed in its own separate sub-enclosure to stop unwanted vibrations in the box reaching the rear of the tweeter, a piece of foam-rubber is pressed between the back of the tweeter and the rear of the sub-enclosure to stop any vibrations in the tweeters rear cover. Fig. 8. Inside the top enclosure
The "collar" around the sides and rear of the cabinet is made of 30mm thick MDF making the total wall thickness here 74mm. The front baffle has angled edges to cut down diffraction interference.The total weight of the drivers + cabinet is 30 kg (66 pounds). This cabinet is placed on the woofer cabinet using three gold-plated SPS-10/GO spikes and protectors by Monacor.