A Clear Case for a Subwoofer
A Subwoofer For The Cube
Monday, May 14, 2012
A few years ago I bought an Apple G4 Cube system. I had wanted one when they came out back at the turn of the century, but they were too expensive, and I had a serviceable Mac. Time and obsolescence brought the price down and I got one to experiment with.
The Cube’s USB audio system is one of its idiosyncrasies. There are two small clear acrylic balls fitted with 1.5” aluminum dish Genesis drivers made by Harman/Kardon. The appearance of these is so striking that half of the computer speakers to come out of China since have had shiny concave cones. The H/K balls sound remarkably good with surprising bass output for their size. But tiny cone area and a 10-W/Ch T-amp can’t overcome the laws of physics, so I started thinking about a subwoofer.
I had an IBM subwoofer from a flea market which had sat in a closet for many years, mainly because it had no bass. It put out mostly lower midrange. I tried it with the Cube and was reminded how disappointing it was. However, the woofer inside was obviously moving at lower frequencies, so perhaps in another box, properly tuned...
As it happened, a cube chassis and enclosure was up for bids on eBay, so I put in a bid and won it. That took care of the enclosure. A Cube’s clear case has a raised skirt in back for wiring to exit, and that suggested itself as the port of a bandpass design. Bandpass cabinets are basically acoustic suspension systems with an added chamber which filters out high frequencies acoustically.
When I bought an Apple G4 Cube computer system a few years ago, I was intrigued by the clear round ball speakers and digital amplifier which came with it. Nice sound, but lacking in deep bass, as with all small computer speakers.
The first step in building was to cut a piece of wood to block off the holes in the top of the cube enclosure. The wood was covered with sticky-back foam to ensure a good seal. Another piece of wood was cut to hold the driver and electronics. It is about 1/4” smaller than the inside dimensions of the case, and wrapped with weather seal foam.
The electronics are on two circuit boards. One is the amplifier itself, the other contains the input and power jacks and a level control.
After an initial listen, the electrical crossover point was lowered two octaves from IBM’s settings. The H/K balls go much deeper than IBM’s cheap speakers in their monitors. A piece of carpet was added in the top and back side of the upper chamber, and silver paper was added to dress up the system and match the paint of the computer.
This makes a really good sounding system. This woofer is tuned to 34 Hz and produces sound down below 30 Hz. 25 Hz is audible at the keyboard without doubling. I have since picked up an Apple - H/K iSub, which is the intended subwoofer for the Cube and iMac, and this woofer beats it on low frequency extension.
