Monday, May 19, 2008
Every couple of years I get the urge to build a set of speakers. In California I attempted to build a copy of the famed Gallo Nucleus speaker. I did get the same bass drivers, but I used 2 plastic bowls instead of the recommended stainless steel spheres, and used some ribbon tweeters from Electron Hut instead of the handmade cylindrical driver. Needless to say, no one was worried about copyright infringement.
Then 2 years ago I built a couple of garage speakers based on plans from Parts Express. They work just great out in the garage, but I was getting itchy to build something better for the living room. This time around I thought I would head into the world of full-range drivers. Full range is kind of a step backwards - instead of having a separate woofer, midrange and tweeter to handle the audio spectrum, full range attempts to get all 3 out of a single driver. The idea is that by avoiding a crossover point between drivers, you get a more natural sound.
Needless to say, this poses problems. A big cone can't really move fast enough to reproduce high frequencies accurately, so you either have to stick to drivers 4" and under, or put a "whizzer" cone onto a larger driver, which kind of makes the highs shouty and directional. But, how much bass can you get out of a 4 inch driver, a stiff 4 inch driver at that? As it turns out, a surprising amount.
The driver I'm using is the Fostex FE126E. As with all things obscure and inscrutable, it hails from Japan. It's a 4 inch diameter driver with a cone made of banana pulp fiber. Its Xmax (how much the cone can move back and forth) is an amazingly small 0.3 mm. Picture a boom-boom subwoofer pumping air, and this is the exact opposite. It has a magnet nearly as large as the cone itself, which means it's a very tightly controlled driver - when it's time to move, the cone is going to move, with almost no slop.
So where does the bass come from? There are a few options, all of which revolve around horns or resonators. When you put your hands up to your mouth and shout, you are forming a horn. Your voice becomes louder in the direction you point your hands - you are sacrificing coverage for intensity in a specific area. When you put your mouth at the end of a paper towel tube and speak into it, that's more of a resonance effect - the tube is amplifying certain frequencies due to its resonating like an organ pipe.
So, by having the back of the speaker fire into an appropriately shaped chamber, we can exploit the resonance and horn effects to effectively amplify and direct the tiny bass output of the driver into usable bass that should mesh with the mid and high frequencies coming from the front of the driver.
After looking at several options, I chose the BIB design, or "bigger is better". This design is brilliantly simple, involving a box with no top and a single internal piece. This forms a folded horn/resonator. The driver is mounted in the skinny end pointed forward, and the horn ends up firing up. The speaker is placed against a wall to further reinforce the bass frequencies.
I've already prototyped a pair in cheap plywood, and now am building the real thing. A side effect of the horn pointing up is that it disperses the bass and midbass vertically, giving a very open, natural sound. The bass is ample, to the point where you're looking for a hidden subwoofer.
Wow, can I go on about speakers or what...