spj@hou3c.UUCP (Stephen P. Johnson) (02/06/85)
My recent article generated a flurry of questions, which may be of general interest. The address for Speaker Builder magazine is PO Box 494 Peterborough, NH 03458. This is the bible for novice speaker builders. I recommend buying all the back issues, which are available. What kind of equipment is needed to build speakers? That depends. For closed box systems you might not need any. If you want to do ported designs you need a good multimeter and an audio signal generator. I also find a digital capacitance meter (about $50) very helpful for putting together caps for crossovers ( get to within 1% instead of guessing what the values really are). I also have a pink noise source and 12 band spectrun analyser (Technics about $250). A microcomputer or programmable calculator running one of the Thiele Small design aids is a big help. What kind of crossover? Someone suggests using 6db networks because higher order networks are more succeptible to driver impedance variations. I suggest that higher order nets are less succeptible because each driver works over a narrower range. Of course, unless you want to go crazy you will use drivers with flat impedance curves, or equalize the impedance with Zobel networks (see Speaker Builder). Otherwise, the perf. of your speaker is a very chancy proposition. One thing everyone seems to miss about 6db networks, the effect on the radiation pattern at high freq. The directivity of a dome radiator at high freq. is primarily determined by the SIZE of the radiator, that's why tweeters are generally smaller than about 1", to get good dispersion. So what happens when you use a 6db network? The woofer is radiating substantial energy in the high freq. range. This increases the apparent size of the tweeter, which causes beaming in the high freq. response. Note that the "perfect" characteristics of 6db nets are only present when the two drivers are physically coincident. Using higher order nets also reduces various forms of distortion, although there is undoubtedly more group delay with higher order nets. What kind of system do I use? Three way with sixth order Thiele Small woofer (10" ported in a 4 cu ft box), and a small hexagonal cylinder on top with a 6 1/2" ferrofluid mid and 1" tweeter). The crossover is 24db per octave active with tri-amping. It sounds very good. I've also built several other systems for rich friends that were better, and more expensive! A question about performance changes in an old ADS810 speaker; the high freqs have gradually disappeared over the years. Two probable causes: 1) The electrolytic capacitors (if any) in the crossovers have dried out and changed value. Most electrolytics have a lifetime of only about 5-10 years. 2) The tweeters have undergone physical changes over the years. Try testing/replacing the electrolytics with film caps first. Good luck.... Maybe someday we'll have net.audio.speakers! Steve Johnson