Sound Practices Magazine Online!
Recommended Components: Altec Lansing Corporation
Here are a few horns that worked for me. These are subjective impressions and somewhat surprising in light of Altec's intended applications. Among common and reasonably proportioned Altec units, the 311-60 and the 311-90 are your best bet. Surprisingly good sounding despite their rather cheapo cast construction - apply damping to exterior. A 288 bolts right on. The sheet metal 203 is a nice long two cell horn which avoids many of the problems presented by shorter horns with more cells in the home listening context. Intended as a "long throw" horn but sounds good short range. The Altec horn that works best in my system is the large cast aluminum 329A. It is rather unhandy to install but it sure sounds good.
Perhaps the best option is to build the right kind of horn from scratch. Simple equations for horn design can be found in Audio magazine articles from the 1950s and a number of textbooks. Use of Altec hardware and throats can facilitate the mechanical interface with your driver. I'm working on this project myself — results forthcoming in a future installment.
Low Frequency Horns
As far as cone material is concerned, paper is it. The combination of a light paper cone and a big magnet is difficult to beat. Harder stock usually provides more detailed reproduction through the midbass. The shape of the cone can be important. Straight wall cones have better tone and detail through the mid bass and curved wall cones have better top end extension. With a 500 cycle X-over, straight wall cones are preferable.
Woofer suspensions also have a great effect on sound quality. In high efficiency speakers there is little coil overhang in the gap. The coil must be returned to the exact center of its travel. If this doesn't happen, the sound is muddied because of asymmetry in the magnetic field. I like the old Lansing spiders which are made of Bakelite. They hold the cone in a rigid grip and provide for controlled excursion. In a proper enclosure, total cone travel is only a small fraction of an inch so excursion limiting is no problem.
The most common really great old woofer is the Altec 515 or 515A. It was in production for about 40 years beginning in the mid-1940s. The older ones used bakelite spiders and a straight walled cone of hard stock with a half-rolled paper surround. This is how speakers should be built. The later 515 is the B model. During the 1980's this speaker was made with a ferrite magnet which is clearly inferior to the awesome Alnico magnet of its forbearers.
Other woofers worth checking out are the Altec 416 and 803. The 416 is a late 803 with a different part number. The 803 is similar to the 515 but designed for use with an 800 cycle crossover. The 803 woofer with the 825 cabinet, the 803 compression driver, and the 811 horn made up Altec's A7. This was the "baby" VOT.
Be sure to take pains to phase your speakers correctly. You will have to experiment because drivers are not all phased the same electrically. Go for the connection which produces the most output at the crossover point. Get a friend to switch the connections while you listen. Make sure the drivers are aligned such that the ear-speaker distance is the same for each driver.
Because of the complicated nature of crossover design, I am inclined to gloss over it in this introductory article. In a follow-up piece, I will provide tips for bringing old crossovers in line with modern practice and a design for a high quality line level crossover for a bi-amplification application that will really get those old drivers working. Stay tuned.
One of the major challenges in setting up a horn system is finding an amplifier which will work with the horns. High power solid-state amplifiers sound best when mated to inefficient speakers. With horn speakers they tend to sound really awful. The efficiency of horns is so high that your average big hog solid state amp never really gets turned on. Likewise, big tube amps using banks of 6550s just don't sound that clean when operated in the milliwatt (mW) power range. Horns work best with low power tube amps.
I hope this introductory survey will help some experimenters avoid a few of the most tempting wrong turns. Most of the information you really need to know will not be found in any books. Good "pro" sound practice differs from the best strategy for hi-fi horns in the home. Some suggestions you read here would be a mistake in the theater context. But recognize that what excels in the auditorium won't necessarily soar in your listening room and you're on your way.
Greg Boynton is proprietor of Rapid Sound, provider of audiophile quality sound reinforcement and audio consulting for the Grand Rapids, MI area.