The following is a recollection
by one of the pioneers in tour sound - Ralph Morris. Ralph was Director of
Marketing of Tychobrache Sound Company.
© 2003 Ralph Morris
I designed the M-1 enclosure in 1971 during my tenure as Director of Marketing for the Tycobrahe Sound Company, which manufactured portable concert sound systems using all JBL transducers. The design specification was for a reference monitor large enough for a tuned port equal in area to the effective area of the 15-inch cone transducer. An internal volume of 19 cubic feet was selected, which allowed tuning at 40 Hz with a non-ducted port, and at 20 Hz with a ducted port. The transducers specified were a 2215 low-frequency driver, a 2420 mid-range driver with the 2391 straight horn and plastic slant-plate lens, and the (then) new 2405, the high-frequency transducer with more horizontal distribution than the earlier “bullet” type. These were the similar components used in the three-way JBL “studio monitor,” Model 4332/3. Those systems replaced the 2215 with the 2231 woofer.
I was challenged to a 50-dollar bet, regarding the difference between a ducted port tuned at 20 Hz and a non-ducted port tuned at 40 Hz. One of each configuration was constructed out of the finest one-inch Finland Birch, and the pair were tested on a calm day in an open parking lot with the speakers facing the sky, and a microphone suspended above them. The enclosure with the larger, non-ducted port benefited from the balanced loading of the transducer, and it was more than 3 dB louder than the enclosure with the ducted port tuned at 20 Hz.
The frequency response of both was within +/- 3 dB of the source from 20 Hz to 20 KHz, so either design would have been acceptable, but the more efficient one was chosen. The outcome of the wager is still in question, given the non-laboratory nature of the test. The pair of these enclosures, wired for bi-amp, was powered by Crown DC-300 amplifiers for the showing of the Rolling Stones film, “Gimme Shelter,” to the Academy of Motion Picture Arts and Sciences, in 1972. The nomination of that film for an Oscar must have riled some of the old guard in Hollywood, because the film contained some violence, nudity and lots of loud music from the free concert given by the group at Altamont, California in 1969.
The Stones wished to make an impression, so they asked for the biggest speakers available from Tycobrahe Sound Company, which provided the sound equipment for their tour of USA and Europe that year. The M-1s were set up on either side of the screen, with two standard Tycobrahe columns at the rear corners to provide four-channel “surround sound” in the small theater. There had been some bad press about the film by some of the critics, and whether the Stones figured they didn't have a chance of winning an Oscar for it, or maybe they just wanted to send a message to the Academy (or the establishment in general), you will have to surmise for yourself. For whatever reason, the Rolling Stones’ staff in charge of the presentation turned up the sound level about as high as the four Crown DC-300s would provide. Most of the Academy members who were assembled for the showing immediately got up and walked out of the theater.
Smaller enclosures of this type were used in studio systems for Stevie Wonder and Frank Zappa. They have a very clean, smooth low-end response that I have never heard from any other speaker, including 16-ft straight horns powered by a compression driver with two 15-inch cone speakers. You would never believe that size doesn't matter, once you have heard these monitors. Since 1972 the original pair have been used for my living room stereo. The low-frequency transducers have been re-coned at JBL, and the aluminum diaphragms in the 2420s have been replaced, the last time in 1997. The old diaphragms appeared to be o.k., but one figures that Aluminum gets hardened after a few years, even at living-room levels.
© 2003 Ralph