jazzmans
Super Member
As anybody who has gotten to this point in the thread will know, I learned a lot about the RS1b during this project. I also had some close knowledge of Infinity starting about fifty years ago and, thus, know what their design goals were. I have seen descriptions of the RS1 design as being either a "quasi line source" or a "pseudo-point source." I think it is something else, and here is what I think it is, and why...
Arnie Nudell started playing with line source techniques at the very beginning of his career. While developing the Servo Statik 1, he worked with RTR to develop a thin, long “ribbon electrostatic” tweeter to improve the dispersion of the high frequencies over what was available from other contemporary electrostatic panels, which were square. I can’t find any photos now, but the original Servo Statik had an electrostatic tweeter that was about an inch and one-half wide, and 30 inches high. This design, and subsequent work with line arrays, was based on page 36 of the 1957 Second Edition of Harry F. Olson’s “Elements of Acoustic Engineering,” where you will see proof that a driver like the one in the original Servo Statik will have very good horizontal dispersion (because of the small width compared to the wavelength of high frequencies), but a lobe-shaped dispersion for vertical dispersion (due to the line array behavior of the 30? tall tweeter).
At the beginning of the 1970s, the best speaker designers, were contemplating the difference between a loudspeaker and a symphony orchestra. I know that sounds glib and superficial, but the basic problem is that an orchestra (and almost everything else in the real world) has constant power output (power into a hemisphere) with frequency, but the power output of a loudspeaker falls off rapidly as frequency increases, because of baffle and driver size limitations (baffles too small, and drivers too big). Bose “solved” the problem by using an active device to boost high frequencies and then, spraying them all around the room by aiming the drivers at the wall. Infinity started experimenting with line arrays as a more elegant way to control power output by frequency.
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Referring again to page 36 of Olson, you will see that by the time the length of the array is equal to the wavelength of the frequency, you achieve a nicely directional radiation pattern with little radiation beyond +/- 60 deg of horizontal. Thus, with a 54-inch line (the approximate length in the RS1b EMIM array), you can get a good directional pattern starting at about 250Hz. But with a 54-inch line, you get severe beaming by 2,500Hz (line size 6x the wavelength), and it only gets worse the higher you go. This kind of beaming can be observed by moving your head above and below the midpoint of the array, where you will notice a fall off of the higher frequencies. The solution to this problem is the secret to the design of the RS1b; it is a five-way speaker, utilizing successively smaller line arrays as the frequency increases. The first band is handled by the active crossover and the woofers from about 30Hz to 150Hz. The radiation pattern is essentially omnidirectional. The next band is from 150Hz to 700Hz and is handled by the 54? EMIM array (#s 1, 2, 4, 5, 6, 7). Above that is the upper midrange (700Hz to 3000Hz) array handled by a single 7-inch EMIM driver (#3), then the lower EMITs from 4000Hz to 8000Hz (using “1/2 line source” standard EMITS of about 2 inches, and finally, the upper high frequencies from 8000Hz to about 32kHz, using a “1/4 line source” EMIT (a standard EMIT with one-half of the radiating area taped over, having a line length of about 1 inch). The fifth part of the crossover is the rear tweeter. This was required, as power output for a speaker in a room, is defined as the radiation into a hemisphere. The EMIMs were dipoles and provided plenty of rear radiation. The EMITS only radiated in one direction, thus the need for a rear-facing EMIT.
And that is most of the story. I think the RS1b speaker was one of the most elegant of the classic Infinity speakers, as it used solid engineering to develop a speaker based on solid theory. Perhaps, it is an intellectual speaker. It certainly sounds better than the Bose 901. And while it may not have mid-bass oomph, and the effortless dynamic range of other top high end speakers, it does solve the problem of constant power output very well. Although it doesn’t make much difference, due to the incredible number of drivers in the fabled IRS Vs, if you move your head up and down from the middle of the array, you can hear high frequency beaming, which you will not hear from the RS1bs.
I remember reading an early review of the RS1bs where the writer declared that the tape on the upper tweeter was to tame the highs by reducing output. That, of course, is ridiculous. That wasn’t the point of the tape at all. By shortening the length of the array, the power output at the high frequencies was actually increased. I wonder if Arnie ever sent a note to the reviewer to correct the mistake, or if he just let it go.
Could you elaborate (in as much depth as you wish) on the last paragraph? I just don't 'get it' the taped over portion of the Emit is still driven, right? where does that energy go?
Thanks. I find this fascinating