The "Infinite Slope" technology is back. Remember the JSE speakers from the mid-1980's? They were based on the same crossover design, conceived and patented by Richard Modafferi, our recently recruited RF consultant. (Yes, he knows filter theory like the back of his hand. Yes, this is a serious engineering concept, not a marketing gimmick.) For reasons that had nothing to do with the inherent merit of the design, JSE went out of business. Now JosephAudio (headed by Jeff Joseph) has taken on the cause and is determined to put the Infinite Slope crossover back on the map again. With Richard Modafferi's help, the company has developed new and significantly improved models, of which this one represents the bottom of the line.

The Joseph RM7si consists of a bookshelf-size (15 by 9 by 11 inches deep) vented box, a 1-inch silk-dome tweeter, a 6 1/2 inch woofer with fiberglass cone and "phase plug," and the Infinite Slope network. See David Rich's side bar for an explanation of how the crossover works. Here I shall restrict myself to the performance of the speaker.

My alleged curmudgeonly inclinations notwithstanding, I am always happy to come across an audio device that performs in every way as represented by its makers, and this is one of them. The RM7si may be a bit pricey for a small speaker with OEM drivers, but the sophisticated filter technology it incorporates probably justifies that, and the sound it produces is truly excellent. I cannot think of any comparably small speakers in my recent, and even not so recent, experience that pleased me as much.

The quasi-anechoic (MLS) frequency response of the speaker, taken at 1 meter, shows basically flat characteristics (+/- 1.5 dB) for both woofer and tweeter, except that the woofer appears to be set 2 or 3 dB higher than the tweeter. I couldn't tell for sure whether this was deliberate "voicing" or some sort of step in the response of the 6 1/2-inch driver itself. The Vifa silk-dome tweeter (also used in a number of ultrahigh-priced speakers) remains dead flat right out to 20 kHz. The crossover frequency is approximately 2 kHz. The filter skirts are very steep indeed but not "infinite' - 42 dB per octave was the steepest I could read with an admittedly crude electro-acoustic nearfield measurement. The vented box is tuned to 41 Hz, and the maximum output from the ducted port is at 52 Hz. The summed response of woofer and port is dead flat (+/-0.25 dB) down to 55 Hz and -3 dB at 43 Hz. There is still useful output (-12dB) at 33 Hz. This is very respectable bass response for a 6 1/2 inch driver in a box of this size. The box itself is very solidly built and appears to be quite dead when knuckle-rapped.

The impedance curve of the RM7si is unusually flat in magnitude above the box-influenced frequencies, varying only from 7 ohms to 12 ohms. In the entire audio range; the phase variations are also small, with +/- 20 degrees the biggest swings above the tuned-box range. I bet Rich Modafferi did that on purpose, to allow you to use just about any half decent amplifier to drive the speaker. (In fact, David Rich connected our test samples to his bargain-basement Pioneer SX-203 receiver - which he reviews in this issue - and obtained the same audible results as he did with his big-bucks amplification system.)

I took distortion readings only on the woofer, since I know from previous experience that the Vifa silk-dome tweeter has entirely negligible distortion in the range used here. Over its midrange band, namely 200 Hz and above, the woofer stays in the 0.2% to 0.35% THD range at 1-meter SPL's of 90 to 95 dB (I generally don't measure at unbearably high sound pressure levels). Below 200 Hz the THD climb rapidly but is still in the 0.25% to 1.4% range at 90 to 95 dB, right down to 57 Hz, at which point it shoots upward and passes the 10% mark at 40 Hz. Again, this is quite respectable (and then some) for a bookshelf-size unit. If you were to cross the RM7si over to a subwoofer, I would suggest a crossover frequency of 80 to 90 Hz, where the speaker is still very flat and the THD is low. The audible outcome of all these good design characteristics is very favorable, the speaker has a basically neutral tonality, with very smooth highs, a thoroughly transparent midrange, and a surprisingly solid satisfying bottom end. The soundstage is open and plausible; the imaging is all it should be. The total noninterference of the two "brick-walled" drivers is clearly an advantage, with no negative aspects that I can discern. (All you first-order crossover cultists take note - maybe you've got it all wrong, huh?) The totally low-end protected tweeter can be played louder, and the absence of interference patterns in the crossover region appears to remove a layer of veiling that I often hear in speakers with lower-order crossovers. So - what we have here is indisputable high-end loudspeaker sound in a compact package, at a price still well below the loony category. Only the deepest bass is missing, but you know that coming in, just by looking at the size of the box. The JosephAudio RM7si is therefore highly recommended.

- Peter Aczel

Well, if you have a very good understanding of network theory, have a good knowledge of computer optimization and are very clever, you can use finite zeros in a passive crossover network. Of course the aforesaid job requirements are not going to be met by your typical loudspeaker company designer. It took someone called Richard Modafferi to figure out how to make a practical crossover network with finite zeros. The method of synthesis is so clever (it uses a transformer, among other things) and elegant that it is patented, and Rich derives an income from the royalties. Now, the slope of the Modafferi crossover is not "infinite" but it goes down real fast... this is a major advance in crossover design.