... and then there was the keepsake that we tailor-made for the Midwest Audiofest.

We had successfully avoided taking part in the contest, but since we were still barely known as speaker builders in the United States, we wanted to make a positive impression. That meant we couldn’t be obnoxious show-offs, but at the same time we didn’t want to be dismissed with an “Oh, that’s nice…” It was the same danger that faced all of the large freestanding boxes and the medium-to-small shelf boxes, which couldn’t provide the necessary element of surprise that generates rapt attention. Understatement may be more of a British virtue, but even in America, people are more receptive when the format and the sound create an unexpected contrast.

We spent around two months thinking about what we could bring along to the party in Ohio; the only thing we knew for sure was that it would be equipped with a Dayton chassis. While flipping through the PE gallery, we noticed that there were many compact two-way speakers, and that almost all of the speakers from the Reference series were already represented. There were hardly any huge, bombastic boxes, so we would have had plenty of room for an original creation in that category. But the shipping to the US alone would have cost more than the equipment, not to mention the immense risk of breakage. It would have saved us a lot of time to dip into the reservoir of existing assembly instructions – in terms of price and sound, the Dayton 17 AL would have been okay. We even have a couple of pictures from Albrecht and Moritz featuring attractive designs. But anyone would expect to hear exactly the kind of sound that it produces. What we were looking for was something crazy like the Needle – Michael’s amazement during the first listening test was exactly the effect we had in mind. But alas, we were just another consumer of the ingenious Cyborg construction. An ACL with RS 125-8 and DC 28 F-8, as a remake of the SB 12 ACL, didn’t quite do it for us either. But since we already had the Needle and ACL in mind, we gave birth to the Bluestone Twins, whose name actually came from a mistake. We sprayed the first version with a blue stone-finish paint, which immediately gave rise to the English name. We liked the name, but ultimately not the color. It was replaced by an almost-white shade, but Whitestone just didn’t have the same potential for a double meaning.

The speaker was based on the RS 100-8, which we had already successfully sold in the FT 9. The four-ohm version would have given us a little more volume with the same amplifier output, but it’s already being used in the Needle, which we only wanted to use as inspiration in terms of size and tone. We wanted to be able to send a completely independent, even sleeker project to America. We chose the ACL principle as a construction without any fixed theory behind it, because that allowed us to test a couple of things that would tell us more about how the enclosed chambers worked in practice. First we simulated a reflex volume that was much too large for the four-inch speaker, but promised an enormous bass response. LSPCad gave us just under 10 liters for -3 dB at 55 Hz, with an even decline in volume under 120 Hz, but smugly only calculated a “correct” reflex volume of 2.8 liters. That gave us the size of the top chamber, followed by three others that were slightly smaller, each with 90% permeability of the membrane surface. This construction has a couple of things in common with a TL with a prechamber, but the line is significantly shorter and it ends in a reflex tube. So much for the theoretical approach to the Acoustic Chamber Line – we still need to do a precise test and possibly some optimizations. Still, we can already see that it is different from a large reflex system; we can admire a large impedance peak and two smaller ones in the range up to 130 Hz. That in turn is reminiscent of a double-ventilated reflex box, like the construction of our FT 9, although its upper peak is a good bit higher under the given circumstances. Still, looking at the curve under a big magnifying glass, a mini-peak can still be made out here with a little effort. That makes sense too, since after all we’re dealing with four chambers here. Next we tried closing off the reflex opening; now the impedance showed two peaks instead of the usual single peak for closed cabinets. We will refrain from using these measurement results to develop a complete theory, let alone argue that we have developed anything useful. Still, we are glad that the Bluestone Twins are now the third design to benefit from the chamber solution.



We don’t want to leave out the SketchUp assembly plan for you to download. For the sake of completeness, we will also give you the technical data for the RS 100-8.

Dayton Audio RS 100-8

Parameters:

Now it was time to build the boxes so we could use the reality to check the measurements of our earlier theories. One difference from the assembly plan was using 16-mm MDF, which was entirely mitered. We cut grooves for the inside boards, which gave us a good grip for gluing them in with joint glue. Clear packing tape also kept them from sliding around unexpectedly. The pictures of the assembly process should speak for themselves.






The constant increase in volume at high frequencies, and the small peak created by the baffle step, forced us to develop a small corrective circuit that didn’t push everything down to the lowest level right away. Under 15 degrees, at an almost consistent 83 dB, it gives us a smooth line between 150 and 10000 Hz except for a very narrow peak at 17 kHz. Around the bottom, the proximity to the wall quickly adds the missing 3 to 5 dB..




Measurements:

Frequency curve	Impedance	Frequency curve under 0/ 30/ 60°°
Distortion for 90 dB	Step response	Waterfall


The crossover was glued in above the terminal on the back and side walls of the box. A bit of caution is advised here, since hot glue can leave ugly blisters on your fingers.



In order to thread the wire through the narrow gaps, we pushed a string tied to a screw through the BB cutout and let it work its way down. Once they were attached to the string, arranging the two stranded wires was child’s play. Half a mat of Sonofil was enough to insulate the upper chamber. It is arranged in such a way that it left the pass-through free at the bottom. The HP/ BR 35 reflex tube was shortened to 5.6 cm. Thanks to its slats, it can be closed air-tight in the cutout. After a couple of days to let it warm up, I still wasn’t used to the color. So I took everything back out of the wooden boxes, down to the corrections, sanded them again, primed them with white wall paint and then sprayed on the final white stone-finish paint.

A day later, Andreas Wolf insisted that we get the Twins ready for their trip to America. When he came into the listening studio, Pink Floyd’s “The Wall” was playing, with the DTA-1 being used as an amplifier. The volume knob was at 12 o’clock. He was a little confused because he could see the two useless-looking toothpicks standing there in front of all the other boxes, but the music definitely had to be coming out of much larger speakers. We admit that the volume wouldn’t have been enough for a big party, but it wasn’t exactly quiet, either. The good resolution, proper dynamics and adequate bass reproduction meant that we didn’t hear a single four-incher in each box. With a good 40 m² in area and a 4-meter ceiling, the listening room was probably too big. Still, classical, jazz and rock all came across fine as long as there wasn’t too much more membrane surface counterbalancing it. Naturally, the strengths of the Bluestone Twins lie in the direction of a man with a piano or a woman with a guitar; they are tonally balanced, so they shine when the stage is relatively bare, which is usually considered the specialty of a wide-range speaker. Their very accentuated speech reproduction, without any annoying hissing, suggests that they would also be good for a flat-screen TV; there could hardly be a more cost-effective solution. Still, we should mention that very deep and extremely high-pressure basses are not the Bluestones’ thing. After all, as their name suggests, they prefer the blues tones.

At the Midwest Audiofest in Ohio, we set up the Bluestone Twins in a fairly unassuming spot in the hall that visitors had to walk through to get to the large listening room. A not especially high-quality portable player and the DTA-1 were switched on during programming breaks, and brought in a constant stream of listeners. They were always amazed that all of the sound was coming from these elegant sticks; a couple of visitors even checked the wiring on the devices. There must be a subwoofer playing somewhere, they often said. When they found none, many listeners just shook their heads. The general consensus on the Twins was that they sounded round and pleasant. That’s what we had been hoping for.

Udo Wohlgemuth


Technology