Fellow Open Baffllers,
Below is a graph of the Visaton B-200 on Richard's open baffle. We measured it together using the same tone burst techniques that were used at Beveridge on the legendary model 2 SW cylindrical sound electrostats). Bev (Harold Beveridge) was a great fan of tone burst testing and so am I. With a burst you can see the startup at any frequency and the ringing that follows the cutoff of the burst. When I see 2 cycles go into the speaker and 4 come out, I have to wonder what’s going on. The worst speaker I ever tested for this problem was the 4 way KLH electrostat that came out around 1978. It’s crossover was 4 tuned resonant circuits in series. They though it was clever, and it was, until you saw the effect of the resonant elements. That speaker put out 4-5 cycles with only 2 cycles in over the whole audio band. Needless to say, nobody is hunting those speakers these days and their production life was very short lived. I wonder what their test methods were? Surely a large company like KLH has some good facilities.
Another benefit of tone burst testing is that the room goes away because you just ignore anything past 2 milliseconds (that’s anything within 2 feet of the driver. The floor is the first thing to come into play as the speaker is only 3 feet (3 mS) above it. I find tone burst testing to be more enlightening than MLSSA and other computer tests. I've heard a number of speakers designed by computer testing and find all of them to be unmusical. I fear the designer is tweaking to satisfy the computer. I don’t think a flat FFT makes a good sounding speaker, not at all. There are too many other variables.
The graph below is a first run and Richard and I have not moved the mic around to check the response off axis or to see how distance affects the response. It was done at 1 meter on axis, just as a starting point. I was so intent on explaining the procedure to Richard that I forgot to confirm the sensitivity of the speaker. That will be given in our next test. What the curve does confirm is that there is a rising high end going up to 8 KHz before the fall. I find it most remarkable that a speaker without a whizzer cone could produce such good highs. This performance along with the high rated sensitivity of the speaker has certainly gotten my attention.
We can also see from the graph that the output falls rapidly after 300 Hz., confirming what I heard in my living room the first time we played the speaker (read the beginning of this thread for Richards report on the activities of that day). That day we only pushed the crossover up to 200 Hz. With a pair of my subs I will try going to 300 Hz and see if the blending is favorable.
Some notes on the graph: I put 0 dB at1250 Hz because there was a flat area between 1250 and 3150 (the longest one, almost 2 octaves) and not much rise (5dB from there to 8KHz.). The data points are the standard 1/3 octave numbers used in this industry for many years. I always look for anomalies between data points as my oscillator is continuously variable. I was happy to see there were no severe ones. This is rare. Most speakers have some little peak or dip that gives them their signature. A peak is much worse than a dip because we hear it like a bell ringing. A tight dip just means we miss a little and that is not nearly as noticeable.. Think about it for a moment, I have not seen this discussed before and I find it a most important point.
I am still a bit perplexed about the peak from 300-450 Hz. and the 8 dB dip at 630 Hz. There is a 0.5 ohm rise in the impedance at 450 Hz which may explain the acoustic rise in that area. The impedance rise implies that the driver is loaded less in that region. Anyone have any ideas about that?
At these lower frequencies the room does start coming into the measurements. Richard and I plan to do another run of tone burst testing in a week or so. We had to take a break and I have been busy in the interim finishing the prototype of the lower cost 245 amp, which I have promised Richard a first listen on his B-200’s.
Roger
Ps. The RM-10 just got a nice review in 6 Moons Audio. Locals can join us for the Champagne.
