I’ve made some extensive measurements on the RAAL 140-15D ribbon tweeter and before the measurements I installed brand new replacement ribbons in both the RAAL tweeter samples and tightened up the ribbons as in this instruction video from RAAL.
As instructed, it worked perfectly. 🙂
Since it’s possible to shape the frequency response both vertically and horizontally with the supplied “foam deflection pads”, it seemed as the tweeter measurement setup could almost be done in an infinite number of variations.
I finally decided and ended up with the following tweeter configurations:
Upper Left: DPN = No “deflection pads”.
Upper Right: DP1 = With “deflection pads” and 7cm visible ribbon.
Lower Left: DP2 = With “deflection pads” and 5cm visible ribbon.
Lower Right: DP3 = With “deflection pads” and 3cm visible ribbon.
The best overall balance and linearity between the horizontal and vertical frequency response is to use the “deflection pads” in the DP1 or DP2 setting e.g. 7cm or 5cm visible ribbon.
I wouldn’t recommend using the RAAL ribbon without the “foam deflection pads”. Without it the performance decreases in all aspects, both in frequency response and in distortion performance. Nevertheless I made a lot of measurements without the foam pads to have as a baseline to compare with.
Even though the RAAL ribbon is a very robust ribbon tweeter I made all frequency measurements with a 44uF protection cap connected in series with it.
Flush-mounting or not?
I’ve tested the RAAL ribbon with and without flush-mounting it on the baffle and the test results shows that there is no big diffraction problem and frequency response impact if it’s not flush-mounted.
Blue = No flush-mount
Red = Flush-mount
Upper Left: DPN
Upper Right: DP1
Lower Left: DP2
Lower Right: DP3
The face-plate of the RAAL ribbon is only 2mm thick and flush-mounting it is more or less a cosmetically decision rather than technical. All my published measurements are made with the RAAL ribbon flush-mounted using a 2-3mm thick felt carpet covering the entire baffle.
What about distortion?
The distortion figures isn’t up to the very best dome tweeter’s performance e.g. ScanSpeak D3004/664000 Beryllium tweeter, but for a ribbon tweeter still a respectable performance.
Upper left: RAAL 140-15D Ribbon tweeter (DP2) @ 1m 90dB
Upper right: ScanSpeak D3004/664000 Beryllium tweeter @ 1m 90dB
Lower left: RAAL 140-15D Ribbon tweeter (DP2) @ 1m 95dB
Lower right: ScanSpeak D3004/664000 Beryllium tweeter @ 1m 95dB
As can be seen on the distortion charts the gap between them narrows down with increasing SPL. Also the harmless second-order harmonics is actually lower for the ribbon. And obviously the dome tweeter can be crossed-over much lower without distortion issues.
When using the supplied “foam deflection pads” it actually helps lowering the distortion as well as controlling the vertical horizontal frequencies dispersion.
Based on the distortion figures I would recommend as many other people have for the RAAL ribbon tweeter, a cross-over point of about 3kHz or higher. If the max SPL requirement isn’t a concern or if you use steep filtering a 2-2.5kHz cross-over point could possibly work.
With the Sequence Three – Grand Reference loudspeaker design I’m aiming for a cross-over point of around 3kHz, so it’s seems to be working as I planned and expected. Further measurements and of course listening test will finally decide the exact cross-over point. With the AT mid-range and the RAAL ribbon I will have a possible cross-over range somewhere between 2.5-3.5kHz. Not a super wide range, but wide enough to make it work.
To sum up, I think the RAAL 140-15D measurement performance lives up to my expectations and I’m eager to continue with the Sequence Three – Grand Reference loudspeaker design. Next up is to publish the results from the AudioTechnology mid-range measurements.
For further detailed measurements see: RAAL 140-15D Measurements