Morel MDT-20 Tweeter Measurements and Tweaks
David L. Ralph
23 Jan 2002
I purchased a pair of MDT-20 tweeters quite a while ago. I used them in a 2-way design with a Dynaudio 20W75, but was never happy with the combination. The problem was likely a non-optimal box and crossover for the Dyanudio. I've always felt that the MDT-20 should be a one of the better cost/performance leaders, but there has been little use by the DIY community, from what I've seen.
More recently I purchased three more units used. A quick check of the impedance shows them all to be very close, even though there was a large time difference between purchases as new units. This page shows the impedance of the original two units purchased new and the raw SPL of four of the five drivers when mounted on my "quasi-IEC" baffle.
The SPL measurements shown in the next graph were made at a distance of 0.5m on the baffle, which is 1m x 1.2m, with the driver centered on the baffle. This helps to lower the frequency at which the measurement is still valid and excludes any diffraction effects when windowed properly. They are all very close from 2k-10khz. Above 10khz the grouping of the three older drivers is also close. The new one is a bit higher with less variation. Considering the time difference in the purchases, I feel that this is very good consistency.
The next graph has three curves for one of the newer drivers, all taken on the quasi-IEC baffle. The on-axis, 15 degree and windowed measurements show that even though the on-axis response is ragged, the power response (approximated by the windowed average) is not that bad. The variation is about +/- 2db above about 12khz.
I can't come to any conclusions as to why the response is as it is above the 12k point. I guess that it has to do with the dome material and doping compound. The internal damping of the material/doping must be very frequency dependent and changes dramatically. The shape has something to do with this as well, but the shape is very similar to many other drivers.