12 Feb 2002
In another page I explained a problem I had with one of a pair of Morel MDT-30 tweeters. The impedance and frequency response were both affected by the foam disc in the pole piece vent. This was apparently placed there to reduce the secondary impedance and extend slightly the low end rolloff of the driver. It does have a chamber. What surprised me later was that when I started testing the lamb's wool in the pole piece vent, the anomoly had returned, but it wasn't as severe as before. I don't know the specific reason, but I make a guess that it has to do with the foam changing over time after re-insertion. This was disappointing. My next step was to replace the foam with the lamb's wool.
I re-measured the driver impedance prior to inserting the wool to be able to make a valid comparison of recent measurements. This was how the anomoly was found to have returned. I then replaced the foam with the lamb's wool for both drivers. The result of the impedance change is shown below.
I have not yet made the new SPL measurements, but I suspect there will be little change. The curves below are for the case of the original foam when functioning properly.
The impedance curves of the two units are very close again. I think that there will not be a deviation over time as there was with the foam in the one case. As with the MDT-20, I believe that there are no negative effects to replacing the foam with the felt and and I am quite sure that there are positive results, long-term consistency being at least one.
