Lamb's Wool Tweak - Morel MDT-20

The first tweeter with which I experimented was the MDT-20. I have five of them, two purchased new and three used. They are all reasonably close, but even the two newer ones are not as closely matched below 2khz as I would like. There is also a double hump in the impedance curve which has a matching dip in the SPL curve, somewhere in the 1900hz area. This dip was much more pronounced in one of the two new ones. These drivers have a chamber and should already be optimally damped.

The graph below shows the impedance of the two new ones, before and after the lamb's wool was placed into the pole piece vent. The two curves with the high maximums (20 and 25 ohms, red and dark blue) are for the case as received, with no stuffing in the pole piece vent. Unlike the MDT-30, there is no foam disc in it. Note the second peak in the 1900hz area. I believe that this is due to imcomplete suppression of a pipe resonance which all tweeters will have if the pole piece vent is not somehow handled. This can be seen in the modification testing I made on the Vifa D27TG-45 in the Miscellaneous section.

The second set of curves (yellow and cyan) are for case of lamb's wool stuffed into the pole piece vent. The two curves are now very, very close. The peak of each one is within 1/2 db of 15 ohms, the Fs has dropped about 50hz or less, the Q at Fs is lower, the secondary resonance has been lowered to about 1500hz and has a very low Q. Next we look at the SPL frequency response curves.

The graph below shows the SPL frequency response curves for both drivers before and after the modification. The changes are restricted primarily to the area below 2khz, though for the case of the tweeter with more non-linearity there appears to be some improvement between 2-3khz.

The dip in the response below 2khz (red and dark blue) is closely matched to the impedance Fs and secondary resonances. Note the larger dip of the secondary resonance for the driver with the exceptionally high impedance peak. Both drivers start to roll off at about 2200hz. Now look at the two curves for the drivers after the modification (yellow and cyan). The two curves are nearly identical, quite a difference from the unmodified case. The rolloff is extended to a point below 2khz since the secondary resonance is now much lower and of lower Q. There is a slight loss in the 1khz area, but since this is nearly always going to be in the stop band, the smoother rolloff should be of benefit, especially if a low crossover point is desired.

Finally we have the Cumulative Spectral Decay (waterfall) plots. The two graphs below are for one of the two newer units. The first one is for the unmodified driver. The second one is for the driver with the lamb's wool. There is only one significant difference. This is the area below 2khz. The decay is much faster and much smoother with the wool in place.

This is supported by the relative lack of change in the total SPL response anywhere except for the area below 2khz. Ultimately I would say that there really is no down side to inserting lamb's wool into the pole piece vent.

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