Dave's Speaker Pages

Pole-Piece Vent Felt Tweak - Seas T25C003 tweeter

6 November 2004

Mark Krawiec sent me a Seas T25C003 tweeter to test. I took initial measurements on my 2m x 2m baffle as a baseline. Then I experimented with replacing the foam plug in the pole-piece vent with lamb's wool as I have with others. This driver showed no benefit from that. However, it still can be improved with another modification.

Overall this is an excellent driver. It is, in large part, due to the quality of the dome assembly, shown in the first pictures below. I can't see anything unusual about the motor assembly, but I also can't see into the area below the gap.


Seas T25C003 without Face Plate


Seas T25C003 Dome Assembly

First up is the measurement on-axis as new. Look closely at them and see if you can identify what the problem is and how it is to be corrected. I placed markers so that the actual SPL variation is clearly defined. It's really pretty severe, though it is in the range where it could be benign, depending upon your hearing ability.

Next up are the measurements on-axis before/after the modification. The problem area is made very apparent.

Here are a couple of shots of the front plate and pole-piece vent. They provide more food for thought on the source of the problem.

Next up are the two CSD curves, again before and after. This just makes the differences more apparent.

Finally we have the family of curves (with the modification), both on-axis and off-axis angles, taken at 0.5m on my 2m x 2m baffle.

If you haven't figured out the source of the problem and the fix, here is what was used - 100" wool cut into a ring and placed onto the pole-piece. Unfortunately, I failed to make a photo of it on the T25, so this is the same thing on a Parts Express Euro tweeter that I experimented with as well (it's nowhere near the same quality as the Seas). This is nothing new, It's found even on many inexpensive drivers. I'm baffled as to why Seas did not include this themselves.

Just for clarity, here's a photo of the two drivers. The PE driver looks rather poor due to the chamber having been removed and replaced. I had actually hoped that I could use a T25 replacement voice coil in the PE driver, but they are definitely not compatible due to the mounts on the top plate.

My conclusion for the T25C003 is that there is room for improvement by tweaking and it's a simple and fairly easy one. Care must be taken when cutting the felt ring to ensure that it doesn't come in contact with the dome and the center opening should be roughly the diameter of the vent opening, but that's it. The results are good and worth the effort. All that's needed is one piece of 1/8" 100% wool felt (available from McMaster-Carr as usual) with a self-adhesive backing.

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