Testing out a modification to an AMT diaphragm to reduce harmonic distortion
- The harmonic distortion (measured as a percentage to the fundamental) in one of my AMT tweeters rises as frequency goes down, hits a peak, but then lowers it's magnitude after passing the peak.
My hypothesis was:
- on the lower-frequency range of the AMT driver, physical resonances end up propagating/traversing the diaphragm folds (in ways I have not yet visualized).
- To reduce the amplitude of these resonances, a viscoelastic compound could be applied to the diaphragm in small amounts, across the tops of the folds, to help absorb the energy of the resonances, thus damping them and the harmonic distortion associated with them.
- This leaves the pleats largely unaffected, and free to contract/expand normally.
- use a brand new diaphragm and apply two thin threads to the back of the diaphragm, thinly coated with silicone sealant, across the tops of the folds, in an X-shape. Re-install and test under same conditions.
Before and after frequency response. Same drive level (approx 2.82V RMS), same setup, mic distance, location,placement. Just from one day to the next after installing the modified diaphragm into the same AMT tweeter body. Tweeter is Hygeia RT-5002. Response is pretty much identical for all practical purposes. On the after (red) I forgot to raise the HPF on the dbx 234xs to 1000Hz though. I believe that is why it is showing just a hair more output below 2k.
Here is the harmonic distortion data, before & after diaphragm modification. I used slightly more drive level than in the FR graphs, but the same for both HD tests.
Doesn't look like much, but here is the quantitative data.
- I "used up" one spare diaphragm in the process, but I do not feel it was a waste.
- Further tests will include different orientations of the silicone treated thread across the diaphragm folds, both on the rear and also on the front.
- For my purposes, it works out well since I am looking to mate an AMT tweeter (not necessarily this one) to a 10" midbass driver, using a large horn, so obtaining the best possible low-end extension and performance is a primary objective. I used this diaphragm/tweeter as the test piece because it was relatively cheap.
- I suspect other larger [rectangular] diaphragms also suffer from un-necessary harmonic distortion content as a consequence of physical resonances traveling along the folds, and treating them in a similar fashion would also decrease distortion as a whole.
- I also slightly suspect that square diaphragms might also suffer from resonances, but to a much lower degree.
- There is an increase in THD near 3,2 kHz. Hopefully with more trial-and-error, that HD peak can also be tamed. Time to use up more spare diaphragms.
Modified RT-5002 with Dayton H110 horn.
~2.83V/1m, no EQ FR. Bone stock vs Horn loaded with modified Diaphragm.
Harmonic Distortion, modified diaphragm, no EQ, 2.83V measured 10cm from horn. Also +5 dB.
Bone stock Frequency Response (black) vs modified diaphragm (red) with EQ, HPF ~1kHz.
Harmonic Distortion, bone stock no EQ vs modified diaphragm horn-loaded with EQ, ~90 dB reference sensitivity.