Back Plate Frequency Response
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Back plate frequency response with the thickness nearly final.
Mode 1 tap tone at 116 Hz and Mode 4 at 290 Hz were not detected.
The center point where the driver coil was attached is on a nodal line for
back plate modes 1, 3, and 6, and close for mode 4, so these modes of vibration
are not excited (it's not clear why mode 3 is so strong here;
my nodal lines may be sloppy or asymmetric).
Mode 5 is the most important.
This is a "drumhead" mode where the nodal line is an oval around the center.
(Measured peaks are 5-10 Hz lower than the tap tones
probably due to the extra mass of the driver coil attached at the
center of the plate when measuring the response electronically.)
165 Hz is E, 223 is A+,
339 is between E and F, and 494 is B (middle-C is 262).