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• electrostatic calibrator measurement

  DeepSOIC • 4 days ago • 0 comments

  Here ("es-cal"), overlaid on the electrodynamic calibrator ("ed-cal"). First amplitudes, then phases.

  Electrostatic calibrator measures +-3db over the full audio range (the falloff that starts at ~18 khz is the soundcard itself), VERY IMPRESSIVE! The amplitude results match with the electrodynamic cal up to ~500 Hz.

  The phase differs quite significantly in the infrasonics. I think it is most likely due to trapped air leaking, as with its stiffer diaphragm, the electrostatic calibrator should be more sensitive to it.

  Going into high frequencies, the phases also start to go apart only at 500 hz. And oddly,  the phase of the electrostatic calibration also starts to notably grow negative from ~200 hz. Odd. I'm not sure, what is the cause of this downward slope.... the distance from the radiating diaphragm to the microphone's capsule is about 3 mm, which is a bit too small to account for this phase shift. Maybe it's the microphone's cavity causing the effect somehow... 

  So for my sound energy measurement, i chose electrodynamic calibrator data, because a) it has less noise, b) lower phase error in infrasonics. But thanks to this electrostatic calibrator, i am very confident that the calibration is actually correct.

• why electrostatic

  DeepSOIC • 5 days ago • 0 comments

  so my speculation for the origin of the peak at 1.8kHz on the magnetic driver calibrator is:

  * heavy cone

  * cone is not infinitely stiff

  OR

  * motor force is not evenly distributed over the cone surface

  Initially i was thinking of using a planar driver, from/for planar headphones. They are quite expensive, and the force is still not really evenly spread over the surface. But after a bit more searching / thinking, it struck me that i can maybe use a capsule of an electret microphone, as a speaker. This one should have nearly perfect force distribution, and a very lightweight diaphragm,

  So it should significantly push on both points, and hopefully move this resonance to much higher frequencies. I only had doubts if it would work at all, as in, will it be loud enough from a normal signal source or a simple amplifier. And i went ahead and bought some.

  It turned out that even the output of a cheap usb soundcard (~2.5 Vpp) is sufficient to make a measurement.
  

  For explanation on how i made the coupling, please see the video linked in the project description. Here i'll just post the CAD screenshot of the coupling. The model is attached to the project.

• first measurements with a magnetic driver

  DeepSOIC • 5 days ago • 0 comments

  this is why i decided to try electrostatic.

  So, first i made a microphone calibrator based on a regular headphone driver.

  It works. 

  This is the frequency response of Earthworks M30 with my magnetic-driver calibrator. It does indeed go down to 3 Hz, as promised*. And there is a huge peak at 1.8 kHz, probably a result of cone flexibility. Or the volume of trapped air being too large, with its compliance causing a resonance together with cone's mass. Or a little bit of both. 

  But i was a little concerned about phase inaccuracy - there was some noticeable downward slope of the phase, there is non-negligible negative phase at 200 hz already. While it seemed like it is not really caused by a resonance peak at 1.8 khz, it might be. So i thought, that i need a calibrator that works up to higher frequencies.

  *: the M30 i bought actually was rolling off much sooner. I complained to the company, and they replaced it. Unfortunately, the bad one i had didn't arrive to them (was stolen), so we'll never know what has caused it.

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