Very Low Resonant Frequency Pickup

[heart worm]

I come in search of knowledge.
I have for some time been interested in making simple electrified percussion instruments. I have made a few, one a metal box with piezos in it feeding a low voltage tube preamp and another a speaker in a box feeding a low voltage tube sallen key resonant low pass filter circuit and on into a preamp. In my research of various existing technologies along this line I have found one that has fascinated me and which has existed some quarter of a century. I am speaking here of the Porchboard Bass percussion stomp box. It interests me for this reason: it uses a variable reluctance transducer (ie a pickup) and the manufacturer claims that the pickup has a resonant frequency of 33 cycles and a second peak around 100. Having tried to model what such a pickup would have to look like in the falstad analog filter simulator, the only way this is possible that I can think of is an insanely high inductance and winding capacitance with a relatively low resistance. And to have two peaks would require two pickups. In the patent for said device it says that the box contains only a single sensor which connects to a 1/4 jack. In reference to the pickup the patent says only:

Though different active or passive magnetic sensors may be appropriate for use as the sensor 42, a suitable sensor is manufactured by SSI Technologies, Inc., Model No. C-6066.6

I can find no definitive information on the referenced sensor but from what I can gather it is likely a passive automotive speed sensor(which is functionally identical to a pickup, I surmise).

My question to this community is this: how is such a miracle pickup possible? I would like to add the disclaimer that I have no intention of manufacturing a similar device to the aforementioned product for profit, but it would greatly benefit me as a tinkerer and musical piddler to be able to obtain or create pickups of any given crazy resonant frequency.

I thank you for your time.

Patent: https://patents.google.com/patent/US5627336A/en

[Helmholtz]

The patent doesn't mention resonant frequency.
From the Porchboard site info I gather that the 33Hz is not a PU resonance but a mechanical system resonance.

A PU with a 32Hz resonance could be made e.g. from a 5H coil with an (external) 5µF tuning capacitor.

[heart worm]

Thank you for your reply. You are correct the patent does not mention resonant frequency and indeed the patent has little to say about the electronics therein at all, but it does not mention a loading capacitor or two coils or anything else. This can only lend credence to your proposal that we are talking mechanical resonance here. I am not sure how specific a patent needs to be. If 33Hz does refer to the mechanical resonance of the wooden box, that would make sense, but there is a video in which the inventor is asked how it works and he says a tuned/tank circuit. Additionally I've read where he referred to the magic being the result of a 'tune proximity sensor'. This, plus the fact that the patent only shows a sensor attached to a jack in the way of electronics lead me to believe that the single pickup and its parasitic properties forms this "tank circuit" with its two peaked resonant frequency response. I probably let my imagination get carried away with me there and departed rapidly from reality with my thinking.

As for me missing a simple combination of values that could easily produce the desired resonant frequency, I reckon I was just lost in the sauce tweaking values in the simulator. Thank you for the common sense.