What is inductance (NOT impedance!) of typical dynamic microphone? - Page 1
Forum section: electronics / general

author: dusan
author_rank: Regular Contributor
timestamp: May 16, 2023, 09:16:05 am
content: I'm trying to create model of typical dynamic microphone, I don't have one, and I'm trying to find out what is typical inductance of dynamic microphone. Everybody only posts impedance, for example Behringer SL-84C has 250Ohm:https://mediadl.musictribe.com/media/PLM/data/docs/P0E7R/SL-84C_QSG_WW.pdfIt's like closest guarded industry secret nobody wants you to know.Code:---*---sinewave--inductor--resistor---*---|_____ capacitor___________________|What values should I choose for voltage amplitude, inductor, resistor and capacitor values?

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 16, 2023, 06:23:31 pm
content: Here's a sweep of a a t.Bone MB-75, a cheap and cheerful SM-57 clone from Thomann. Impedance is specified as 300 ohm. It's a lot flatter and more resistive than I expected.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 16, 2023, 06:50:13 pm
content: It's not really a simple RLC circuit because the mechanical parts of the microphone affect the electrical circuit due to back-emf. Those mechanical parts have frequency dependent behavior that isn't as clean as capacitors and inductors. So you can approximate it, but it's not going to be great.Also, note that your model is potentially misleading. You show a voltage source as part of an RLC network. That's fine as far as it goes, you can define a lot of equivalent circuit networks. But if you are expecting that you can model it as an ideal frequency independent pressure-to-voltage converter modified by an RLC network that matches the microphone impedance, that's probably not correct. For that you need the overall sensitivity curve which is in the data sheet, and includes the transducer sensitivity as well as the circuit loading.

author: dusan
author_rank: Regular Contributor
timestamp: May 17, 2023, 05:49:58 am
content: What are those two curves? What's on the axis?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 17, 2023, 06:48:47 am
content: Magnitude in ohms and phase in degrees of the impedance vs frequency.

author: dusan
author_rank: Regular Contributor
timestamp: May 17, 2023, 07:47:44 am
content: How can I calculate inductance from it?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2023, 07:29:26 pm
content: Quote from: dusan on May 17, 2023, 07:47:44 amYou can't, because it's more complex than a simple RLC circuit. The inductance, resistance and capacitance are frequency dependant.

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 18, 2023, 11:06:21 pm
content: The microphone looks more resistive than inductive over most of the audio band. This was a quick attempt at roughly matching the behavior with an LCR circuit from 10 Hz to 100 kHz.

author: dusan
author_rank: Regular Contributor
timestamp: May 19, 2023, 05:21:52 am
content: Thanks, now if I want to "reverse that", i.e. now this subcircuit became source and powers the amplifier, where should I put the signal source, in series?

author: VK3DRB
author_rank: Super Contributor
timestamp: May 23, 2023, 11:22:14 am
content: About 12 years ago I had to design a piece of test equipment to emulate 400W of audio into a programmable array of dynamically switched 50W "speaker" loads, with programmable crossovers too. Obviously, using real speakers was not practical.I don't have the circuit now, but I recall a decent single voice coil speaker emulator was more than just an RLC circuit; it was an array of R's, L's and C's for each speaker. Maybe 8 or so passives for each speaker.A microphone emulator could be similar but on a much smaller scale and not emulated by a simple RLC circuit either, but could be approximated in a model. The technology in the microphone needs to be known. Eg: electret/crystal/voice coil/In practical terms, assuming it is a voice coil microphone, you could use a complex impedance meter assuming the L is quite prominent over the C and you know the R. ie: work back to get the L. It will only be an approximate.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 23, 2023, 06:40:39 pm
content: Entire books have been written about making electrical models. I usually model PZT devices like crystals, for which there are simple widely published formulas. Not sure about a dynamic, but it's going to be similar to a speaker. I've always liked this diagram from an old General Radio manual because the presentation is easy to understand. Note that "c" means Hz. Don't be surprised if an equivalent network uses impractically large inductances. I'd start by measuring reactance within the frequencies of interest. If you need to convert them to a more useful form I've got an impedance converter here, first item-http://www.conradhoffman.com/chsw.htm

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 23, 2023, 08:00:19 pm
content: A dynamic mic and a loudspeaker are really the same device. In fact the reciprocity principle is used to calibrate microphones. This is a good read-https://en.wikibooks.org/wiki/Engineering_Acoustics/Transducers_-_Loudspeaker

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2023, 08:16:13 pm
content: Quote from: Conrad Hoffman on May 23, 2023, 08:00:19 pmTrue. A dynamic microphone is more like a small headphone transducer, as it's roughly the same size.

author: dmills
author_rank: Super Contributor
timestamp: May 28, 2023, 03:19:19 pm
content: And in fact if you plug an SM57 into the headphone out on the desk with a bodge lead you can use it to swear at the talent for behaving badly on stage...Who? Me? Never.Regards, Dan.