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External Ultrasonic Microphone

Hello! We are hoping to trial various external ultrasonic microphones with the new AudioMoth this field season and were wondering what people have for recommendations for microphones? We will be using the devices to record bats in California. Thank you for any input!

24 comments

24 Comments

Edwin Houwertjes

Oct 26, 2021

A while ago I suggested hooking up an SPU0410LR5H-QB with just a simple resistor to steal some power from the mic/bias pin. I have the feeling the load of the microphone is not enough and we need to create a little voltage drop in order to lower the bias voltage. Otherwise the audio might not pass through the microphone switch circuit. If you tied and found it was not working, try to add a 10k resistor over mic and gnd connection. (10k is not critical, a higher value might be a bit better for less influence on the audio)

Oh, and if I ever did sent any of you a 7mm round PCB with SPU0410LR5H-QB microphone on it, it probably has 1nF capacitors and no 1uF capacitors. 1nF creates a highpass filter effect so these only work on ultrasound. Sorry for that mistake. (8mm round versions were fine)

Edwin Houwertjes

Oct 24, 2021

It would be nice to have the schematics, it looks like there is a SN74LVC1G3157 SPDT switch or something similar to switch between internal and external microphone and a MAX4466 opamp to amplify the audio. The small finger on the jacklug socket seems to connect directly to the switching line of the SPDT switch. Somehow it seems that U5 could be switching the power for the external microphone, but I can not see how what is actually switching U5, or maybe I just got confused switching back and forth between layers.

stuart.newson

Jul 12, 2021

Thanks Alex for getting back. That's a shame that the 1.2.0 schematics haven't been released yet. I think that I would need these to ensure the work on an external microphone for bat recording is done properly and consistently between the versions. Is there a plan to release these at some point?

Alex Rogers

Jul 12, 2021

No, we haven't released the 1.2.0 schematics yet.

stuart.newson

Jul 08, 2021

Many thanks Alex. Are the schematics for the 1.2.0 available anywhere? I have found a copy of the schematics for 1.1.0.

Alex Rogers

Jul 08, 2021

Hi Stuart, Yes, the analog circuitry is identical between 1.0.0 and 1.1.0. Both very different from 1.2.0 though. Alex

stuart.newson

Jul 08, 2021

Does anyone know if the analog input circuitry is the same with the older 1.0.0 as the 1.1.0? I think that I am safe with testing that I am doing with external microphones with the 1.1.0 (the 1.1.0 schematics are available), but don't know about the 1.0.0.

Edwin Houwertjes

Apr 13, 2021

I sent some of my elements to David in Belgium, he tested the microphone and compared to the internal (which is te same element) It seems to be working about just as well. I don't know if he has compared the recordings to something that uses an FG element. I wish I had the 1.2.0 version so I could do some test.

I just tried to mod my 1.1.0 so it might look more like te 1.2.0. I recorded a sweep from 150-20Khz which I played with a homebrew speaker, so we can only compare the two (maybe the mics work on higher frequency that te speaker). The SPU was used recording on middel gain setting, the FG on the highest setting.

Unfortunately connecting the FG did not work on my modded 1.1.0 board, so I attached V+ of the microphone to 3v3.

Edwin Houwertjes

Apr 13, 2021

Connecting the Knowles SPU and FG seems possible by using the circuit I described earlier. The FG element however is very much less sensitive than the SPU so the FG might need like 20-30db more amplification. (I don't know any exact numbers) A recording will be a whole lot softer when you use an FG element. I have no idea of the gain setting in the audiomoth can compensate for that, maybe running on the higest gain setting will be good enough. The FG has a huge price compared to the SPU, I think the FG is around half the price of an audiomoth. It is also difficult to mount and you would probably double the price of the audiomoth. The knowles SPU element seems to be doing quite a good job on the ultrasound range.

stuart.newson

Apr 13, 2021

Replying to

Many thanks Edwin. That's really helpful. I know that colleagues in Belgium have been planning on testing the Knowles SPU0410LR05-QB in an external microphone. I would be really interested if you get any feedback on the performance of this microphone as an external microphone. As I say for me, the main barrier for me is the attenuation of weaker ends of calls at higher frequency, which is a particular problem for Myotis.

Edwin Houwertjes

Mar 14, 2021

I heard people that were not that enthusiastic about the Primo element. Unfortunately like the good old WM61, where you probably only find clones slold todat, you might also run into a fake Primo. If you want to try the SPU0410RL05-QB you could consider this model. ELV sells these with good documentation, adding just one resistor (about 5 kilo ohms) seems to work fine on my computer. I don't know the hardware of the 1.2.0. version board but what is described here in the link below, lets me think this will work. The attached picture shows a similar microphone to ELV Bausatz Mikrofon MEMS1. I will sent out one to a friend soon to test but maybe someone else has the chance to test this as well, specially a comparison beween the Knowles SPU and the Primo element would be nice. Micbooster also sells the SPU element but seems to need more external components.

hinojosalangosto

Nov 24, 2021

Replying to

@Edwin Houwertjes Thank you very much for the information. Yes, the Kwnoles have 3 pins...

I will try to use one of that Kwnoles, I have the Audiomoth dev, wich have a 3v out conector. Get the extra energy is not a problem.

But i have a doubt. If the mic would have 4 pins, two for the energy and two for the signal it would be easy to conect.

But in this case, i see you conected a energy wire to a foot of the conector. + or -? And the other wire?

Sorry because I don´t know much electronics, but I want to try this things...

Edwin Houwertjes

Nov 24, 2021

Replying to

@hinojosalangosto Maybe this makes things easier to understand. The 3.5mm jack connector is a 3-conductor connector wit a switch contact. Tip Ring and Sleeve, Sleeve is ground (0v) Tip is the microphone signal. The Ring contact is not used. I put 3V (via a 1000 ohm resistor) on that pin. The connector on the PCB has 6 pins, two are on the Tip contact, two are the Sleeve, one is the Ring contact and one is a SWitch contact. The switch contact senses the external microphone, when you place the connector the switch contact pulls to GND. The FG elements mic wire (usually yellow) does not like the bias voltage that is on the tip contact so it needs a capacitor. 1uF is fine but this is not critical 100nF oe 100uF will also work. The bias volatge needs to be lowered, for that I use a 10000 ohms resistor between the mic lin (Tip contact) and the GND (Sleeve contact) . This should make your 3-wile microphone work.

hinojosalangosto

Nov 24, 2021

Replying to

@Edwin Houwertjes Thank you very much! I think i undertand it.

Alex Rogers

Mar 14, 2021

AudioMoth should work well with the Primo microphones - https://micbooster.com/primo-microphone-capsules/97-primo-em258-mono-module-with-35mm-plug-10m.html

Robert Manzano

Mar 14, 2021

Replying to

@Alex Rogers The main issue with the EM258 is the max. freq being a bit too low for surveying european bats, for which you need to get up to 120kHz at least. But of course depending on the specific purpose of it might be enough.

I'll be trying the external Knowles micro soon.

Robert

hinojosalangosto

Nov 22, 2021

Replying to

Good afternoon,I'm still looking for an external ultrasound microphone for the audiomoth.I have seen the Primo EM258 capsules.I have seen that they sell them soldered and not soldered. Without soldering they are cheaper. You just have to solder them to the mini jack connector?

On the other hand,I have read that the PRIMO microphone is not capable of recording bats that emit at higher frequencies. Do you know one that reaches at least 110Khz?

Edwin Houwertjes

Mar 13, 2021

I don't have the audiomoth to test it, but a Knowles SPU0410LR5H-QB wired like this works fine on my PC soundcard. Maybe this is something you could try. I think the SPU0410LR5H-QB is the same element as on the PCB of your 1.2.0 version board. Kind regards, Edwin

Justin Halls

Aug 27, 2021

Replying to

I use a similar technique on my external microphones, but since I build them into a 3.5mm jack plug there is very little space for the additional components. I therefore use a 1uF tantalum capacitor from Vdd to ground and connect Vdd directly to the ring of the jack plug - which is normally supplied with a bias voltage in most input sockets. Then I use a small 0.1uF coupling capacitor from signal to tip. Both of these can, with care, be accommodated in the body of the jack plug. In order to use these microphones (which preceded the design of the audio moth) you need to add a resistor on the audio moth from v+ (3V) to the ring connection of the added jack socket, which is unconnected in the moth.