[AudioI] Advice for low-frequency project

[Matt Flax]

On 16/2/22 10:08, John McManigle via People wrote:
 Hello all, 

 
 

Hi John,

 


 

 
I am working on a project to record/analyze low-frequency noise with a Pi Zero. Currently early in the prototyping stage, so want to see whether this plan seems feasible:
 


 

 

Sounds like a nice project, how low are you wanting to go in frequency ?
 

 


 

 

 
 
ICS-40300 analog MEMS microphone (which has extended low-frequency response), planning to use the pre-fab test board (https://www.mouser.com/ProductDetail/410-EV_ICS-40300-FX) during prototyping.
 
which would connect to...
 
I guess would need a custom op-amp circuit to go from mic level to line level (looks like that mic outputs at 0.355 Vrms at 200Ω impedance; not sure how close that is to line-readable).
 
which would feed...
 
The line input on a Audio Injector Zero Sound Card. (Planning to use line in even if it needs amp rather than mic input because future versions might require two channels.)
 

 
 
I am pretty comfortable with basic digital work and programming, but this is one of my first forays into the analog world. So my questions are:
 

 
 
1. Does this seem like a reasonable approach overall?
 


 

 

1. Yes that sounds like a reasonable approach. Because you are recording from electret microphones (which typically have high noise floors), the noise from power supplies and things like that should be manageable.
 

 


 

 
2. Does anyone know of a MEMS microphone with extended low-frequency response that outputs I2S or otherwise digital, as that would save some design headache obviously? (My target sounds are 20-150 Hz.)
 


 

 

2. There are MEMS microhpones which are digital, however I am not familiar with them - also there is the added complication of using a codec or other device to translate from PDM to I2S. WE have a product with two SPU0410LR5H MEMS microphones onboard. That may be of interest to you, it is the Ultra soundcard.
 

 


 

 
3. Is there a built-in highpass filter in the Audio Injector Zero card that would limit input performance in the 20-150 Hz range?
 


 

 

3. For the stereo/zero soundcard, the electret microphone has an AC coupling capacitor which would have a low frequency cut off, not sure exactly what it is, but you probably can't short it because there is the DC voltage to power the microphones on the signal. The line inputs have AC coupling capacitors C11 and C12 which could be shorted to remove AC coupling and essentially allow DC operation - that requires tinkering and experimentation.

 

The Ultra soundcard has two lines from the main board which jack into the codec's microphone preamplifier. They have two AC coupling caps there C22 and C21. Shorting them would give you a direct path to the codec's mic in pins, however that may not be necessary. The preamplifier board has the two SPU0410LR5H MEMS microphones onboard. Their signal pins go directly to the C22 and C21 on the main soundcard board. The line inputs of the Ultra soundcard do have a header on the preamplifier board which have no AC coupling and can go to DC. However, it may not be necessary to use line inputs if the microphone input lines go below 20 Hz - you could test that out.
 

 

If you don't want external preamplifiers and still want to use the other MEMS microhones you mention, the Ultra soundcard may be the way to go. If you want to look more into the capability of the soundcard, the cs4265 is the audio codec on the main soundcard board.
 

 


 

 
4. Any recommendations on the mic-to-line amp circuit? I was planning on approximately following https://www.st.com/resource/en/application_note/an4598-preamplifying-the-analog-output-of-a-mems-microphone-stmicroelectronics.pdf
 

4. That looks like it would work. There are some pre-built electret microphone preamplifier boards out there.
 

 


 

 
5. Other general advice?
 


 

 

5. Yes, but it is a little off topic.

 

Matt
 

 


 

 

 
 
Thanks very much,
 

 
 
John
 

 
 

 
 
 




 

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