# Infrasound - Erebus microphones ?



## frankienones (Mar 25, 2011)

Hi
To measure anything below 20 hz seems to be difficult with a "standard" mic.
What about looking at the guys (?) who do measure activity of volcanoes , they have the same frequency range (below 30 , and towards 0 hz)
I could not find info of anyone who have used their technic to measure soundpressure from VERY deep subwoofers .. or , i might have missed it.
Am i missing something here ? I'm not sure whether REW could be used to handle such measurements , but measurement could be catched by oscilloscope...??
Interesting enough.. the same people have often used the panasonic electret mic (or a handful of them) to make their mics able to capture those very low frequencies
Info can be found here

..or , there is nothing to be learnt by these guys ?
Frank


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

Perhaps John can comment, but I see no reason why REW can’t measure ultra-low frequencies, given the right input. I expect that would start with a mic that has really low frequency response, then get it calibrated so that its ultra-low response is flat. Mics like the Behringer ECM8000 or Dayton EMM6 we normally use for audio probably won’t do, as their response falls off too fast below 20 Hz. With those mics, ultra-low freq compensation ends up merely boosting noise.

Regards,
Wayne


----------



## JohnM (Apr 11, 2006)

REW can measure to below 2Hz provided the rest of the measurement chain has response there.


----------



## frankienones (Mar 25, 2011)

thanks to both ..

This was what initially catched my interest : They showed a frequency response of the mic , to be very flat.. down to a selected high pass filter value.
yeah.. calibration would be an issue , even if the response is flat , but it could be used for a relative measure ?
From what i know so far.. the transducers by Honeywell do look promising (attached extract) ,they would be "easy" to setup , as they have a built in amp.

This is just an idea.. but if the signal from such mic , was "calibrated" on the high end , with respect to a known response curve from a std calibrated mic , would it be possible to "learn" - from the readings at where they overlap each other ?
I'm new to this , so plz forgive my ignorance of the subject..

Regards
Frank


----------



## Anechoic (Jan 16, 2009)

frankienones said:


> Hi
> To measure anything below 20 hz seems to be difficult with a "standard" mic.


It depends on what you mean by "standard" mic. Most precision (ANSI/IEC-rated) measurement mics are flat well below 5 Hz, many are flat down to 1 or 2 Hz - I have mics from ACO Pacific, Rion, PCB and BSWA that are all flat down to 3 Hz or better. The only issue is that these mics cost money and require pre-amplifers and power supplies. You can save a little bit of money by buying used equipment.

With regards to the consumer measurement mics like the Behringer ECM8000, those of course are rolled-off significantly in the low frequencies but that's not an inherit problem with the map capsules, the manufacturers have put capacitors in the preamp circuits to reduce the low frequency response (presumably to reduce excessive low frequency levels from wind, background noise, etc). It may be possible to actually remove the cap to improve the low frequency response of these mics, but I'll leave that as an exercise for an EE.

If you actually need sub-1 Hz sound measurement data, you start running into problems finding adequate transducers, but even moreso, you run into problems trying to develop measurement procedures to get adequate data - afterall, when you get that low you're basically measuring the comings and goings of various high/low pressure weather systems.


----------



## frankienones (Mar 25, 2011)

Anechoic : I'll receive a calibareted mic from you , next week.. and then i will have a reference down to 10hz..
- do you have a comment on the possibilty to read signal from another mic - on the frq below 10 hz ?
i.e - use the signal from the calibrated one , on say 20hz - and use that as a comparative value for th vlf mic ..
The VLF mic will have a flat response below 100 hz
Anyway .. it would be possible to read the signal with a scope , and record it manually.. 
I'm not in a position to use equipment costing 1k$+ - and are looking for other options..
TIA
Frank


----------



## Anechoic (Jan 16, 2009)

frankienones said:


> Anechoic : I'll receive a calibareted mic from you , next week.. and then i will have a reference down to 10hz..
> - do you have a comment on the possibilty to read signal from another mic - on the frq below 10 hz ?
> i.e - use the signal from the calibrated one , on say 20hz - and use that as a comparative value for th vlf mic ..
> The VLF mic will have a flat response below 100 hz
> ...


I'm not sure what you are trying to ask here. Do you want to use a calibrated mic to figure out the response of a second mic?


----------



## frankienones (Mar 25, 2011)

Anechoic said:


> I'm not sure what you are trying to ask here. Do you want to use a calibrated mic to figure out the response of a second mic?


Not exactly.. I'm asking if i can use the calibrated mic - to figure out the sensitivity of the second mic , using comparative method.. 

as in wiki :"A comparison calibration determines a microphone’s sensitivity by comparing its electrical response to a sound field (ie. its sensitivity) against that of a previously calibrated microphone (a reference microphone). Since the sensitivity of the reference microphone is already known, if the 2 microphones are exposed to the same stimulus (sound-field), then the responses can be directly compared"

i know there are issues involved.. but the second mic do have a flat response from 0,5-30 hz, with high accuracy. .. but to put the measurements into a db scale -(for sound pressure reading) - the measurement would need sensitivity adjustment.

You could tell me if i'm way out now 
I tried to describe it in the attached graph.. the yellow line is a reading from the VLF mic , and the green graph from a calibrated mic.
The second graph - shows the reading from the VLF mic - shifted - to correspond with the measurement from the cal. one , at 20hz
Sorry if this was an awkward way of asking a simple question..

Regards


----------



## Anechoic (Jan 16, 2009)

frankienones said:


> Not exactly.. I'm asking if i can use the calibrated mic - to figure out the sensitivity of the second mic , using comparative method..


Yes, in theory you can use the comparison method to derive the frequency response of another mics (in fact that's how I do my calibrations). In practice however it becomes very difficult to use the comparison method for low-frequencies in a free-field environment which is why I use a pressure chamber for the low-frequency portion of my mic data.


----------



## frankienones (Mar 25, 2011)

Thank you Anechoic


----------

