How Does REW or audiotool Measure SPL Without Knowing the ADC Reference? (Dayton iMM-6C Calibration)

[gpinna]

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Hi everyone, I'm new here.

I’ve just bought a Dayton Audio iMM-6C USB-C calibrated measurement microphone and I have a question about how SPL (dB) is calculated.
The calibration file of the microphone has in the first line 1000Hz -37.4 that i suppose it's the sensitivity at 1kHz.
However, since the iMM-6C is a USB microphone, its built-in ADC sets its own reference voltage. My question is: How does REW or app like audiotool determine the absolute SPL if they don’t know the ADC’s reference voltage? Do they rely solely on the calibration file for frequency correction, while requiring a separate SPL calibration (e.g., using an external SPL meter) to establish the actual decibel level?
I also can't find any info on the reference voltage in the datasheet of the microphone.
I’d appreciate any insights into how REW handles USB measurement microphones like this. Thanks!

 

[staticV3]

Hi @gpinna! Welcome to ASR.

The calibration file determines what dB SPL at the microphone input corresponds with what dB FS at the microphone output.

Using this information, REW can determine the SPL at any arbitrary level.

 

[DVDdoug]

I also can't find any info on the reference voltage in the datasheet of the microphone.

We don't care about the voltage which is internal to the microphone. Only the digital output is important.

And you may know this, but once you have a reference any dB change in the acoustic level results to the exact same change in the digital level. The dB difference is constant so finding the new acoustic level is simply a matter of subtraction.

 

[NTK]

USB mics are usually supplied with a cal file that includes the sensitivity of the microphone. (Source: link)

For serious SPL measurements, the calibration of the mic is checked before every measurement session against a calibrator. (Picture source: link)

 

[DVDdoug]

For serious SPL measurements, the calibration of the mic is checked before every measurement session against a calibrator.

A year or so ago, I bought a calibrator (around $100 USD) because I have a 40-year old Radio Shack SPL meter and I've read that electret mics can lose sensitivity as they age. (It does have an electret mic.) It was still "perfect" as accurately as I could read the analog meter!

 

[gpinna]

Thanks a lot. I kind of got it i but I'd like to understand it fully. I thought the sensitivity on the calibration file was just the analog sensitivity while actually it includes the digital part as well. I tried to calculate the formula from basic principle and i got that dB(spl) = dB(FS) + sensitivity_all + 93.98 where the sensitivity_all is 20*log(sensitivity_analog*V_ref). Where sensitivity_analog is measured in Pa/mV. Is it the right formula? Using the latter formula I tried to calculate the v_ref the case of iMM-6C. I took the the analog sensitivity from the datasheet of the iMM-6S that it's the same mic without adc part (10mV/Pa) and the sensitivity_all from in the calibration file (-34.4 dB) and I got 1.9 Volt that sound reasonable.

 

[phoenixsong]

​

Hi everyone, I'm new here.

I’ve just bought a Dayton Audio iMM-6C USB-C calibrated measurement microphone and I have a question about how SPL (dB) is calculated.
The calibration file of the microphone has in the first line 1000Hz -37.4 that i suppose it's the sensitivity at 1kHz.
However, since the iMM-6C is a USB microphone, its built-in ADC sets its own reference voltage. My question is: How does REW or app like audiotool determine the absolute SPL if they don’t know the ADC’s reference voltage? Do they rely solely on the calibration file for frequency correction, while requiring a separate SPL calibration (e.g., using an external SPL meter) to establish the actual decibel level?
I also can't find any info on the reference voltage in the datasheet of the microphone.
I’d appreciate any insights into how REW handles USB measurement microphones like this. Thanks!

Hi slightly off topic, but I can't help but ask you since you have experience with this microphone. Can I use it to record singing? Will its calibration be active in such a scenario, affecting the recorded file itself?

 

[AnalogSteph]

Can I use it to record singing?

Sure, as long as you don't exceed its maximum SPL capabilities (120 dB for 1% THD)...

Obviously it'll sound about as exciting as a good phone or headset mic, but shouldn't be affected by handling noise overly much.

Will its calibration be active in such a scenario, affecting the recorded file itself?

No, but you could use REW to generate a PEQ preset from the calibration if need be (I've forgotten the specifics but it is possible).