We have not done any official noise or voltage fluctuation testing. But I can say one factor of the noise floor is the battery voltage. As the battery voltage starts sagging, the SMPS has a harder time quietly boosting the voltage. Another factor is the load on the SMPS. The higher current draw the of the microphone, the noisier it gets for the same phantom voltage. With higher current draw mics, we suggest you lower the phantom voltage. We originally designed the Espresso for use with our Cortado contact mics, which use only about .5mA of current, but they can work with mics up to around 5mA as long as the phantom voltage is lowered enough to keep the noise floor down.
If you want to try to test the noise floor of the Espresso, you need to put an electrical load on the Espresso similar to the typical mic you use (such as a condenser mic with no mic capsule attached and the input shunted to ground). You would record the noise you hear with some recording software on your computer (hopefully your ADC is very quiet) and then measure the amount of noise. Assuming no other noise is coming from any part of the system, you can find the noise floor of the Espresso this way. This is much trickier said than done. I’ve always just used my ears to determine how acceptable the noise is from the Espresso…which often slightly changes through out a typical recording session due to the battery voltage falling.
I’m sorry I can’t give you more of a quantitative answer.