With FFT sofware / hardware you can measure a microphone’s frequency and phase response easily. How? You need a speaker that can reproduce full range audio (doesn’t have to be flat or necessarily good although that is a good thing in general). Send pink noise through the amp / speaker / self powered speaker / etc…

Place you mic out in front of the speaker. 1 meter is a good distance since that is the distance speaker manufacturers typically use to explain how the speaker was measured by them.

1 meter = 3.28084 feet

Let’s call that 3 feet for this purpose. Probably doesn’t matter but why not try right!

Ideally you don’t want the speaker sitting on the floor (you’ll get extra low end but maybe that’s a good thing for your speaker) and you don’t want your speaker reflecting off nearby objects / walls / ceiling / etc… although even this isn’t a huge deal for this purpose.

Unless you start from an optimized situation acoustically, electronically, mechanically, etc… you’re not going to get an absolute result anyway. Instead you’ll get a response that relates to nothing. What good is this? Now substitute out microphone A with a mic that you know the frequency and phase response for (either one that came with a measurement file or at least a print out). In my case I have a good selection of Earthworks omni mics to use as a reference.

When you compare mics from different brands and models, you may have to make gain adjustments between each mic to maintain the trace upon the 0 db / 0 phase reference lines. Once you have matched basic gain between the first mic and the next, IF your unknown mic measures identical to your known mic, you’re golden. There are some variables to consider. If the mics are not in the exact same position, you will get slightly different measurement results. If the distance between the speaker and the mic changes (even slighting) it would be wise to recalculate the delay offset to make sure your phase trace is correct. If your known mic measures X and your unknown mic measures X, they’re matched and you can assume that the known mic has the same frequency and phase response as the known mic.

Take this a step further and you can test all of your mics periodically (using the same distance, same acoustic setup, same speaker / same settings, etc…) to see if you’re mics are functioning correctly. Unless you have some way to measure a very flat speaker in a very flat acoustic environment, don’t expect a flat response but if each time you test a microphone, it’s measurement result matches the previous one and so on, you at least know that the mic is still functioning. Obviously it would be wise to build a data library for each of your mics as they are purchased so you have a base line of what how that mic was performing when it was new. Mics get dropped, wacked, fall off stands, mic cables get tripped over. If you can test your mic selection and verify them against a measurement you made when they were new, that is great. How many old SM58s would test the same way decades later? How would an SM58 made this year test against one that was made 10 years ago? 20 years ago? 30 years ago? etc…

Ignoring the effect of age, moisture, internal windscreen deterioration, etc… I bet the sound of an SM58 has changed over it’s 50+ years of production. Do you know?

I only own a few SM58s. Nothing really old and nothing really new. Next time I have my rig set up I’ll test the ones I have and see how close they are to each other.

Here is another use for FFT software / hardware. You can compare mics that should measure the same to see if they do. Unless you are using mics that come with a custom frequency response print out like high end mics do (per serial number), the frequency and polar response shown in the literature is either a sample from one batch, an average of multiple mics or even worse, a mic that was hand picked because it measured the best. No way to truly know even if you ask. The trace we see in the Shure literature for the SM58 may be from the 60s. Do they update that frequency response and polar response each time they shift the manufacturing process? Do they measure sample mics from each batch? Do you know?