Comparing two record sounds' FFTs

I am doing some sound analysis on Audacity and have a question about the dB reference point while recording.

What I did was recorded a sound with and without a sound filter attached to the same microphone. I then analyzed using the plot spectrum feature to create two FFTs. There is a difference in the two FFTs, but my question is whether or not both sounds when recorded had the same reference point. That is to say that a reading of 10 dB on one FFT will be equal to a 10 dB reading on the other.


That’s the beauty of FFT and spectrum analysis and the dB range. If you have lumps and bumps in the response that are, say, 10 dB away from each other, they will stay 10 dB apart no matter how loud the whole thing is. So trumpets at -10 and violins at -20; and trumpets at -30 and violins at -40 are the same reading. The trumpets are still 10dB louder than the violins.

If you are after strict graphic presentation, then yes, you’ll have to choose one frequency and use that as a reference point between the two analyses and keep jockying the test loudness until you get them to match. But overall volume doesn’t make the slightest difference to the electrical readings–within reason.

It’s not unusual to rate acoustic material two different ways. “Putting the small, furry-animal blast filter over my microphone dropped the overall sound level 5 dB, but dropped the evil wind noise 23 dB without affecting the quality of the show.”