Below is a recording of a piezo (contact microphone) in contact with your bagpipe bag. It has 4 parts, depending on where I put the microphone. I am playing at low A = 440 Hz (MacLellan A440 chanter) and my drones are just slightly out of tune (a fact that I regret, sort of – it gives you an idea of how much one can hear from the piezo’s perspective if they are a little out of tune). Anyways, the first part is the piezo in between the blowpipe stock and chanter stock, right on top (bagchanter). The second part, the piezo is moved to in between the blowpipe stock and the drone stocks (bagblowpipe). The third part, the piezo is placed underneath the bag, behind the chanter (bagbottom). The fourth part is my reaching behind me and putting the piezo on the back of the bag (bagback). The chanter is playing high A the whole time. I play a Medium Swan Monarch Synthetic bag with a TrueFit cover. The drone reeds are MG bass and Selbie tenors. They all are attached to the drone reed seat via an extender to tune low enough in pitch to play with the MacLellan A440 chanter. Thus, the drone reeds slightly poke out the end of the drone stocks, which may have a huge effect (or no effect), on how much sound is contained in vibrations of the bag.
Here is a frequency analysis of all 4 positions with no normalization.