Trying to compare two speech samples that have been recorded by smartphones in pockets I'm trying to compare two audio samples of speech from two different Android smartphones to determine if the people are co-located. I'm doing this using the frequency bins of an FFT between 300 Hz - 3400 Hz. When the two devices are sitting on a table, I have good results.

However, when the devices are placed within pockets, the audio is recorded differently than when they are placed on a table.

Does anyone know exactly what happens to the audio frequencies/signal when the microphones are covered by a pocket or a bag? Are certain frequencies filtered?
The distance emitter-receptor and the presence of obstacles in the trajectory(clothes for example) will afect the recorded signal for sure. So some frequencies can be attenuated or just disappear.