My first thought is acoustics. Empty spaces are resonant spaces, and this can be heard when the phone's speaker(s) are playing sounds and/or the microphones ar

My first thought is acoustics. Empty spaces are resonant spaces, and this can be heard when the phone's speaker(s) are playing sounds and/or the microphones are microphoning. Mechanically damping resonances is always superior to handling them with DSP -- a DSP doesn't need to account for resonances that simply aren't problematic to begin with, freeing it for other duties. Such resonances can also be heard when handling the phone -- even when it is off. Tapping on the phone and hearing/feeling a dull, well-damped thud inspires more of a perception of solidity than a lingering resonance does, even though such foam itself contributes nothing to to the structure of the device at all.