I found this paper while doing some research. And was fascinated (and scared) by what it revealed. You can measure acoustics by using the gyroscope on your smartphone. Which at first may sound cool but think about what that means in terms of privacy. Since, as of this writing, manufactures and development guidelines do not require users to grant permissions to access the gyroscope, they could potentially “listen” in on conversations to be used for other purposes.
We show that the MEMS gyroscopes found on mod- ern smart phones are sufficiently sensitive to measure acoustic signals in the vicinity of the phone. The re- sulting signals contain only very low-frequency infor- mation (<200Hz). Nevertheless we show, using signal processing and machine learning, that this information is sufficient to identify speaker information and even parse speech. Since iOS and Android require no special per- missions to access the gyro, our results show that apps and active web content that cannot access the micro- phone can nevertheless eavesdrop on speech in the vicin- ity of the phone.
Read it for yourself and be amazed and frightened!