Devices can be made more intelligent if they have the ability to sense their surroundings and physical configuration. However, adding extra, special purpose sensors increases size, price and build complexity. Instead, we use speakers and microphones already present in a wide variety of devices to open new sensing opportunities. Our technique sweeps through a range of inaudible frequencies and measures the intensity of reflected sound to deduce information about the immediate environment, chiefly the materials and geometry of proximate surfaces. We offer several example uses, two of which we implemented as self-contained demos, and conclude with an evaluation that quantifies their performance and demonstrates high accuracy.