Basically, through the manipulation of soundwaves that are emitted by your smartphone’s speaker, one would be able to come up with buttons, knobs, or sliders, among others, which will be able to interact with your device without the need for any kind of power source or wired connection.
An app will need to be launched that will emit a tone via your handset’s speaker, and whenever you push a button or twist a knob, the “acoustrument” will modify the tone in the same manner as that of your finger changes the tone of a flute whenever it moves to block a hole. Should you push the button, the air pathway will change, resulting in a change of the tone, too. In this manner, your handset’s microphone will be able to detect the tone change and react accordingly. Interesting, no?