A system for determining a user input which consists of a disturbance of a propagating signal in a medium with a surface, the system comprising: a communication interface configured to send a first signal to be used to propagate from a first transmitter , a first freely propagating signal through a propagating medium having the surface, and configured to send a second signal to be used to propagate from a second transmitter , a second freely propagating signal through the propagating medium , wherein the freely propagating signals have been allowed to freely propagate through the propagating medium to have a first received signal and a second received signal, both including the first freely propagating signal and the second freely propagating signal that have been disturbed by the user input, be received at a plurality of sensors of the propagating medium ; a processor coupled to the communication interface and configured to process the first received signal to at least in part determine the user input associated with the disturbance; wherein the propagating signal is acoustic; and wherein the transmitter and sensor comprised in the communication interface are coupled to the medium ; wherein determining the user input associated with the disturbance includes correlating with a first reference signal at least a portion of the first received signal including a first pseudorandom binary sequence content to determine a first correlation result, the first reference signal includes the first pseudorandom binary sequence content and was determined using information determined during calibration performed using a test signal propagated through the propagating medium, and correlating with a second reference signal at least a portion of the second received signal including a second pseudorandom binary sequence content different from the first pseudorandom binary sequence content to determine a second correlation result. a communication interface configured to send a first signal to be used to propagate from a first transmitter , a first freely propagating signal through a propagating medium having the surface, and configured to send a second signal to be used to propagate from a second transmitter , a second freely propagating signal through the propagating medium , wherein the freely propagating signals have been allowed to freely propagate through the propagating medium to have a first received signal and a second received signal, both including the first freely propagating signal and the second freely propagating signal that have been disturbed by the user input, be received at a plurality of sensors of the propagating medium ; a processor coupled to the communication interface and configured to process the first received signal to at least in part determine the user input associated with the disturbance; wherein the propagating signal is acoustic; and wherein the transmitter and sensor comprised in the communication interface are coupled to the medium ; wherein determining the user input associated with the disturbance includes correlating with a first reference signal at least a portion of the first received signal including a first pseudorandom binary sequence content to determine a first correlation result, the first reference signal includes the first pseudorandom binary sequence content and was determined using information determined during calibration performed using a test signal propagated through the propagating medium, and correlating with a second reference signal at least a portion of the second received signal including a second pseudorandom binary sequence content different from the first pseudorandom binary sequence content to determine a second correlation result. <EOS>