This project has been realised in the context of the European research project TAI-CHI (Tangible Acoustic Interfaces for Computer-Human Interfaces). It shows how a common object can be transformed into a drum pad by the mean of acoustic technologies. Four piezo sensors are attached to a plastic tray and connected to an electronic device for computing the position of taps or sticks hitting the tray’s bottom . The location is performed using an adaptation of traditional in-air TDOA algorithms (Time Delay of Arrival) to be compatible with surface-wave propagation. The electronic device is based on SenseWeb, a modular hardware platform for embedded interactive systems that is developed at MIS. The configuration used here is composed of a specifically designed preamp board (8 channels), a high sampling rate acquisition board (up to 384 KHz), a digital signal processing (DSP) board, and a MIDI board for connecting to a sound module. A tracing paper with a printed grid of 16 pads is glued on the tray to have a visual reference. The tray is used up side down, and since it is slightly transparent, the tracing paper is visible from the opposite face. To show the mapping flexibility of TAI instruments, two different layouts are proposed. For the first one, each pad is assigned to a MIDI note, with the intensity of the impact controlling the sound velocity. In the second layout, the 16 pads are grouped in four columns of four pads. Each column is assigned to a MIDI note and the vertical position inside the column is assigned to a MIDI Continuous Controller, varying the cut-off frequency of a filter applied to the sound.