quaudio™ comprises four omnidirectional microphones located at or near to the corners of a regular tetrahedron. Since these capsules are omnidirectional they can be located at the opposite corners of a cube with no loss in generality. This arrangement is straightforward to achieve in a standard PCB assembly line by soldering two pairs of MEMS or electret capsules on opposite sides of the substrate. Alternatively it is possible to solder three capsules to one side and a single capsule to the other. In both cases the acoustic centre of each sensor should be separated laterally by a distance corresponding to the vertical separation between membranes on either side of the device.