An apparatus for sensing air pressure, comprising: an electrically active layer (401, 1301) having a first plurality of substantially parallel electrodes (305, 1309) and a second plurality of substantially parallel electrodes (306, 1310); a signal generator configured to generate excitation signals and connected to said first plurality of electrodes; a signal detector configured to detect output signals from said second plurality of electrodes, wherein an output signal from one of said second plurality of electrodes is indicative of the degree of capacitive coupling to one of said first plurality of electrodes on application of an excitation signal thereto; a flexible top layer (403, 1303); and a spacer layer (402, 1302) sealed to at least part of said electrically active layer and said top layer; wherein, said first plurality of electrodes are not parallel to said second plurality of electrodes, such that there exists a matrix of intersection points between said electrodes; said top layer is attached to said electrically active layer to define a plurality of hermetic voids (601, 1601) each covering one of a plurality of areas, wherein said flexible top layer may be grounded in at least a part of said top layer that overlays each said area, and either: each said area contains one of said intersection points, or each said area is in a cell bounded by a plurality of said intersection points; and for each said area where part of its top layer is grounded, the degree of capacitive coupling between the intersecting electrodes is indicative of the movement of said grounded part of said top layer overlaying said area towards or away from said electrically active layer, and thereby of the positive or negative pressure applied to the grounded part of said top layer overlaying said area. an electrically active layer (401, 1301) having a first plurality of substantially parallel electrodes (305, 1309) and a second plurality of substantially parallel electrodes (306, 1310); a signal generator configured to generate excitation signals and connected to said first plurality of electrodes; a signal detector configured to detect output signals from said second plurality of electrodes, wherein an output signal from one of said second plurality of electrodes is indicative of the degree of capacitive coupling to one of said first plurality of electrodes on application of an excitation signal thereto; a flexible top layer (403, 1303); and a spacer layer (402, 1302) sealed to at least part of said electrically active layer and said top layer; wherein, said first plurality of electrodes are not parallel to said second plurality of electrodes, such that there exists a matrix of intersection points between said electrodes; said top layer is attached to said electrically active layer to define a plurality of hermetic voids (601, 1601) each covering one of a plurality of areas, wherein said flexible top layer may be grounded in at least a part of said top layer that overlays each said area, and either: each said area contains one of said intersection points, or each said area is in a cell bounded by a plurality of said intersection points; and for each said area where part of its top layer is grounded, the degree of capacitive coupling between the intersecting electrodes is indicative of the movement of said grounded part of said top layer overlaying said area towards or away from said electrically active layer, and thereby of the positive or negative pressure applied to the grounded part of said top layer overlaying said area. each said area contains one of said intersection points, or each said area is in a cell bounded by a plurality of said intersection points; and for each said area where part of its top layer is grounded, the degree of capacitive coupling between the intersecting electrodes is indicative of the movement of said grounded part of said top layer overlaying said area towards or away from said electrically active layer, and thereby of the positive or negative pressure applied to the grounded part of said top layer overlaying said area.