An apparatus comprising: a piezoelectric micromachined ultrasonic transducer, pMUT, including: a piezoelectric membrane disposed on a substrate; a reference electrode coupled to the membrane; the apparatus being characterized in that it further comprises a first drive/sense electrode and a second drive/sense electrode coupled to the membrane to drive or sense a first mode of vibration and a second mode of vibration in the membrane; a first signal generator to output a first electrical signal on the first drive/sense electrode relative to the reference electrode; a second signal generator to output a second electrical signal on the second drive/sense electrode relative to the reference electrode, wherein the first signal generator and the second signal generator are to drive the first electrical signal and the second electrical signal in phase to excite the first mode of vibration more than the second mode of vibration in the membrane and wherein the first signal generator and second signal generator are to drive the first electrical signal and the second electrical signal out of phase to excite the second mode of vibration more than the first mode of vibration in the membrane; and a signal processor coupled to the first drive/sense electrode and the second drive/sense electrode, the signal processor being configured to receive a response spectrum comprising a first frequency component and a second frequency component that is greater than the first frequency component that are output in response to the first mode of vibration and the second mode of vibration in the membrane, wherein the first frequency component is associated with the first mode of vibration and the second frequency component is associated with the second mode of vibration. a piezoelectric membrane disposed on a substrate; a reference electrode coupled to the membrane; the apparatus being characterized in that it further comprises a first drive/sense electrode and a second drive/sense electrode coupled to the membrane to drive or sense a first mode of vibration and a second mode of vibration in the membrane; a first signal generator to output a first electrical signal on the first drive/sense electrode relative to the reference electrode; a second signal generator to output a second electrical signal on the second drive/sense electrode relative to the reference electrode, wherein the first signal generator and the second signal generator are to drive the first electrical signal and the second electrical signal in phase to excite the first mode of vibration more than the second mode of vibration in the membrane and wherein the first signal generator and second signal generator are to drive the first electrical signal and the second electrical signal out of phase to excite the second mode of vibration more than the first mode of vibration in the membrane; and a signal processor coupled to the first drive/sense electrode and the second drive/sense electrode, the signal processor being configured to receive a response spectrum comprising a first frequency component and a second frequency component that is greater than the first frequency component that are output in response to the first mode of vibration and the second mode of vibration in the membrane, wherein the first frequency component is associated with the first mode of vibration and the second frequency component is associated with the second mode of vibration.