Fig. 2: System overview and frequency filtering.

a,b, Photograph (a) and schematic (b) of the sensor system with self-feedback. The inset shows a coloured microscopy image of the transducer. ADC, analogue-to-digital converter; DAC, digital-to-analogue converter. c, Time series of the sound input given by natural sounds (from the natural sound dataset 1 in ref. ⁴⁷), used to study the sensor response to complex inputs. d, Sensor response (in the active, nonlinear mode) to sound input (shown in c) obtained from the measurements in an anechoic chamber with two sensors of different resonance frequencies: 5.19 kHz for sensor 1 (purple line), 3.73 kHz for sensor 2 (red line). e, Frequency responses of sensor 1, sensor 2 and the exciter signal for natural sound input (as shown in c). The filtering effect is clearly visible. f, An enlarged section of the peak in d, to demonstrate the sine-wave response of the sensor despite the complex sound input. Note that the measurements in c–f are performed with a sensor having the design shown in Supplementary Fig. 2.