Figure 5: Experimental demonstration of metamaterial-enhanced acoustic sensing.

(a) SNR measured in the metamaterial compared with that obtained in free space. The input signal from the speaker was a series of Gaussian pulses with a centre frequency of 9.3 kHz and a bandwidth of 500 Hz. The inset shows the Gaussian pulse in time domain (left) and frequency domain (right). The diamond dots are the experimental data and the solid line is the fitted curve with a slope of 114, indicating about 20 dB SNR improvement by using the metamaterial-enhanced sensing system. In both free space and metamaterial, the SNR is calculated by using the total measured signal power W in the frequency domain within the bandwidth of 9.05–9.55 kHz when the speaker was turned on compared with that obtained with the speaker turned off (W₀, as noise floor), that is, SNR=(W−W₀)/W₀. In the grey highlighted zone, the measured free-space SNR is <1, indicating that the input signal is below the detection limit. (b,c) Time-domain and frequency-domain pulse signal measured in free space (top) and in the metamaterial (bottom). The SNR measured in the metamaterial device is 32.7 compared with the measured free-space SNR of 0.27, which corresponds to point A in a.