Broadband sound pressure enhancement in passive metafluids

Acoustic sensors operating in lossy environments, such as water, require significant sensitivity to overcome the sound attenuation in the environment and thus see farther. We show here that a surprisingly large class of passive fluids has the ability to enhance the sound pressure propagating inside them without employing active actuation. Specifically, the general requirements for this remarkable property are fluid impedance higher than the impedance of the environment and negligible insertion loss as sound propagates from the environment into the high impedance fluid. We demonstrate the pressure enhancing effect by designing a broadband isotropic metafluid that increases the pressure of sound waves impinging from water. We validate the design in numerical simulations showing that significant sound pressure level increases are achievable in realistic metafluid structures in large bandwidths covering several octaves. Our approach opens up unexplored avenues towards improving acoustic transducer sensitivity, which is critical in applications, such as medical ultrasound imaging, sonar, and acoustic communications.

Figure 1

The PEPM coats the existing sensor to increase its sensitivity and signal-to-noise ratio. The matching layer matches the impedance of the PEPM and background fluid.

Figure 2

(a) Metafluid made of periodic arrangements of unit cells composed of square steel inclusions sealed in an elastomeric matrix; the inclusions are lubricated with a $1\text{−}\mu \mathrm{m}$-thick film of oil to provide a frictionless steel-elastomer interface. The inset: unit-cell structure with relevant geometrical dimensions; (b) fit between closed-form expressions (dashed lines) and simulated (solid lines) effective bulk modulus and mass density versus linear filling factor $\left(s/D\right)$. All material parameters are relative to those of water.

Figure 3

(a) The pressure field propagating from left to right at 40 kHz features the expected increase inside the metafluid. The outlines of the steel inclusions are shown with white lines; (b) SPL enhancement versus position inside the structure of the metafluid; (c) SPL enhancement versus position inside a continuous fluid whose bulk modulus and density are given by Eqs. (2). The dotted black lines in (b) and (c) represent the expected enhancement $20{log}_{10}\left(\sqrt{z/z_0}\right)$ [see Eq. (1)]; (d) the SPL enhancement versus frequency at the sensor-metafluid interface, i.e., right-hand side edge of (a), is essentially constant in the entire band of 7–105 kHz.

Figure 4

Time domain (top) and frequency spectrum (bottom) of a short Gaussian pulse propagating inside the metafluid. The numerically simulated pressure is obtained at the water-metafluid interface (0 cm), in the middle (6 cm), and at the metafluid-sensor interface (12 cm).