Centre for Metamaterial Research and Innovation

Acoustic metamaterials for listening in flow

Acoustic metamaterials for listening in flow 

A case study by Tim Starkey and Alastair Hibbins, University of Exeter 

 

The problem 

The movement of any object through a fluid, such as air or water, creates a disturbance in the surrounding medium. Up to a characteristic speed, the fluid moves in a smooth path or layer around the object - this is known as laminar flow. However, once a critical limit is exceeded, the disturbance becomes irregular: the fluid's speed continuously changes in both magnitude and direction, becoming chaotic. In turbulent flow, unsteady vortices of many sizes appear, interacting and growing to produce high-amplitude pressure fluctuations at the surface of the body. 

These fluctuations are not just random hiss. As turbulent structures form near a wall, they are carried downstream with the flow, producing a well-defined pattern of pressure at the surface - but it is a pattern of near-field pressure, tied to the flow itself rather than to any sound arriving from a distance. 

A problem arises when you want to listen to a distant object through this mess. The sound you care about - from another vessel, aircraft, or source far away - is a small, coherent signal arriving on top of a much louder blanket of turbulent flow noise right at the sensor. Whether the sensor sits on the body of a car, the fuselage of an aeroplane, or the hull of a ship, it is swamped by the noise of the turbulence, and its signal-to-noise ratio suffers badly. Traditional fixes, such as large power-hungry beamforming arrays, are heavy and computationally intensive - awkward for compact or autonomous platforms. 

Our solution 

In our research we use surfaces and structures that support trapped acoustic surface waves. A metasurface of this kind is patterned with a periodic array of tiny resonant elements, each much smaller than the wavelength of the sound. By tuning the acoustic field right at the surface, we can design a structure that "hears" through the turbulence. 

The trick lies in a difference between the two kinds of pressure hitting the surface. Sound arriving from a distant source is coherent - it drives the whole patterned surface in step, so the surface waves it excites add up. The turbulent near-field pressure, by contrast, is incoherent and jumbled: across the surface it tends to average out to zero. A well-designed metasurface therefore naturally favours the coherent signal we want while suppressing the flow noise we don't. The resonant elements help further by filtering out the very smallest turbulent scales - those smaller than the elements themselves - so the surface responds to the larger, more useful scales. 

Our work is conducted with the Department of Aerospace and Ocean Engineering at Virginia Tech (USA), and has been supported by ONR, Dstl, and Thales. 

Why use a metamaterial? 

As is often the case, new opportunities are found at the intersection of different fields. In acoustics, metamaterials research has opened up new ways to manipulate and control sound fields, including structures that let air flow through but block sound, and the cloaking of solid objects. In aeroacoustics, meanwhile, structured surfaces - ordered roughness, bioinspired canopies, porous treatments - have long been used to control noise and improve aerodynamic efficiency. In both fields, the same underlying trick is at play: a patterned surface is used to influence the local behaviour of some field, whether that field is pressure, velocity, or displacement. The natural question is whether that shared principle can be harnessed for real applications. 

We see several promising avenues: 

  • Reducing flow noise - a targeted metasurface, for example at a trailing edge, could filter or average out turbulent structures and so lower the noise radiated by an aircraft or vessel. 
  • Improving sensors - tailoring the acoustic environment around a sensor to lift its signal-to-noise ratio and dynamic range in exactly the noisy conditions where it usually struggles. 
  • Managing energy - using metamaterial interfaces to convert or dissipate the energy tied up in flow-induced noise, without spoiling the aerodynamics or hydrodynamics. 
  • Understanding coupling - using tailored surfaces to probe the long-standing question of how aerodynamics and acoustics interact at the boundary layer, framed in terms of surface impedance. 

There are clear practical drivers too. Future unmanned underwater vehicles, for instance, must be light, energy-efficient, and compact, which pushes their onboard sonar toward fewer sensors and less processing. Flow-coupled acoustic metasurfaces could offer a passive alternative to traditional beamforming that helps meet those constraints. 

This is early-stage, low technology-readiness research: the work so far concerns low Mach number flow (M < 0.3) in air across a range of Reynolds numbers, and much remains to be understood. Our research in this area is a systematic study aimed at revealing the fundamental physical mechanisms and issues that come into play when metamaterials are used with flow - and we believe the handful of concepts described here only scratch the surface of what flow-driven metasurfaces might do across aero- and hydro-acoustics. 

References 

  1. Damani S, Alexander N, Devenport W J, Pearce B P, Shelley S R, Starkey T A, Hibbins A P and Sambles JR 2021 Excitation of airborne acoustic surface modes driven by a turbulent flow AIAA J. 59 5011–9 
  1. Damani S et al 2025 Kevlar-covered subresonant pressure sensor for flow measurements AIAA Journal 1–10 
  1. Braaten E et al 2023 Directionally sensitive sensor based on acoustic metamaterials AIAA AVIATION 2023 Forum (American Institute of Aeronautics and Astronautics) https://doi.org/10.2514/6.2023-3828 
  1. Galluscio C et al 2024 Wind tunnel testing of the directionally sensitive meander metasurface array 30th AIAA/CEAS Aeroacoustics Conf., AIAA 2024–3025 https://doi.org/10.2514/6.2024-3025