title: Boundary-Layer Effects on Acoustic Transmission Through Narrow Slit Cavities.
abstract: We explore the slit-width dependence of the resonant transmission of sound in air through both a slit array formed of aluminum slats and a single open-ended slit cavity in an aluminum plate. Our experimental results accord well with Lord Rayleigh's theory concerning how thin viscous and thermal boundary layers at a slit's walls affect the acoustic wave across the whole slit cavity. By measuring accurately the frequencies of the Fabry-Perot-like cavity resonances, we find a significant 5% reduction in the effective speed of sound through the slits when an individual viscous boundary layer occupies only 5% of the total slit width. Importantly, this effect is true for any airborne slit cavity, with the reduction being achieved despite the slit width being on a far larger scale than an individual boundary layer's thickness. This work demonstrates that the recent prevalent loss-free treatment of narrow slit cavities within acoustic metamaterials is unrealistic.
authors.affiliation: Department of Physics and Astronomy, Electromagnetic and Acoustic Materials Group, University of Exeter, Stocker Road, Devon EX4 4QL, United Kingdom.
authors.forename: G P
authors.initials: GP
authors.lastname: Ward
authors.affiliation: Department of Physics and Astronomy, Electromagnetic and Acoustic Materials Group, University of Exeter, Stocker Road, Devon EX4 4QL, United Kingdom.
authors.forename: R K
authors.initials: RK
authors.lastname: Lovelock
authors.affiliation: Department of Physics and Astronomy, Electromagnetic and Acoustic Materials Group, University of Exeter, Stocker Road, Devon EX4 4QL, United Kingdom.
authors.forename: A R J
authors.initials: AR
authors.lastname: Murray
authors.affiliation: Department of Physics and Astronomy, Electromagnetic and Acoustic Materials Group, University of Exeter, Stocker Road, Devon EX4 4QL, United Kingdom.
authors.forename: A P
authors.initials: AP
authors.lastname: Hibbins
authors.affiliation: Department of Physics and Astronomy, Electromagnetic and Acoustic Materials Group, University of Exeter, Stocker Road, Devon EX4 4QL, United Kingdom.
authors.forename: J R
authors.initials: JR
authors.lastname: Sambles
authors.affiliation: DSTL, Porton Down, Salisbury, Wiltshire SP4 0JQ, United Kingdom.
authors.forename: J D
authors.initials: JD
authors.lastname: Smith