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. 2022 May 26;8:55. doi: 10.1038/s41378-022-00384-0

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© The Author(s) 2022

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 4 — a Frequency responses of representative acoustic surfaces. Two 10 cm × 10 cm samples (active area: 9 cm × 9 cm) with the same film thickness of 12 μm but different dome radii are compared in the figure. The sound pressure level is measured at a distance of 30 cm and a constant 10 V amplitude of the applied sinusoidal driving voltage. The pitches of the microdome array for the 350-μm-radius (R = 350 μm) sample and the 800-μm-radius (R = 800 μm) sample are 1 and 2 mm, respectively. b Sound pressure levels produced by the samples in response to varying drive voltages (RMS) at f = 10 kHz. c Frequency responses of an acoustic surface fixed on a rigid flat baffle or curved cylinders of different radii ρ under the same test conditions as (a). The dome radius is R = 800 μm, and the pitch is 2 mm. d Sound pressure level produced by the sample fixed on different surfaces in response to varying drive voltage (RMS) at f = 10 kHz. e Schematic showing the measurement of the directivity patterns of an acoustic surface mounted on a rigid flat baffle and an acoustic surface mounted on a rigid cylinder. f, g Directivity patterns of a representative acoustic surface (active area: 9 cm × 9 cm) on a rigid baffle (solid curves) and a representative acoustic surface (active area: 9 cm × 9 cm) on a glass cylinder of radius ρ = 5 cm (dotted-dashed curves) at f = 2 kHz (f) and f = 20 kHz (g), respectively, in terms of normalized SPL. The SPL at different angles to the acoustic surface is measured while maintaining a constant distance of 30 cm and normalized based on the maximum value in space. The dashed curves provide the theoretical directivities of a flat acoustic surface at 2 and 20 kHz, predicted based on the Rayleigh integral.