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. 2022 Feb 7;22(3):1264. doi: 10.3390/s22031264

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© 2022 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Sound waves and spectrograms for a 50-s sound source randomly extracted from videos uploaded to YouTube for sleep induction: (a) the sound of making glossy slime with a piping bag, (b) a voice whispering softly in a hair shop role play, (c) the sound of an oil massage and the whispering sound overlapped, (d) the sound of raindrops falling from the eaves and raindrops pouring, (e) steel wool sponges, the sound of rubbing various brushes, dolls, paper packaging materials, toys, etc., into the microphone.