My takeaway: >The requirement is that the size of the gap is smaller than the wavelength of the sound wave. This effect works not only in audio range of freque

My takeaway: >The requirement is that the size of the gap is smaller than the wavelength of the sound wave. This effect works not only in audio range of frequencies (Hz-kHz), but also in ultrasound (MHz) and hypersound (GHz) frequencies, as long as the vacuum gap is made smaller as the frequencies increase."