Acoustic Levitation at Very Low Frequencies
Acoustic levitation at low frequencies is presented, using a physical device which unlike most others reported in the literature, has linear dimension much smaller than the acoustic wavelength. The experimental apparatus consists of two air cavities, of 8 litre volume each, separated by a 15.24 cm (6 inch) loudspeaker diaphragm; one cavity being acoustically closed, the other opening to the ambient air through a small orifice, 8 mm in diameter. Sinusoidal excitation of the loudspeaker at high amplitudes, leads to a strong periodic flow of air in and out of the orifice, observed as stronger and jet-like during the output phase, which can levitate polystyrene spheres of 2 cm and 2.54 cm in diameter. Levitation occurs at positions 5–10 cm above the sound source, for driving frequencies from 40 Hz to 80 Hz (particularly around the nonlinear harmonics of the nearly 20 Hz Helmholtz resonance), and during periods of time from 1 minute, up to 3–4 minutes. The levitated spheres exhibit oscillations with amplitudes of a few centimeters, before suddenly flying out of the levitation zone. Measurements are presented of sound pressure signals inside and outside of the open cavity, and of acoustic flow velocity signals outside the orifice. Typical sound pressure levels are around 150 dB inside the resonator, 125 dB in the levitation zone, and 90 dB outside the levitation zone. Computer simulations are presented of the open cavity, modeled as a strongly excited nonlinear Helmholtz resonator, which includes a nonlinear resistance term accounting for the transfer of linear momentum to the periodic jet flow out from the orifice. Levitation is finally explained in terms of viscous drag forces from the periodic outward jet flow acting on the spheres.
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Document Type: Research Article
Publication date: 2010-03-01
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- Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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