Study of acoustic resonance in a cylindrical Helmholtz resonator excited by a tangential jet of air through a lateral slit

We report an experimental investigation of a cylindrical Helmholtz resonator with a lateral slit, excited by an oblique air jet generated by a rotational motion of the resonator. Despite the small size of the cavity, the system produces clearly audible tones. The acoustic response can be analysed using a smartphone-based audio spectrum analyser. The dominant resonance frequency is found to vary systematically with the slit length. The experimental results are described by the classical Helmholtz resonance model, which relates the observed frequencies to the geometrical parameters of the resonator through an effective mass--spring description. The apparatus is inexpensive, easy to construct, and suitable for both classroom activities and undergraduate laboratory experiments. The activity provides a clear and quantitative framework for exploring acoustic resonance, connecting concepts from mechanics, thermodynamics, and acoustics.