  Resonance is an ubiquitous phenomenon present in many systems. In particular,
air resonance in cavities was studied by Hermann von Helmholtz in the 1850s.
Originally used as acoustic filters, Helmholtz resonators are rigid-wall
cavities which reverberate at given fixed frequencies. An adjustable type of
resonator is the so-called universal Helmholtz resonator, a device consisting
of two sliding cylinders capable of producing sounds over a continuous range of
frequencies. Here we propose a simple experiment using a smartphone and normal
bottle of tea, with a nearly uniform cylindrical section, which, filled with
water at different levels, mimics a universal Helmholtz resonator. Blowing over
the bottle, different sounds are produced. Taking advantage of the great
processing capacity of smartphones, sound spectra together with frequencies of
resonance are obtained in real time.
