  Wind-driven sound generation is a source of anger and pleasure, depending on
the situation: airframe and car noise, or combustion noise are some of the most
disturbing environmental pollutions, whereas musical instruments are sources of
joy. We present an experiment on two coupled sound sources -organ pipes-
together with a theoretical model which takes into account the underlying
physics. Our focus is the Arnold tongue which quantitatively captures the
interaction of the sound sources, we obtain very good agreement of model and
experiment, the results are supported by very detailed CFD computations.
