Acoustics of Moving Media

a division of acoustics concerned with acoustic phenomena (the nature of sound wave propagation, radiation, and reception) in moving media or when the source of sound is moving. The field of its application is broad, inasmuch as the air and waters in

Figure 1. A diagram of sound propagation under the condition of an increase of wind with height

seas and oceans are moving continuously, thus affecting sound propagation.

Sound waves can be distorted by currents in the medium. Thus, for example, in the atmospheric layer near the ground the wind velocity increases with height (see Figure 1). Consequently, when sound is directed against the wind, the waves are bent upward and may pass above an observer standing on the ground, but when sound is propagated with the wind, the waves are bent downward; this explains why the audibility is better on the leeward side.

The definition of a sound field in a moving medium is based on Galileo’s relativity principle, which states that the movement of a medium relative to a source of sound is equivalent to the movement (at the same velocity) of the source relative to the medium. Many problems are solved on the basis of this principle—for example, the reflection of sound from a wind front and the radiation of sound from a vibrating surface streamlined by flow.

Besides the wind, there are in the atmosphere irregular turbulent flows that cause scattering of sound waves and fluctuations (irregular deviations from an average value) in their amplitudes and phases. The scattering problem was solved by taking into account the inhomogeneities of the turbulent flow and also the viscosity and thermal conductivity of the medium.

The development of high-velocity techniques brought to the fore the study of the sound field of rapidly moving sound sources and receivers which have velocities close to that of the sound in a medium.