Setting the story straight about how vibrating surfaces produce sound

Can all vibrating surfaces produce sound in an adjacent fluid? This is a basic question in acoustics. Books and courses teach that the nature of the pressure wave generated by a vibration depends on the ratio between the vibration's speed and the fluid's sound speed: Supersonic vibrations (where the vibration moves faster than the sound speed) radiate sound waves into the fluid, while subsonic vibrations (where the vibration moves slower) generate evanescent pressure waves that do not radiate and cling to the surface. During an applied research project, however, we found results at odds with this prevailing view, indicating that subsonic vibrations can also radiate sound waves. Furthermore, a handful of articles over the last 30 years have also found similar results. While these results challenge the prevailing view of sound radiation, no full fundamental physical explanation of this phenomenon was available. We provide this missing explanation by taking energy conservation fully into account in a quite straightforward radiation model. This model balances the sound wave radiated into the fluid with a necessary attenuation of the guided wave underlying the vibration. While this attenuation is neglected by the majority of explanations in the literature, it is the key to explaining subsonic sound radiation, because attenuated vibrations produce inhomogeneous sound waves that always radiate. We find that subsonic vibrations can radiate sound waves in many situations, depending on the properties of the guided wave and the fluid medium.