Noise Reduction Using a Spiral Acoustic Black Hole

The use of an Acoustic Black Hole (ABH) was shown to be an efficient means of reducing vibration. In a recent work, we proposed a compact and curvilinear shape of the ABH using an Archimedean spiral, which has a uniform gap distance between adjacent baselines of the spiral. As the length of the spiral was increased, we could obtain better damping performance. In this study, we investigate the sound radiation from the spiral ABHs, as noise reduction is also an important issue together with vibration damping. We perform numerical simulations for the following four cases: the beam only, the beam with the standard ABH, the beam with the spiral ABH, and the beam with the spiral ABH with damping material added to it. The pattern of the sound power level in the far field is similar to the pattern of the driving point mobility of the beam, which implies that the attenuation of structural vibration by the ABHs is the main reason for the reduced sound radiation. Whilst several extra peaks occur in a wide frequency range between the resonance frequencies of the beam only when the ABHs with and without the damping material are attached to the beam, the standard and spiral ABHs show the reduction of the overall sound power levels. In addition, when the damping material is added to the spiral ABH, a further reduction in the noise radiation is achieved. Because the sound from the spiral ABH is propagating through the gap between spiral lines, we analyze a geometrical condition of the spiral shapes in order not to amplify the sound energy.