Internal damping measurements of beams with an acoustic black hole taper

Aluminum beams which taper to a thin central point according to a power law exhibit acoustic black hole (ABH) behavior. As the beam becomes very thin, approaching a non-zero critical thickness, the wave speed decreases, trapping flexural waves to a localized region of high displacement. Typically, damping material is applied to this region for noise control applications. However, recent measurements have shown that increased modal loss factors are achieved even without additional damping. To investigate this, we present loss factor measurements for tapered beams in the absence of dissipative effects, including that of acoustic radiation into the surrounding medium, to determine what proportion of the increased damping is caused by acoustic losses. This is done by exciting the beam inside a vacuum chamber and measuring its flexure using laser vibrometry, then comparing it measurements performed outside of the vacuum chamber. Using beams with different ABH parameters, loss factor measurements are shown to be highly dependent on the critical thickness and greater than those of a non-tapered beam. These results will be discussed in the context of previous work on structures with ABH tapers.