The effect of turbulent approach flow on the radiated sound from a circular cylinder was studied experimentally. The approach flow turbulence was provided by a single stream shear layer produced by an open jet anechoic tunnel facility. An instrumented cylinder was used to measure steady and unsteady surface pressure. The sound radiated from the cylinder placed in both an irrotational approach flow and the highly turbulent approach flow of the shear-layer was measured and compared. The cylinders located within the shear layer produced a less tonal sound with a higher broadband amplitude when compared to that of the free-stream approach flow. Secondly, the radiated sound from cylinders of different diameters was investigated to assess the effect of the ratio of cylinder diameter to approach flow length scale. It was found that as this ratio decreased, the broadband sound levels decreased as well. A simple theoretical model was then used to provide a prediction of the radiated sound for the cylinder within the shear layer. The theoretical model used statistics of the velocity field of the approach shear layer and the radiated sound spectrum the cylinder placed in a free stream. It was found that this method provided a similar spectral shape to the measured radiated sound.

Acoustic ground cloaks, which conceal an object on a rigid surface, utilize a linear coordinate transformation to map the flat surface to a triangular void by compressing space into two triangular cloaking regions consisting of a homogeneous anisotropic acoustic metamaterial. Transformation acoustics allows for the realization of a coordinate transformation through a reinterpretation of the scale factors as a new material in the original coordinate system. An underwater acoustic ground cloak was constructed from perforated steel plates and experimentally tested to conceal an object on a pressure release surface. The perforated plate acoustic ground cloak successfully cloaked the scattered object. There was excellent agreement between the phase of the surface reflection and cloak reflection with a small amplitude difference. Above 15 [kHz], the cloaking performance decreased as the effective material parameters of the perforated plate metamaterial deviated from the required material parameters.

The unique material properties now obtainable with acoustic metamaterials have led to unprecedented control of acoustic wave propagation, resulting in many applications including acoustic cloaking. The two fundamental approaches in the development of a ground cloak are quasiconformal mapping [Li et al. Phys. Rev. Lett. 101, 203901 (2008)] and coordinate transformations [Popa et al. Phys. Rev. B. 83, 224304 (2011)]. The differences in the required material properties prescribed by these two approaches lie in the amount of anisotropy and inhomogeneity, as well as the size of the cloak relative to object. The coordinate transformation approach has been used to produce a realizable anisotropic homogeneous ground cloak in the acoustic domain. This presentation will highlight the findings of work that examined how advances in metamaterial development could lead to the realization of required material properties for ground cloaks, and explore alternative transformations to expand the applications for acoustic ground cloaks.

Previous work in the open literature has described three potential ways to create an acoustic metamaterial with anisotropic mass density and isotropic bulk modulus: (1) alternating layers of homogeneous isotropic materials, (2) perforated plates, and (3) solid inclusions. The primary focus of this work will be to experimentally demonstrate the anisotropic behavior of a metamaterial comprised of a multi-solid inclusion unit cell in water. The two material design of the unit cell consists of one material more dense and one less dense than the background fluid, which results in an effective mass density tensor for the unit cell where one component is more dense and one component is less dense than the background fluid. Successful demonstration of an anisotropic metamaterial with these effective parameters is an important step in the development of structures based on transformational acoustics.

Player perception of the quality of a hand-held sports implement is strongly influenced by the sound generated at impact with a ball. A recent innovative golf club driver design increased the moment-of-inertia of the club, making it more forgiving to hits off center and allowing players to hit balls with greater accuracy. However, this club has not been readily accepted because players hate the loud and annoying sound it makes when striking a golf ball. In this paper, we will discuss the acoustic and vibrational properties of this club as measured through experimental modal analysis, laser Doppler vibrometry, and acoustic field tests. Vibration and acoustic data correlate well and reveal that the annoying high amplitude components in the impact sound are due to vibrational modes in the sole of the club head, at frequencies below that of the “trampoline mode” of the club face. Implications resulting from this study will influence future golf club designs to make them sound more pleasing without altering performance.