Sound diffusers based on sonic crystals

Applied Mechanics and Materials, 2012

Sonic crystals are periodic structures made of sound hard scatterers which attenuate sound in a range of frequencies. For an infinite periodic structure, this range of frequencies is known as band gap, and is determined by the geometric arrangement of the scatterers. In this paper, a parametric study on rectangular sonic crystal is presented. It is found that geometric spacing between the scatterers in the direction of sound propagation affects the center frequency of the band gap. Reducing the geometric spacing between the scatterers in the direction perpendicular to the sound propagation helps in better sound attenuation. Such rectangular arrangement of scatterers gives better sound attenuation than the regular square arrangement of scatterers. The model for parametric study is also supported by some experimental results.

2006

We predict theoretically the nondiffractive propagation of sonic waves in periodic acoustic media (sonic crystals), by expansion into a set of plane waves (Bloch mode expansion), and by finite difference time domain calculations of finite beams. We also give analytical evaluations of the parameters for nondiffractive propagation, as well as the minimum size of the nondiffractively propagating acoustic beams.

The Journal of the Acoustical Society of America, 2006

We report the nondiffractive propagation of acoustic waved in sonic crystals, e.g., acoustic media with periodic modulation of the material parameters (density and bulk modulus). Such novel materials have recently attracted a great interest, because of their potential applications in the control of sound propagation, used as reflectors, focusers or waveguides. All these properties are related with the dispersion introduced by the crystal anisotropy. In particular we consider the case of two-dimensional sonic crystals, consisting, e.g. in an array of steel cylinders in water. It is shown that, for given frequencies and directions of incidence, a narrow sonic beam can propagate without diffractive broadening. Such nondiffractive sonic beams exist in crystals with perfect symmetry, and do not require the presence of defects, differently from other waveguiding phenomena reported previously. The cancellation of diffraction occurs at frequencies and wavevectors for which dispersion curves (isofrequency lines) have zero curvature, i.e, are locally flat. By means of perturbative techniques, a simple analytical expression for the nondiffractive conditions has been obtained. The phenomenon is also demonstrated by numerical integration of the acoustic equations using the FDTD technique. We present experimental evidence of the nondiffractive propagation in a periodic array of steel cylinders in water.

An acoustic metamaterial made of a two-dimensional (2D) periodic array of multi-resonant acoustic scatterers is analyzed both experimentally and theoretically. The building blocks consist of a combination of elastic beams of low-density polyethylene foam (LDPF) with cavities of known area. Elastic resonances of the beams and acoustic resonances of the cavities can be excited by sound producing several attenuation peaks in the low frequency range. Due to this behavior the periodic array with long wavelength multi-resonant structural units can be classified as a locally multi-resonant acoustic metamaterial (LMRAM) with strong dispersion of its effective properties.The results presented in this paper could be used to design effective tunable acoustic filters for the low frequency range.

Physical Review B, 2007

We present an experimental demonstration of the subdiffractive propagation ͑self-collimation͒ of an ultrasound beam in a two-dimensional sonic crystal formed by a square array of steel cylinders inmersed in water. Measurements show that the diffractive spreading of a narrow beam is strongly reduced along the spatially modulated direction of the crystal. The effect of the finite crystal length is theoretically analyzed, resulting in a frequency shift of the subdiffractive point in good correspondence with the experimental results.

The Journal of the …, 2010

Most conventional diffusers take the form of a surface based treatment, and as a result can only operate in hemispherical space. Placing a diffuser in the volume of a room might provide greater efficiency by allowing scattering into the whole space. A periodic cylinder array (or sonic crystal) produces periodicity lobes and uneven scattering. Introducing defects into an array, by removing or varying the size of some of the cylinders, can enhance their diffusing abilities. This paper applies number theoretic concepts to create cylinder arrays that have more even scattering. Predictions using a boundary element method are compared to measurements to verify the model, and suitable metrics are adopted to evaluate performance. Arrangements with good aperiodic autocorrelation properties tend to produce the best results. At low frequency power is controlled by object size and at high frequency diffusion is dominated by lattice spacing and structural similarity. Consequently the operational bandwidth is rather small. By using sparse arrays and varying cylinder sizes, a wider bandwidth can be achieved.

Journal of Applied Physics, 2012

A theoretical and experimental study of the propagation of sound beams in-and behind three-dimensional sonic crystals at frequencies close to the band edges is presented. An efficient collimation of the beam behind the crystal is predicted and experimentally demonstrated. This effect could allow the design of sources of high spatial quality sound beams.

RIASSUNTO Il presente contributo riporta un'introduzione al tema della propagazione del suono nei cristalli sonici e un excursus sulla letteratura scientifica più recente. Si discutono i risultati di alcune indagini sperimentali condotte presso l'Università di Bologna inerenti misure di Insertion Loss, misure effettuate all'interno del reticolo e misure di intensimetria. Infine i valori di Sound Insulation misurati per un cristallo sonico sono confrontati con valori misurati su barriere tradizionali, evidenziando come il cristallo sonico permetta di raggiungere un isolamento confrontabile con il valore soglia di Insertion Loss raggiungibile a causa della diffrazione del bordo superiore della barriera. ABSTRACT This work reports an introduction to the topic of wave propagation in sonic crystals and a review of the recent scientific literature. The paper presents the results of some experimental investigations carried out at the University of Bologna by discussing Insertion Loss measurements, measurements performed inside the lattice and sound intensity measurements. Finally, the Sound Insulation Index measured for a sonic crystal is compared to the values measured for common noise barriers, pointing out that sonic crystals reach insulation values comparable to the maximum Insertion Loss achievable due to the top edge diffraction. Parole chiave: Cristalli sonici, mezzi periodici, isolamento acustico, scattering di Bragg.

The Journal of the Acoustical Society of America, 2008

Since the invention of sound diffusers three decades ago a substantial effort has been made to predict the acoustic behaviour of these structures. BEM methods are well established for this purpose after a systematic comparison between simulations and experimental data. Volumetric methods such as Finite Element Methods (FEM) or the Finite Difference Time Domain method (FDTD) are not often used, due to their large computational cost. However, Near to Far Field Transformations (NFFT) can overcome that problem. Recently some of the authors have shown that the FDTD method is a useful technique to analyse the time domain signature of sound diffusers. In this paper a careful analysis of the performance of diffusers in the time domain ('time spreading') are reported, opening a new field of research.