On the analysis of the time spreading of sound diffusers

ABSTRACT – This research addresses the usefulness of an absolute descriptor to quantify the degree of diffusion in a third octave band basis of a sound field. The degree of sound field diffuseness in one point is related with the reflection's energy control multiplied by the temporal distribution uniformity of reflections. All this information is extracted from a monaural, broadband, omnidirectional, high S/N impulse response. The coefficient range varies between 0 and 1, evaluates the early, late and total sound field, for frequencies above Schroeder's and in the far field from diffusive surfaces, zero being " no diffuseness " at all. This coefficient allows the comparison of different rooms, different places inside rooms, measurement of the effects of different sound diffusers coatings and the resulting spatial uniformity variation, among other applications.

Black sea journal of engineering and science, 2024

This study investigates the aeroacoustic behaviors of a square truncated perforated diffuser under airflow, commonly used in Air Handling Units (AHUs). The design parameters are fundamentally taken into account to unveil the aeroacoustic performance of the diffuser. Initially, unsteady-state Computational Fluid Dynamics (CFD) simulations are conducted based on models that accurately represent the fluid domain of the chamber with the perforated diffuser in the ANSYS Fluent environment. Subsequently, the Ffowcs Williams and Hawkings (FW-H) method integrated into the software is employed to acquire time-dependent signals from microphones placed in three different locations within a perforated diffuser chamber. Finally, the results are converted to a frequency range of 0-1000 Hz using the Fast Fourier Transform (FFT) method, and the SPL values are obtained. The results show that the microphone location is crucially important to determine SPL and the porosity reduction from 0.55 to 0.35 can reduce SPL by approximately 30-40 dB. Variations in wall thickness of the diffuser fluctuated between 5-10 dB at each frequency value.

Acoustical Science and Technology, 2006

MATEC Web of Conferences, 2020

This article compares the values of the normal scattering coefficient measured in a model experiment for two types of diffusers placed on a rigid surface. Wooden diffusers of cubic and pyramidal shapes were tested in a scale model of a room with dimensions of 0.7x0.4x0.4 m. Sound decay curves were measured at frequencies of 4kHz and 8kHz. Two large walls were covered with a porous absorber, on the third, in certain combinations, the investigated diffusers with a characteristic size of 3.5 cm were placed, the number of which varied from 0 to 29. The idea of the applied method is that the sound decay curve in a room with a non-diffuse sound field depends significantly from the scattering properties of surfaces. The decay curve was measured with different numbers of the diffusers on the test wall, which made it possible to determine the influence of the shape of the diffusers and their number on the value of the normal scattering coefficient. According to the results of the measurement...

Most conventional diffusers are used on room surfaces, and consequently can only operate on a hemispherical area. Placing a diffuser in the volume of a room may provide greater efficiency by allowing scattering into the whole space. There are very few examples of volume diffusers and they tend to be limited in design; subsequently a suitable method for their development is lacking. 2D volumetric diffusers are investigated, considering a number of design concepts; namely arrays of slats, percolation structures and cylinder arrays. An experimental technique is adapted for their measurement, and the results are used to verify prediction models for each type. Diffusive efficacy is assessed through a new metric based on an existing surface diffuser coefficient and a measure of scattered power requiring half of the energy to be back-scattered. Single layer slat arrays are formed from optimal aperiodic sequences, though due to the directional scattering from individual slats at higher frequencies, performance is heavily dependent on line-of-sight through the array. This limits the operational bandwidth to approximately 1.5 octaves. Multi-layer structures offer improvements by allowing cancellation of the back-scattered lobe, though at high frequency the specular reflection from an individual slat still dominates. Percolation fractals use slats orientated in multiple directions and by scattering laterally can channel sound and diffuse at lower frequencies. Low frequency diffusion however is limited and the best structures are those which provide a broad range of geometric reflection paths. Through application of number theoretic concepts, arrangements of cylinders are shown to offer more enhanced diffusing abilities than slat and percolation structures. At low frequency scattered power is controlled by cylinder size and at high frequency diffusion is dominated by their spacing. By minimising structural similarity and including cylinders with circumference comparable to wavelength, significant diffusion is achieved over an approximate 5 octave bandwidth.

Boundary Element Methods (BEMs) may be used to model scattering of sound by surfaces such as diffusers, accelerating prototyping of new Room Acoustics treatments. Unlike the more widely used frequency domain method, the time domain BEM is usually solved in an iterative manner so can exhibit instability, a crucial impediment to its widespread use. These instabilities are primarily associated with resonance of the cavities formed by closed surface sections, but may also be caused by discretisation or integration error corrupting physically relevant damped resonances. Previous works on time domain BEMs have focused on idealised surfaces, such as spheres and plates, and little is published on their performance for the more complex surfaces treatments typical to Room Acoustics. Consequentially, this paper will apply the method to model the transient behaviour of Schroeder Diffusers and Binary Amplitude Diffusers. Cavity resonances and integration error are addressed through use of the Combined Field Integral Equation and an adaptive contour integration scheme respectively. Thin and absorbing surface sections are implemented as required by the respective diffusers. Accuracy and stability is tested by comparison to verified frequency domain BEMs.

In the present work the time evolution of the travelling sound waves inside a wind instrument is studied by means of a Finite-Difference Time Domain (FDTD) method. The adopted model for the instrument consists of a resonator with axial symmetry (body of the instrument) excited by periodic air pulses. In order to simulate the excitation mechanism transparent sources should be used at the input of the resonator. The considered calculation technique reduces the spurious oscillations due to the use of staggered grids. This technique has been successfully applied in the last decades in several fields within Acoustics, such as Room Acoustics (1) or Environmental Acoustics (2). The numerical simulation of the instrument makes possible to analyze the influence of the internal geometry (orifice, termination shape, and so on) on the sound propagation and the time evolution of the oscillation regime.

Journal of the Acoustical Society of America, 1991

New algorithms are described that provide insight into linear field propagation and offer significant reductions in computational complexity. The developments presented here include the usage of a recently developed discrete Hankel transform to implement two single step, planar propagation algorithms for baffled, radially symmetric, acoustic pressure or velocity fields; an update on the single step approaches that reduce computational complexity through geometrically determined spatial frequency limitations; and algorithms for extending to multistep propagation. Two equivalent means of introducing arbitrary medium attenuation into the above schemes are presented. Finally, a planar boundary crossing algorithm that accounts for refraction and reflection (but not multiple reflections) is added to one of the multistep propagating algorithms. The resulting algorithm is then used to examine the differences between the corresponding fields of a focused piston source operating in water and in a layered fat/liver (biomedical imaging) medium. The results yield computationally efficient algorithms that can be used for linear propagation of focused or unfocused beams in attenuating, multilayer media, and also provide the basis for a novel nonlinear propagation algorithm.

2012

The sound scattering properties of diffusing panels are actually very difficult to measure and to predict. Furthermore, an unique, commonly accepted "diffusion coefficient" has not been defined yet. In this paper the Wave Field Synthesis approach is employed for the experimenta measurement of a physically defined diffusion coefficient. The results are in good agreemen with the numerical simulations presented in a parallel paper. 1. Precis Although many researches were conducted in the last few years, a consistent definition of a single number (the "diffusion coefficient") for stating the effectiveness of diffusing panels has not been yet obtained. In this paper, moving from the Wave Field Synthesis approach developed at the Tech. University of Delft, it is demonstrated how it is possible t characterize completely the scattering properties of a generic object by a large number of impulse response measurements taken moving the microphone at small steps along a straight line. From this characterization, by comparison with the theoretical behaviour of an absorbing/diffusing object, and after separation of the edge diffraction effect, it is possible to define objectively a surface diffusion coefficient. This new quantity is easy to measure, and ranks correctly diffusing panels of various kinds. Furthermore, this new approach is consistent with the employment of the measured diffusion coefficient in room acoustics simulation programs based on the geometrica acoustics theory, as reported in a parallel paper. The work starts with the theoretica explanation of the method, and prosecutes with experimental measurements on different panels, with validation of the results by comparison with numerical simulations.