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Henry Yakushev
Henry Yakushev

Acoustic Absorbers And Diffusers Theory Design ...

Thoroughly describes the various mechanisms of sound absorption and diffusion. Written by the founder of RPG Diffuser Systems, yet holds nothing back. Very useful for designing your own acoustical treatment.

Acoustic Absorbers and Diffusers Theory Design ...

The literature shows that the models continue in the field of engineering. However, there is not enough information about the transition of these models to architectural applications and their performance. From an acoustic point of view, achieving a wideband absorption, designing for a maximum functional area, and preserving the flexibility of the interiors may not be possible while using permanently mounted thick absorbers such as conventional porous and fibrous materials. An alternate remedy is eliminating the destructive reflections by employing selective absorption or absorption by design [1]. Beyond designing for appropriate reverberation time and speech intelligibility in classrooms, architects ought to make sensible choices regarding the finishing materials. They need to avoid health-harming, toxic components and embrace materials promoting physical, mental, and environmental health.

This pilot work showed preliminary insights into how the parameters could be tuned to handle specific target frequencies. Still, the decision on the 4 n parameters of the proposed structures was based on trial and error procedures, which did not provide the best absorption curve possible. For further studies, it would be beneficial to rely on optimization algorithms, as recent studies have benefited from [59,60] by strongly considering the constraints of existing building and finishing materials. Another interesting trail to follow in further work from the design perspective might be surface design in terms of visual patterns. This approach would behave as a trade-off between visual aesthetics and functional acoustics, which studies have attempted to achieve lately [61,62,63,64].

Abstract:A micro-perforated plate or panel (MPP) is a device used to absorb sound. It consists of a thin flat plate made from several different materials with small holes and a back cavity. Several reported modifications and enhancements to the original design of the MPP acoustic absorber were modified by the holes or the back-cavity shape and sizes following the original idea. The present study attempts to artistically beautify the MPP acoustic absorbers by incorporating dotted arts into the design of MPP. The perforation for micro-perforated panels could be dotted arts with a perforation size smaller than 1 mm for enhanced acoustic absorption performance in the form of various artistic designs. Small LED lights could be placed inside the acoustic chamber to create the color lights emanating from the perforations instead of dots with different colors. Several MPP incorporated artistic designs of dotted patterns were presented and their acoustic absorption performance was analyzed using impedance tube in this paper.Keywords: acoustic absorbers; dotted arts; micro-perforation plates; pointillism

There are many applications in physics and electrical engineering for objects and surfaces that disperse waves. To take a few examples, such scatterers can be applied to sonar and radar camouflage, electromagnetic reverberation chambers and reducing unwanted ultrasound reflections from surgical equipment. To study how metamaterials might create scattering, this study has focussed on sound diffusers applied in room acoustics. This allows the work to build on a large body of knowledge concerning how such surfaces are measured, predicted and designed. Common wall treatments are made of flat panels, leading to specular sound reflections. In critical environments such as auditoria, professional broadcast and recording control rooms, recording studios or conference rooms, such reflections can decrease sound quality due to echoes or cause sound coloration1. Even when these specular reflections are damped by absorption, the sound field inside a room may be non-diffuse, affecting the quality of the listening. In these situations, diffusers can often help by evenly spreading the acoustic energy in both space and time. Specialist diffusers are panels whose scattering function is uniform, so the reflected waves are dispersed in many different directions.

The maximum phase shift of the reflection coefficient achieved by a single well in a phase grating diffuser occurs at its quarter wavelength resonance, i.e., \(L=c_0/4f\) where f is the frequency, L is the depth of the well and c 0 is the speed of sound in air. Therefore, a limitation of Schroeder diffusers is that the depth becomes large for low design frequencies. This results in thick and heavy panels, limiting the use of phase grating diffusers for low-frequencies where the wavelength of sound in air is of the order of several meters. In the context of smart building design and sustainable building, leading-edge technologies can be applied to optimize space and design lightweight materials, improving the performance of the acoustic solutions using less resources.

Local resonances have also been exploited to introduce strong dispersion in acoustic metamaterials15. In these structures the phase speed can be strongly modified and materials with exotic properties as either negative effective bulk modulus or negative mass density16, 17 can be designed. Metamaterials have been widely used to design acoustic absorbers as metaporous materials18,19,20,21, dead-end porosity materials22, 23 or absorbing resonant metamaterials composed by membrane-type resonators17, 24,25,26, quarter-wavelength resonators (QWRs)23, 27,28,29 and Helmholtz resonators (HRs)26, 30,31,32. These last types of metamaterials23, 27, 28, 31, 32 make use of strong dispersion giving rise to slow-sound propagation inside the material. Using slow sound results in a decrease of the cavity resonance frequency and, hence, the structure thickness can be drastically reduced to the deep-subwavelength regime31. Moreover, these structures can fulfil the critical coupling conditions26, having their impedance matched with the exterior medium and resulting in perfect absorption (PA), as recently demonstrated for panels using slow sound and QWRs28 or HRs31.

In this paper, we present deep-subwavelength diffusers based on acoustic metamaterials to reduce the thickness of Schroeder diffusers. The system works as follows: first, we consider a rigid panel of finite length with a set of N slits. Second, we modify the dispersion relations inside each slit by loading one of their walls with a set of HRs, as shown in Fig. 1(b). The sound propagation in each slit becomes strongly dispersive and the sound speed inside it, c p , can be drastically reduced. Each slit behaves effectively as a deep-subwavelength resonator and, therefore, the effective depth of the slits can be strongly reduced as \(L=c_p/4f\) holds. By tuning the geometry of the HRs and the thickness of the slits, the dispersion relations inside each slit can be modified. As a result the phase of the reflection coefficient can be tailored, e.g., to those of an Schroeder phase grating diffuser. Furthermore, by tuning the thermo-viscous losses, which are inherent in the HRs and in the narrow slits, the leakage of the structure can be compensated by the intrinsic losses of the system and PA can be obtained. Thus, the magnitude of the reflection coefficient can be also tuned, and the behaviour of the slits ranges from perfect reflectors to perfect absorbers. Perfect absorbing slits allows the construct of ternary sequence diffusers33 for low frequencies.

While the focus of the study has been sound diffusers for rooms, dispersed, broadband reflections are of interest beyond architectural acoustics. Example of structures creating diffuse reflections are found in nature, for example Cyphochilus and Lepidiota stigma beetles have chitin networks that achieve an exceptionally bright white colour from all observation angles36. A second example would be the use of acoustic camouflage by insects to avoid predation by bats. The latest research suggests that insects look for rough surfaces, ones that create dispersion, to reduce the chances of being detected via echolocation37. We would anticipate applications for deliberately designed dispersive surfaces: in underwater acoustics; in airborne acoustics and for other wave types (e.g. light, seismic waves). As in nature, applications might involve signalling, reducing interference from unwanted reflections and acoustic camouflage.

The Institute for Contemporary Arts is a non-collecting contemporary art institution designed by Steven Holl Architects and located on the Virginia Commonwealth University campus in Richmond.[10] The design by Steven Holl Architects emphasizes the fluidity of interior and exterior space and fosters the connection between technology and natural resources. The design, however, creates reverberant sounds that disturb the experience within the museum.[10] Acoustic plaster was used as a remedy to address the sound environment without compromising design. The application of acoustic plaster significantly reduced the sound reverberation, especially in the 33-foot tall central forum, where or echoing would otherwise occur due to the high ceiling.[10]

The LEDE philosophy is easier to implement at home than a non-environment room because less treatment is needed. Also, the absorbent controlling the earlier reflections can be shallower than is used in a non-environment, although you would also need membrane absorbers elsewhere to control the bass (low-frequency control will be discussed in detail later). However, there are ways of controlling early reflections with less absorption to get a much bigger sweet spot. One disadvantage of LEDE rooms is that the philosophy is not easily extended to deal with surround sound reproduction. A different design is needed, like those outlined next.

On any particular surface, there is a choice about whether to apply absorbers to remove a reflection or a diffuser to disperse the sound. This choice is partly down to the desired amount of reverberance in the room, which sets the total amount of absorption that can be used around the space, and so might well limit how many panels can be absorbent. To minimise the amount of absorption needed, one should make the listening area as small as possible because larger reflection-free volumes require larger absorption patches. Moving between absorption and diffusion on a particular surface also affects the size of sonic images. If absorption is used to remove early reflections, especially near the loudspeakers, then the sonic images in the sound stage will be small, as if sound comes from a point in space. If diffusers are used, the sonic images may be broader, more like a typical home listening environment. 041b061a72


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