Comparison Failure and Successful Methodologies for Diffusion Measurements Undertaken inside Two Different Testing Rooms

The scattering phenomenon is known to be of great importance for the acoustic quality of a performance arts space. The scattering of sound can be achieved in different ways: it can be obtained by the presence of architectural and/or decorating elements inside a room (e.g., columns, statues), by the geometry and roughness of a surface (e.g., Quadratic Residue Diffuser (QRD)) and by the diffraction effect occurring when a sound wave hits the edges of an obstacle. This article deals with the surface scattering effects and the diffusion phenomenon only related to MDF and plywood panels tested by disposing the wells both horizontally and vertically. The test results undertaken inside a semi-reverberant room and inside a large reverberant room have been compared to highlight the success and the failure of the measuring methodologies. In detail, according to the existing standards and regulations (i.e., ISO 17497—Part 2), diffusion measurements have been undertaken on a few selected types of panel: two QRD panels (made of Medium Density Fiberboard (MDF) and plywood) with and without a smooth painted solid wood placed behind the QRD. The panels have been tested inside two rooms of different characteristics: a semi-anechoic chamber (Room A) and a large reverberant room (Room B). The volume size influenced the results that have been analyzed for both chambers, showing an overlap of reflections on panels tested inside Room A and a clear diffusion response for the panels tested inside Room B. In terms of the diffusion coefficient in all the octave bands between 125 Hz and 8 kHz, results should not be considered valid for panels tested in Room A because they were negatively impacted by extraneous reflections, while they are reliable for panels tested in Room B

Acoustic diffusion and scattering coefficients for room surfaces

This project concerns quantifying the diffuseness of sound reflections fromsurfaces by means of a diffusion coefficient. Afthough it is now acknowledged thatdiffuse reflections are important in determining sound fields within rooms, nostandardised diffusion coefficient currently exists. Definition of a universalcoefficient would permit comparison of different surfaces and aid theunderstanding of diffusion. It would also benefit diffuser designers and roomacoustic computer modellers. Previously proposed diffusion parameters for roomsurfaces are investigated and new ones developed.One approach is to parameterise the uniformity of the scattered energy measuredas a polar response; a number of such parameters have been previouslypublished. These are appraised using measured and predicted 2D and 3D polarresponses for a diverse range of sample surfaces. The situations in which theparameters succeed and fail are discussed and it is demonstrated that none isideal. A new polar response coefficient, superior to those previously published,is presented. This satisfies many criteria of the ideal diffusion measure and islikely to be standardised by the Audio Engineering Society. It is shown that theapplication of all polar response diffusion parameters is, however, limited.Two recently proposed alternative approaches to evaluating a scatteringcoefficient, which involve measuring the invariance of the energy reflected froma surface to its orientation, are discussed. One of these is a free field techniqueand the other requires reverberant conditions. Practical analysis shows that thereverberation chamber method is superior. It is likely to be standardised by ISO.An empirical reverberation chamber technique is also investigated, as is thepossibility of quantifying the diffusion efficacy of surfaces from their effect onsound field diffuseness. Both of these approaches require further research.It is concluded that to provide maAmum benefit, the choice of diffusion coefficientis application dependent

Improving phase grating and absorption grating diffusers

This thesis investigates room acoustic diffusers based on number sequences, exploring theirshortcomings and presents improvements.Standard Phase Grating Diffusers display frequencies where they act like flat plates and failto diffuse. To overcome this, two new sequences (Luke and power residue) are introduced.The diffusers based on these sequences display extended frequency range compared tostandard ones such as Quadratic Residue and Primitive Root Diffusers. Their performance isstudied using Boundary Element Modelling which shows that they can avoid flat platephenomena in the audible frequency range. Furthermore, it is shown that by taking advantageof their inner symmetries Quadratic Residue and Primitive Root Diffusers can be createdfrom smaller components thus allowing for the flat plat effect to be mitigated.Next, Absorption Grating Diffusers are investigated. They consist of ideally absorbing andreflecting elements. For their implementation heavily damped Helmholtz Resonators areinvestigated showing that they give an approximation of the required distribution ofadmittance on the surface.Then the performance of ideal Absorption Grating Diffusers is investigated using BoundaryElement Modelling. Even with idealised completely absorbing elements, the performance ofthe diffuser is shown not to achieve substantial diffusion. This arises because edge diffractionfrom the reflecting elements weakens at high frequencies. At frequencies where smallerelements are creating substantial scattering, larger elements are producing specularreflections. Furthermore, due to the lack of cancellation, the specular reflected lobe isinsufficiently attenuated, because it can only be changed through absorption.Improvements to the original design are suggested. By changing reflective elements toreactive ones, scattering can be extended to higher frequencies. This allows for a range offrequencies were more reflecting elements display substantial dispersion. Also, implementingthe absorbing elements using porous material in a shallow well allows some reflection,resulting in cancellation in the specular reflection lobe due to interference.Measurements of the scattered pressure distribution of absorption grating surfaces are carriedout and then compared to Boundary Element Modelling simulations using surface admittancedata measured in an impedance tube. The agreement between measurement and simulation is excellent proving the accuracy of this simulation method for these applications. The resultsshow that the samples tested perform as two level Phase Grating Diffusers, with some energyloss, while their diffusion characteristics are shifted to lower frequencies. This arises becauseof the lower speed of sound in the porous medium. This implementation is shown to absorb50% of the incident sound while the rest is scattered uniformly but only over a limitedbandwidth

Volume diffusers for architectural acoustics

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

Acoustic Properties of Absorbing Materials

Thanks to the progress made in materials research and to the introduction of innovative manufacturing technologies, a wide range of sound-absorbing elements are currently available to adjust the acoustic features of an environment. Nowadays, performance is only one of the required specifications, together with environmental compatibility, longevity, and affordable cost. This book collects the most recent advances in the broad-spectrum characterization of sound-absorbing materials used in civil, industrial, and tertiary applications, by means of experimental, numerical, or theoretical studies

Low-frequency Acoustic Noise Mitigation Characteristics of Metamaterials-inspired Vibro-impact Structures

Acoustic absorbers like foams, fiberglass or liners have been used commonly in structures for infrastructural, industrial, automotive and aerospace applications to mitigate noise. However, these conventional materials have limited effectiveness to mitigate low-frequency (LF) acoustic waves with frequency less than $\sim$400 Hz owing to the need for impractically large mass or volume. LF acoustic waves contribute significantly towards environmental noise pollution as well as unwanted structural responses. Therefore, there is a need to develop lightweight, compact, structurally-integrated solutions to mitigate LF noise in several applications. Inspired by metamaterials, which are manmade structural materials that derive their unique dynamic behavior not just from material constituents but more so from engineered configurations, tuned mass-loaded membranes as vibro-impact attachments on a baseline structure are investigated to determine their performance as a LF acoustic barrier. The hypothesis is that the LF incident waves are up-converted via impact to higher modes in the baseline structure which are far more evanescent and may then be effectively mitigated using conventional means. Such Metamaterials-Inspired Vibro-Impact Structures (MIVIS) could be tuned to match the dominant frequency content of LF acoustic sources in specific applications. Prototype MIVIS unit cells were designed and tested to study the energy transfer mechanism via impact-induced frequency up-conversion, and the consequent sound transmission loss. Structural acoustic simulations were done to predict responses using models based on normal incidence transmission loss tests. Experimental proof-of-concept was achieved and further correlations to simulations were utilized to optimize the energy up-conversion mechanism using parametric studies. Up to 36 dB of sound transmission loss increase is obtained at the anti-resonance frequency (326 Hz) within a tunable LF bandwidth of about 200 Hz while impact-induced up-conversion could enable further broadband transmission loss via subsequent dissipation in conventional absorbers. Moreover, this approach while minimizing parasitic mass addition retains or could conceivably augment primary functionalities of the baseline structure. Successful transition to applications could enable new mission capabilities for aerospace and military vehicles and help create quieter built environments.Mechanical & Aerospace Engineerin

STUDIO DEI DIFFUSORI DI SCHROEDER E MODELLAZIONE MEDIANTE DWM (DIGITAL WAVEGUIDE MESH)

Sono stati trattati due differenti argomenti: lo studio della diffusione delle onde acustiche per mezzo dei diffusori di Schroeder e la modellazione acustica tramite la Digital Waveguide Mesh (DWM). E’ stato prima affrontato lo studio delle sequenze numeriche a spettro costante e il modo in cui esse vengono generate. La progettazione dei diffusori è stata studiata, partendo da una formulazione semplificata in campo lontano e variando i parametri di progetto sono stati ricavati i diagrammi polari di diffusione. In particolare, mediante i diagrammi polari in campo lontano è stata considerata la variazione della diffusione al variare della classe di sequenza (QRD, PRD, MLS), della lunghezza N della sequenza, della frequenza f0 di progetto e del numero di periodi np della sequenza base di cui è composta l’intero diffusore. Sono stati descritti quelli che sono i metodi adottati per valutare la diffusione e la sua quantificazione mediante il coefficiente di diffusione. La parte centrale della tesi ha riguardato la realizzazione del modello di simulazione basato sulla DWM. Il modello 2D è stato realizzato adottando per la mesh una topologia rettangolare interpolata, che coniuga semplicità nella realizzazione e basso errore di dispersione in frequenza. I particolari dei diffusori di Schroeder sono stati rappresentati efficacemente con una tecnica di suddivisione del dominio in sottodomini a differente densità. Questo metodo rende più fitta la mesh solo là dove la geometria lo richiede e più larga nelle regioni di spazio vuote prive di bordi. Nella simulazione, la valutazione della diffusione è stata fatta secondo la tecnica standard prevista per le misure reali, così come descritta nel documento (AES 4 ed. 2001). In questo modo i risultati ottenuti possono essere confrontati con quelli fatti su diffusori reali. Tutti i programmi di simulazione sono stati sviluppati in ambiente LabVIEW