Diffuse Field Conditions Research Articles

Eco-Friendly and Biocompatible Material to Reduce Noise Pollution and Improve Acoustic Comfort in Healthcare Environments

Noise pollution negatively impacts people’s mental and physiological health. Unfortunately, not only is noise present in hospital environments, but its level frequently exceeds recommended thresholds. The efficacy of passive acoustic absorbers in reducing indoor noise in these scenarios has been well-documented. Conversely, given their inorganic composition and their origin in the petrochemical industry, most of these materials present a risk to human health. Over the last few years, there has been a notable increase in research on eco-friendly, low-toxicity, and biocompatible materials. This work outlines a methodology for fabricating recycled acoustic panels from plastic bottles and PET felt composites. This study encompasses three key objectives: (i) a comprehensive biocompatibility assessment of the panels, (ii) an evaluation of their thermal and acoustic properties, and (iii) their applicability in several case studies to evaluate potential acoustic enhancements. Specifically, antifungal resistance tests, Volatile Organic Compound (VOC) emission assessment, and cell viability experiments were conducted successfully. Additionally, experimental procedures were performed to determine the thermal conductivity and thermal resistance of the proposed material, along with its sound absorption coefficients in diffuse field conditions. Finally, the potential benefits of using this biomaterial in healthcare environments to reduce noise and improve acoustic comfort were demonstrated.

Acoustic metaslit for regional sound insulation for a three-dimensional diffuse sound field incidence

An acoustic metaslit model designed for regional sound insulation, employing waveguide modes and the directivity of openings, is introduced. This model characterizes the frequency responses of numerical simulations. The performance of metaslits, made from steel plates, is experimentally validated under diffuse sound field conditions and assessed by measuring acoustic power differences using intensity probes. The frequency response of single and multiple-unit metaslits shows peaks at multiples of the target frequencies. Additionally, experimental results demonstrate that the three-dimensional structure does not influence the two-dimensional regional sound insulation phenomenon. Furthermore, comparisons between experimental and numerical results indicate that obstacles near the outlets of metaslits can impact their performance.

Sound transmission loss of periodic Mindlin plates with non-uniformly spaced mass attachmentsa).

The sound transmission loss (STL) of wall partitions, especially in the coincidence region, is investigated. A Mindlin plate with periodically attached masses in a periodic "supercell" pattern is analyzed theoretically and experimentally for sound attenuation. Modeling the masses as points, analytical expressions for predicting the dispersion relation and frequency bandgaps of the plate are developed. The results show that varying the distances between the masses or the masses themselves can lead to the emergence of additional lower-frequency bandgaps and slightly decrease the bandwidth of the primary complete bandgap. Additionally, a triangular periodic pattern of point masses can provide a larger complete bandgap than the conventional rectangular pattern. The results are validated by numerical analyses using the wave and finite element method. Experimental testing is conducted on large-scale plates (2.4 m × 1 m) with periodically attached masses under diffuse field conditions, demonstrating the benefits of utilizing multiple scattering to increase the STL in the coincidence region of the bare plate. The proposed approach is seen to significantly increase the STL of wall partitions in the coincidence region and provides insights into the fundamental principles of sound and vibration attenuation in complex structures based on multiple scattering.

Dispersion relation and sound transmission loss of a Mindlin plate with an array of attached masses

The paper concerns increasing the sound transmission loss of wall partitions in the coincidence region by the addition of an array of point masses. Both theoretical and experimental analysis of sound and vibration attenuation in a Mindlin plate with masses attached in a periodic "supercell" pattern are performed. Distances between the neighbouring masses within the supercell are not restricted to be equal, and masses themselves can be non-identical. By modelling the masses as point attachments, analytical expressions are developed for predicting the dispersion relation and frequency bandgaps of the plate. It is shown that wave propagation in two primary (orthogonal) directions in the plate and the corresponding frequency bandgaps have the most pronounced effect on the sound transmission loss. Variation of the distances between the masses, or masses themselves, can lead to additional lower-frequency bandgaps. The effects of the frequency bandgaps on the sound transmission through the plate are validated by numerical analysis conducted using the wave and finite element method. Experimental testing in a transmission suite under diffuse field conditions of large-scale (2.4m by 1m) plates with periodically attached masses is conducted, showing benefits of utilizing multiple scattering to increase the sound transmission loss in the targeted region.

Improving sound insulation near ring and coincidence frequencies of cylindrical sandwich shells

This paper proposes an impedance-based design methodology for cylindrical sandwich shells, with the aim to improve the sound transmission loss properties near the ring and coincidence frequency regions. The approach enables to systematically address the poor acoustic performance, characteristic of these problematic frequency regions, while retaining the mechanical properties of these structures. This is done by seeking to suppress the mass-controlled region in the frequency spectrum, with properly tuned characteristic frequencies of the structure, completed by a degree of damping treatment. The impedance-based approach allowing this tuning is derived from the canonical wave equation with a view to sound transmission through cylindrical shells. In addition to offering fast, early design possibilities, the method provides physical insights into the vibroacoustic performance of the shell, for instance introduced to estimate the sound transmission loss of shallow curved sandwich panels in the low-frequency range. Oblique- and diffuse-field conditions are investigated, validating the analytical developments against finite element calculations.

Direct application of the diffusers’ reflection patterns in geometrical acoustics simulations

Using the combination of specular and Lambert reflections is a common technique for modelling reflection in room acoustics simulations. Diffusing surfaces are simulated indirectly using scattering coefficients, describing the ratio between a specular and Lambert part. Such technique has two main flaws. The ideally specular reflection cannot simulate the reflection redirection, where a strong reflection appears on direction other than mirror. Simultaneously, the Lambert model is accurate for scattering simulation only in diffuse field conditions. Otherwise, the actual scattered distributions may differ significantly from Lambert model. This paper focuses on the direct usage of diffuser reflection patterns in the geometrical acoustics simulations. A general approach for the application of any reflection pattern in computer simulations is described and the procedure for creating reflection models based on the surface geometry is presented. The results obtained using the combination of specular and Lambert reflections are compared with the ones obtained using the proposed approach. We also analyse the impact of the chosen diffuser and modelling technique on the sound pressure level distribution and common acoustic parameters within the simulation. The results prove that the proposed method can reproduce reflections with significantly higher accuracy compared to use of specular and Lambert model combination. It is also shown that using more exact reflection model significantly affects the early stage of acoustic field formation in a room.

Empirical study on the correlation between measurement methods under diffuse and direct sound field conditions for determining sound absorption and airborne sound insulation properties of noise barriers

In the frame of the SOPRANOISE project (funded by CEDR in the Transnational Road Research Programme 2018) the database of the European noise barrier market developed during the QUIESST project was updated with newly acquired data. This database gives the opportunity for an empirical study on the correlation between the different measurement methods for the acoustic properties of noise barriers (according to the EN 1793 series) to further investigate the interrelationships between these methods by using single-number ratings and third-octave band data. First a correlation of the measurement methods for sound absorption under diffuse field conditions (EN 1793-1) and sound reflection under direct sound field conditions (EN 1793-5) is presented. Secondly, a correlation of the measurement methods for airborne sound insulation under diffuse field conditions (EN 1793-2) and airborne sound insulation under direct sound field conditions (EN 1793-6) is shown. While for airborne sound insulation a distinct correlation is found due to the wide data range, for sound absorption no robust correlation can be found.

SOPRANOISE - update and analysis of noise barrier database including new current results

In the frame of the SOPRANOISE project (funded by CEDR in the Transnational Road Research Programme 2018) work package 2 focused first on providing theoretical and practical background information on measurement of the acoustic performance of noise barriers due to a state-of-the-art regarding correlations and possible trends between diffuse (EN 1793-1, EN 1793-2) and direct sound field methods (EN 1793-5, EN 1793-6). After that, the objective of this research was to extend and update the database of the European noise barrier market developed during the QUIESST project, including more detailed analyses on single-number ratings as well as third-octave band measurement results. The data collected and the analysis performed show relevant facts and figures about acoustic performances of noise barriers measured under diffuse and direct sound field conditions, together with a better understanding of the respective significance, similarities and differences of these standardized methods, improving data analysis and correlations between these methods. This paper gives a general overview of the data collected, summarising the main results of the statistical analyses performed. Overall results and comparisons between results of measurements performed under diffuse and under direct sound field conditions are shown. Finally, conclusions and possible outlook of the research are presented.

Proposal of numerical models to predict the diffuse field sound absorption of finite sized porous materials – BEM and FEM approaches

The prediction of the sound absorption coefficient in diffuse field conditions is of great interest to an acoustic project. Foams and granular materials like porous concrete have been used in many internal or external applications as sound absorbers. However, many doubts have regularly been discussed in scientific literature. The simple use of experimental impedance tube methods, that consider only the incident acoustic plane waves, is not equivalent to the experimental measurement in a reverberant room, which is necessary for a correct characterization of a given constructive solution. The scientific community has explored several methods to predict and approximate the sound absorption coefficient in diffuse field conditions, using only acoustic plane wave data. However, many of them do not allow a realistic approximation of the reverberant room measured data. Therefore, two numerical models are herein proposed which consider the porous media on the fluid-equivalent theory, first, a two-dimensional (2D) finite element method, and, second, a three-dimensional (3D) boundary element method (BEM). These methods are compared with analytical equations and experimental data from the reverberant room, studying the influence of the porous material panel size in the sound absorption coefficient in diffuse field conditions. To illustrate the proposed method’s practical interest, the BEM 3D model was used to determine the diffuse field’s sound absorption coefficient for finite-sized panels of porous concrete. This test was done for plane panels and for slit-like surfaces to highlight the possibilities of the proposed method.

Mean local frequency-wavenumber estimation through synthetic time-reversal of diffuse Lamb waves

Inspection of the elastic properties of plate-like structures is an important and challenging problem in many engineering disciplines. There exist multiple non-destructive inspection techniques based on active ultrasonic transducers to tackle this problem. In recent years, the usefulness of passively induced waves has also been explored. It has been shown that the diffuse waves, often referred to as coda waves, carry profound information about the elastic properties of the supporting medium. They can effectively probe the volume of a medium due to multiple scattering and long propagation paths. Moreover, the measured diffuse field can be synthetically focused using the correlation of wavefield’s time histories. The local elastic properties of the medium can be inferred from the focal spot size.In this work, we demonstrate the algorithm of mean local frequency-wavenumber estimation through synthetic focusing of diffuse Lamb waves. We provide theoretical foundations for expanding the concept of mean frequency estimation in purely temporal signals to mean wavenumber estimation in a spatiotemporal Lamb wave field. The applicability of this approach for the damage detection in plate-like structures is demonstrated on aluminum and carbon fiber reinforced polymer (CFRP) plates with localized defects. Test samples are excited with sequences of white noise signals, to create optimal diffuse field conditions, and wavefield is captured using a Scanning Laser Doppler Vibrometer (SLDV). We show that the spatial maps of mean wavenumber allow for precise localization and sizing of damage.

Multiplying resonances for attenuation in mechanical metamaterials: Part 1 – Concepts, initial validation and single layer structures

Worldwide increases in noise levels due to growth in urban population, traffic and machinery have serious implications for health, productivity and quality of life. Prevention of sound transmission through walls and ceilings, particularly towards the lower frequency range of human hearing is important both because of its recent increase in levels and the challenges faced when providing sound insulation of long wavelength noise with existing construction methods. Mechanical metamaterials have the potential to address these challenges by enabling the creation of an artificial medium that generates far more attenuation of wave transmission than any existing material system. In part one of this work we design, model and test several local resonance structure (LRS) systems to provide the basis for future acoustic insulation systems. Three different LRS families were studied: spherically symmetric, flexural single-resonance and flexural multi-resonance. Some resonator elements showed a peak effective mass up to fifty times greater than their rest mass and achieved peak transmission losses 10s of dB greater than a non-resonant structure of equivalent surface density within the designated frequency range. By arranging sets of resonators with closely spaced resonance frequencies the transmission loss gains were spread over a wider frequency range and a reduction in the transmittance peak at the upper end of the band gap was achieved though variations in damping and mass of the resonators. A large (2.5m2) test article was constructed and tested under full scale diffuse field conditions such as are found in buildings. The results confirmed that the band gap observed in impedance tube measurements of small-scale LRS specimens survives. Modelling and testing results for more multilayer and multi-resonance systems are presented in part 2.

Towards more reliable measurements of sound absorption coefficient in reverberation rooms: An Inter-Laboratory Test

The internationally recognized procedure ISO 354:2003 for measuring sound absorption coefficients under diffuse field conditions is now under revision. The main reason for this revision is the limited reproducibility of absorption coefficients measured in different laboratories that may have significant implications spanning from room acoustic design to material selection. A network of Italian laboratories have come together to carry out an Inter-Laboratory Test (ILT) to assess and compare the measurement uncertainties resulting from the application of the current version of ISO 354:2003 and of the new ISO/CD 354:2019. After detailing the methodological aspects, the paper presents the results of the measurements, discussing the compliance of the laboratories to the standard requirements and new qualification tests, and, more importantly, providing a quantitative estimation of their effects on measurement uncertainty and accuracy.

Impacts of Casting Scales and Harsh Conditions on the Thermal, Acoustic, and Mechanical Properties of Indoor Acoustic Panels Made with Fiber-Reinforced Alkali-Activated Slag Foam Concretes.

This paper presents experimental results regarding the efficiency of using acoustic panels made with fiber-reinforced alkali-activated slag foam concrete containing lightweight recycled aggregates produced by using Petrit-T (tunnel kiln slag). In the first stage, 72 acoustic panels with dimension 500 × 500 × 35 mm were cast and prepared. The mechanical properties of the panels were then assessed in terms of their compressive and flexural strengths. Moreover, the durability properties of acoustic panels were studied using harsh conditions (freeze/thaw and carbonation tests). The efficiency of the lightweight panels was also assessed in terms of thermal properties. In the second stage, 50 acoustic panels were used to cover the floor area in a reverberation room. The acoustic absorption in diffuse field conditions was measured, and the interrupted random noise source method was used to record the sound pressure decay rate over time. Moreover, the acoustic properties of the panels were separately assessed by impedance tubes and airflow resistivity measurements. The recorded results from these two sound absorption evaluations were compared. Additionally, a comparative study was presented on the results of impedance tube measurements to compare the influence of casting volumes (large and small scales) on the sound absorption of the acoustic panels. In the last stage, a comparative study was implemented to clarify the effects of harsh conditions on the sound absorption of the acoustic panels. The results showed that casting scale had great impacts on the mechanical and physical properties. Additionally, it was revealed that harsh conditions improved the sound properties of acoustic panels due to their effects on the porous structure of materials.

An investigation of multi-rate sound decay under strongly non-diffuse conditions: The crypt of the Cathedral of Cadiz

Multi-rate sound decays are often found and studied in complex systems of coupled volumes where diffuse field conditions generally apply, although the openings connecting different sub-spaces are by themselves potential causes of non-diffuse behaviour. However, in presence of spaces in which curved surfaces clearly prevent diffuse field behaviour from being established, things become more complex and require more sophisticated tools (or, better, combinations of them) to be fully understood. As an example of such complexity, the crypt of the Cathedral of Cadiz is a relatively small space characterised by a central vaulted rotunda, with five radial galleries with flat and low ceiling. In addition, the crypt is connected to the main cathedral volume by means of several small openings. Acoustic measurements carried out in the crypt pointed out the existence of at least two decay processes combined, in some points, with flutter echoes. Application of conventional methods of analysis pointed out the existence of significant differences between early decay time and reverberation time, but was inconclusive in explaining the origin of the observed phenomena. The use of more robust Bayesian analysis permitted the conclusion that the late decay appearing in the crypt had a different rate than that observed in the cathedral, thus excluding the explanation based on acoustic coupling of different volumes. Finally, processing impulse responses collected by means of a B-format microphone to obtain directional intensity maps demonstrated that the late decay was originated from the rotunda where a repetitive reflection pattern appeared between the floor and the dome causing both flutter echoes and a longer reverberation time.

A spherical harmonics basis for quantifying the isotropy of sound fields in reverberant enclosures

This study examines an experimental method for evaluating isotropy in reverberant sound fields, based on an analysis in the spherical harmonics domain. The methodology relies on estimating the wavenumber (or angular) spectrum of the sound field in the room, to characterize the magnitude of the waves arriving from definite directions at the observation point. Subsequently, the obtained wavenumber spectrum is expanded into a series of spherical harmonics, and the multipole moments from the spherical expansion are used to characterize the isotropy of the sound field. This spherical harmonic basis is best suited for characterizing isotropy, as it provides an unequivocal characterization of the symmetry of the wave field. The work examines how theoretical considerations compare with experimental results obtained in various rooms with diverse diffuse field conditions. The experimental results are based on automated measurements using a scanning robot. In addition, the corresponding spatial distribution of the active soundintensity field is determined, making it possible to benchmark the proposed methodology with direct observations of the energy flows in the sound field.

Estimating the diffuseness of sound fields: A wavenumber analysis method

The concept of a diffuse sound field is widely used in the analysis of sound in enclosures. The diffuse sound field is generally described as composed of plane waves with random phases, which wave number vectors are uniformly distributed over all angles of incidence. In this study, an interpretation in the spatial frequency domain is discussed, with the prospect of evaluating the diffuse field conditions in non-anechoic enclosures. This work examines how theoretical considerations compare with experimental results obtained in rooms with various diffuse field conditions. In addition, the paper investigates how the results relate to the modal theory of room acoustics, based on the conception that any mode, also in non-rectangular rooms, can be expanded into a number of propagating waves.

A new perceptive method to measure active insertion loss of active noise canceling headsets or hearing protectors by matching the timbre of two audio signals

Attenuation of passive hearing protectors is assessed either by the Real Ear Attenuation at Threshold subjective method or by objective Measurements In the Real Ear. For Active Noise Cancelling headsets both methods do not practically apply. Alternative subjective methods based on loudness balance and masked hearing threshold techniques were proposed. However, they led to unmatched results with objective measurements at low frequency, diverging in either direction. Additionally, they are relatively long as frequency points of interest are measured one after the other. This paper presents a novel subjective method based on timbre matching, which has the originality of involving other perceptive mechanisms than the previous ones did. The attenuation performance of ANC headsets is rated by the change in pressure level of eight harmonics when the active noise reduction functionality is switched on. All harmonics are played at once, and their levels are adjusted by the test subject until he perceives the same timbre both in passive and active modes. A test was carried out by a panel of people in diffuse noise field conditions to assess the performance of personal consumer headphones. Early results show that the method is as repeatable as MIRE and lead to close results.

Improvement of the measurement of the sound absorption using the reverberation chamber method

The random incidence absorption coefficient is measured in a reverberation room according to ISO 354 or ASTM C423-09a. It is known that the inter laboratory accuracy under Reproducibility conditions of these results is still not very well. It is generally assumed that the limited diffusion properties of reverberation rooms, especially with a strongly sound absorbing sample, are the main reason for the bad reproducibility values for the sound absorption between laboratories. Reverberation rooms should be made much more diffuse to reduce the interlaboratory differences. However there are practical limitations in quantifying and improving the diffuse field conditions. The measured sound absorption still seems to be the most sensitive descriptor of the diffuse field conditions. A way to further reduce the inter-laboratory differences is the use of a reference absorber to qualify a room and to calibrate the results of a sound absorption measurement. In the presentation an overview will be given of the research performed and some suggestions for the new version of ISO 354 will be given.

Accuracy of speech transmission index predictions based on the reverberation time and signal-to-noise ratio

This paper examines the accuracy of the speech transmission index (STI) calculated from the reverberation time (T) and signal-to-noise ratio (LSN) of enclosed spaces. Differences between measured and predicted STIs have been analysed in two rooms (reverberant vs. absorbent), for a wide range of absorption conditions and signal-to-noise ratios (sixteen tests). The STI was measured using maximum length sequence analysis and predictions were calculated using either measured or predicted values of T and LSN, the latter assuming diffuse sound field conditions. The results obtained for all the conditions tested showed that STI predictions based on T and LSN tend to underestimate the STI, with differences between measured and predicted STIs always lower than 0.1 (on a 0.0–1.0 scale), and on average lower than 0.06. According to previous research, these differences are noticeable and therefore non-negligible, as 0.03 is the just noticeable difference in STI. The use of either measured or predicted values of T and LSN provided similar STI predictions (i.e. non-noticeable changes), with differences between predictions that are on average lower than 0.03 for the absorbent room, and lower than 0.01 for the reverberant room.

A study of a real world transmission loss chamber and the Kinetics UniBrace-L technology

This study performed at Columbia College Chicago (CCC) had two purposes: The first was to test the functionality of the recently developed real-world transmission loss (RWTL) chamber, while the second was to evaluate the performance of the Kinetics UniBrace-L product on a double wall construction assembly. CCC’s RWTL chamber is designed to illustrate issues that have influence when testing partitions in the field. It is smaller in volume than a certified STC chamber, which results in modal effects on both sides of the chamber. Numerous microphone positions are available and are used to display the modal effects of the rooms and to determine an average sound pressure level in both spaces during testing. Absorption values are also substantially different between the sending and the receiving spaces (each side can be switched as either sending or receiving room) and diffuse-field conditions are not achieved in either side of the chamber. As such, the RWTL chamber will typically yield a lower STC value than a certified STC Chamber and will also yield lower values than what may be experienced when performing a test that would follow the standard ASTM E-336 or associated procedures.