Reverberation Time Measurements Research Articles

Understanding the influence of impedance transitions of acoustically reflective vertical surfaces on the frequency-dependent effectiveness of absorptive ceilings in large non-Sabinian empty spaces

An unexpected phenomenon was observed during measurements of reverberation time in empty spaces to be fitted out, in which the only sound absorbing treatment consists of mineral fiber tiles on the ceiling, a material typically highly absorbing at mid and high frequencies during laboratory measurements: this involves a systematic increase of the reverberation time in octave bands between 1 kHz and 4 kHz in the measured empty spaces. The physical understanding of this surprising observation has focused on the question of the influence of impedance transition on sound scattering, and the influence of the frequency-dependent scattering properties of acoustically reflective vertical surfaces of different natures (glazing, plasterboard lining, masonry) of these empty spaces on the efficiency of a sound absorbing ceiling. These scattering phenomena were studied and modeled in a computer model in order to verify the model's adjustment hypothesis, an essential initial step before the study of the acoustic parameters in future fitted spaces and the effectiveness of acoustic solutions for space planning of open plane offices. Good consistency between the measurements and the empty model was observed when the frequency-dependent scattering behaviour of the vertical reflecting surfaces of different nature was taken into account in the simulation.

Selection of the averaging time for the interrupted noise method of measuring reverberation time

The author wrote the requirements for the selection of the averaging time for the interrupted noise method of measuring reverberation time in the international standard ISO 354:1985 "Acoustics-Measurement of sound absorption in a reverberation room". When this standard was revised to become ISO 354:2003, the evaluation range used to measure the reverberation time was changed from 30 dB to 20 dB. Because the working group which revised the standard did not know the reasons for the averaging time requirements, they incorrectly changed them to supposedly take account of the change to the evaluation range. The purpose of this paper is to give the reasons for the averaging time requirements so that they are corrected and not incorrectly changed again. It is necessary that the averaging time is not so large that information is discarded and so that, in the case of exponential averaging devices, the measured decay rate is not biased by the decay rate of the exponential averaging device. On the other hand, it is desirable that the averaging time be as large as possible so that the ensemble standard deviation of the measured reverberation times is as small as possible.

Case study of simplified reverberation time measurement under multiple conditions using a sound level meter

Recent advancements in the computational capacity of sound level meters have facilitated the simultaneous recording of diverse calculation results and waveform recording data. Traditionally, reverberation time determination entailed manually adjusting a linear regression line to the reverberation decay curve derived from recorded sound levels, followed by reverberation time computation based on the gradient of said line. However, automated techniques have made reverberation time calculation via a sound level meter feasible. This paper discusses a case study evaluating the efficacy and considerations of simplified reverberation time measurements conducted across various conditions, encompassing scenarios with multiple sound sources and diverse indoor environments. The study leveraged recorded data from a sound level meter and employed automated methods simulated on a personal computer.

Determination of reverberation time in large educational auditoriums

According to the current regulation, acoustic comfort in educational buildings in Lithuania is ensured by applying sound insulation of structures and limiting excessive room reverberation. The maximum reverberation time values described in the document depend on the building's acoustic comfort class, room volume and frequency range. In order to determine the design values of the reverberation time for larger auditoriums, there is a need for consultations with an acoustic expert. This paper presents some typical examples of reverberation time measurements in new, old and modernized large lecture halls. The measurement results indicate good practice in the design and installation of new auditorium, as well as the nature and importance of acoustic problems in old and modernized ones. The analysis of the measurement results allows us to assume that the success of the project's acoustic quality often depends on the designer's ability to properly assess the design indicators and the properties of applied materials.

Exploring sound diffusion in learning environments through experimental and wave-based analysis

Acoustic design guidelines in learning environments generally involve specific requirements of reverberation time (T20), speech clarity (C50), and speech transmission index (STI) to ensure adequate quality for spoken communication. Predictive formulas recommended by international standards rely on ideal diffuse field theory as a crucial factor for meeting adequate room acoustics criteria. However, such theoretical assumptions are barely encountered in real-world rooms, undermining the predictions' accuracy. The present work explores the main factors influencing sound diffusion in classrooms by means of in-field acoustic measurements and numerical models. In detail, different desk configurations and the installation of materials with varying acoustic impedances at the suspended ceiling have been studied to evaluate the associated increase in sound diffusion. Among the several metrics to quantify sound diffusion, the standard deviation of measured reverberation time values has been selected in the present study. Experimental and simulated data findings reveal that the furniture layout with a single desk arrangement and the suspended ceiling's treatment with porous and perforated ceiling tiles increase sound diffusion by exploiting edge diffraction and material discontinuities, respectively.

Investigating architectural acoustics modeling with the summer undergraduate research or internship experience in acoustics program

Dante Christian was chosen to participate in an inspiring 10-week experience with the Acoustical Society of America’s Summer Undergraduate Research Experience in Acoustics (SURIEA) Program. Given his physics major and architectural background, SURIEA coordinators felt that he would thrive best as an intern at Acentech (an architectural acoustics firm). The research project that Dante landed on was an auralization of the Warner Theater’s rehearsal room—a structure in Erie, PA. Two of his mentors from Acentech visited to make reverberation time measurements and to ensure that the rehearsal room was behaving as it was intended to. Thus, these measurements came to serve as the backbone of his project’s structure. For Dante’s summer research, he went through the processes of modeling in SketchUp and in Trebel (a geometric and wave-based solver) to gather and simulate impulse responses for the Warner Theater’s Rehearsal room. He worked iteratively to assign absorption and scattering coefficients to the model that best fit the measured experimental results. This is because, especially, absorption coefficients of every material in the room were not specified. Overall, this research project taught Dante how to critically think about a space, its materials, and how its materials interact with the soundscape.

The Historical Building and Room Acoustics of the Stockholm Public Library (1925–28, 1931–32)

The Stockholm Public Library was realized in two distinct phases of construction in the 1920s and early 1930s, and remains a well-known work in twentieth-century architecture, with a heritage status today. While previous studies have focused on the library’s architectural design, particularly its lighting, acoustics were also an important aspect of the building’s design and construction. This study marks the first detailed investigation of the library’s architectural acoustics, with a suite of standard measurements performed to assess and characterize the library’s historical room and building acoustics. Reverberation time measurements in the library’s reading rooms yielded results of about 1.5–2 s for frequencies associated with speech. A significantly longer reverberation time of 5–6 s was measured in the library’s central rotunda, confirming a prominent acoustic issue in the library, where appropriate heritage discussions are needed in the future as the library undergoes a major renovation in the coming years. A comparison of the measured airborne and impact sound insulation of the 1920s and 1930s reading room ceilings also yielded interesting results. While the materials in library’s two construction periods are notably different, the airborne sound insulation performance of the 1920s and 1930s floors or ceilings was comparable and in line with contemporary standards. Impact sound insulation results from the 1920s and 1930s floors, however, differed significantly, with the latter displaying a relatively poor performance. Flanking transmission effects related to historical construction details and deviations from archival plans were investigated and discussed. This work emphasizes the practical and academic importance of conducting on-site measurements, and the close mutual development of modern architecture, construction, and architectural acoustics.

The impact of heat and humidity changes on laboratory acoustic measurements through perturbation method

The paper relates to the determination of the influence of changes in temperature and relative humidity on the laboratory acoustic measurements results. A series of direct reverberation time measurements was carried out, applying different thermal and humidity conditions in the empty reverberation chamber and reverberation chamber with a sound absorber. The reverberation chamber is mainly used to tests aimed at assessment construction materials and products. The results of acoustic measurements and the related classification of construction materials and products ultimately translate into the comfort and acoustic quality of rooms with special requirements as well as living rooms. Based on the analysis of the results, the significance of the problem related to the influence of air parameters on the results of measurements was demonstrated. The effect of changes in temperature and humidity on the results of calculating the equivalent sound absorbing surface was estimated using the n-perturbation method.

Building structure-borne noise measurements and estimation due to train operations in tunnel

The perception of structure-borne noise is particularly salient when train passes through the tunnel under the buildings, which has a negative impact on human health. In the process of constructing buildings along metro lines, it is crucial to estimate indoor structure-borne noise levels in order to enhance design and prevent any negative impact on human comfort. This study conducted measurements of structure-borne noise, reverberation time, and train-induced vibrations in Guangzhou, China to investigate the generation, propagation, and dissipation mechanisms of structure-borne noise. An approach based on Short-Time Fourier Transform and Schroeder integral was proposed for obtaining frequency-dependent reverberation time. Additionally, a deep learning-based approach incorporating indoor vibrations, frequency-dependent reverberation time, and room parameters as inputs was proposed based on Genetic Algorithm-Artificial Neural Network. The estimated structure-borne noise levels demonstrated good agreement with measured values, indicating the feasibility of the approach. The finding of this research facilitates a clear comprehension of the generation, distribution, and dissipation mechanisms of indoor structure-borne noise for engineers while also enabling convenient acquisition of indoor structure-borne noise. The estimated noise levels can be effectively utilized during building design processes along metro lines to mitigate adverse impacts on human comfort.

Robust 3D localization of anomalies in reverberation time measurements using the sound field scanning method

Reverberation time measurements form the basis for room acoustic optimizations of existing building structures. During the verification of the achieved room acoustic improvements, anomalies may appear in the reverberation time signal which may be hard to spatially localize, especially in spaces with demanding acoustic requirements such as large, open workspaces, or concert halls. This contribution focuses on the application of the Sound Field Scanning technology to the fast spatial localization of room reflections. In this process, an omnidirectional sound source is positioned at an observation point in the room and periodically excited with band-limited pulses. At the same observation point, an acoustic camera system consisting of a rotating linear microphone array is oriented towards the preferred spatial direction. The emitted pulses and associated room reflections are captured on the measurement surface of the rotating microphone array. Acoustic images with high depth resolution are generated in parallel planes to the measurement surface. In complex situations, the task of spatially localizing anomalies in the reverberation time signal can be reduced to a few measurements from different viewing angles, thus, significantly accelerating the problem-solving process with high confidence. The method is exemplarily described through the room acoustic analysis of a University Lecture Hall.

Increasing the accuracy of ISO 354 and ASTM C423 through modal identification

Measuring the acoustic properties of materials is a highly challenging task. Previous studies have highlighted errors in the absorption coefficient above 200 Hz, leading to absorption coefficient values higher than one. These errors are often attributed to differences in modal decays with and without the specimen. This effect is amplified in the case of highly reactive materials, such as innovative materials, that significantly impact the acoustic field and, consequently, the modal behaviour of the room. This work aims to reduce the difference between measured reverberation time values, with and without the specimen, through Data Augmentation. Modal identification was performed on measured impulse responses of porous specimens, where the alpha is known, allowing the determination of modal decay times and reverberation times within the frequency range of 70–180 Hz. A statistical analysis of natural modes and numeric solutions enables the identification of outliers. In this way, even in a non-well-optimized reverberation room, it is possible to enhance the accuracy of acoustic absorption measurements.

Assessing acoustical quality in university classrooms in Brazil: Measurements and simulations

The acoustic quality of learning spaces, such as classrooms, is a worldwide issue. Some countries have regulations or guidelines for these environments, which does not happen properly in Brazil. Beyond the lack of nationwide research related to classroom acoustics, Brazil has two standards with a few information related to classroom acoustics. One of them was written in 1992, having a single demand concerning an appropriate reverberation time for conference rooms at the 500 Hz frequency, without detailed requirement, in contrast to other national guidelines. Another one, updated in 2020, recommends the sound pressure levels for indoor environments, having classrooms in its list. The aim of this study is to analyze the acoustic quality of two Brazilian classrooms over a public university in the south part of Brazil. Several impulse responses were taken within the learning environments and then processed in Matlab with the ITA ToolBox. The classrooms were also modeled into RAVEN and I-SIMPA software aiming to identify different sound absorption schemes to improve their speech intelligibility. The findings showed that the classrooms had a bad acoustic environment, with a measured reverberation time of approximately [Formula: see text]. However, the simulation suggested that implementing sound absorption strategies could reduce the reverberation effects by half, bringing it closer to optimal levels. The estimated costs for implementing sound absorbers were [Formula: see text] for Classroom 01 and [Formula: see text] for Classroom 02.

Influence of Test Room Acoustics on Non-Native Listeners’ Standardized Test Performance

Understanding the impact of room acoustics on non-native listeners is crucial, particularly in standardized English as a foreign language (EFL) proficiency testing environments. This study aims to elucidate how acoustics influence test scores, considering variables overlooked in prior research such as seat position and baseline language proficiency. In this experiment, 42 Japanese university students’ performance on standardized EFL listening tests was assessed in two rooms with distinct acoustic qualities, as determined by the speech transmission index (STI) and reverberation time (RT). The rooms differed significantly in their STI values and RT measurements, with one exhibiting high speech intelligibility qualities of ≥0.66 STI and RT0.5–2kHz < 0.7 s and the other falling below these benchmarks. The findings revealed that listening test scores were consistently higher in the acoustically favorable room across all participants. Notably, the negative effect of poor acoustics was more pronounced for students with lower baseline language proficiency. No significant score differences were observed between front- and rear-seat positions, suggesting that overall room acoustics may be more influential than individual seating locations. The study concludes that acoustics play a significant role in the standardized EFL test performance, particularly for lower-proficiency learners. This highlights the necessity of standardized testing environments to be more carefully selected in order to ensure the fair and reliable assessment of language proficiency.

Reverberation time measurement issues

The current ISO attempt to revise ISO 354 has ended in failure because the experts on the working group could not reach agreement in the required time frame. However, several important issues were identified. Some of these issues are discussed in this paper. The current ISO 354:2003 changed from using 30 dB of decay to using 20 dB of decay to determine the reverberation time. Unfortunately, the conversion of the theoretical formulae for the spatial and ensemble standard deviation from 30 dB to 20 dB of decay was not correct. The current version of the standard incorrectly changed the maximum linear averaging time and changed from specifying the maximum exponential averaging time to specifying the maximum exponential time constant but did not halve the maximum value. There is also evidence that the maximum exponential averaging time can be increased by 50% without biasing the measured reverberation time. When a discrete decay curve is analyzed, there are issues when deciding the first and last point to use when determining the reverberation time. During the attempted revision of the standard, it was proposed to specify a minimum amount of empty room sound absorption. This did not reduce the spread of results in the round robin and this paper gives theoretical results which suggest that this is to be expected.

Theatres from roman age to renaissance: a short survey on the meaning of reverberation time measurements

In the mind of C.W. Sabine, reverberation time was thought as a numerical index of what was happening in a closed hall when a sound source acting within was suddenly stopped: the original idea was that sound rays where travelling in any direction reinforcing residual sound energy, but at the same time overlapping audible messages that these rays where carrying to the listener's brain. As well known, he stated a formula linking the R.T. value to the hall volume and to the capacity of the impinged surfaces to keep a fraction of the sound energy: from one hundred years to now, many authors researched in the field and stated the best R.T. values for the listener of different kinds of sound. Surely Greeks and Romans did not know the possible existence of such a parameter, as they acted in open spaces, neither Vitruvius and, successively Alberti, Milizia, Poletti and so on, even if the tile cover utilized by the Romans to preserve from sun light and rain was avoiding that some sound energy dispersed in the sky. Surely the modern computer assisted measurement techniques are able to keep some kind of sound decay even in an ancient theatre, but we are aware, as Greeks were, that they are derived only from reflections travelling quite horizontally, between vertical structures, or inclined between actors and spectators via orchestra floor, when not occupied from public. This paper will present a specific and synthetic survey on the subject.

Efficient deep learning-based prediction of acoustic parameters in atrium spaces using BIM files

Acoustic simulation tools, although common in the acoustic evaluation of buildings with large atrium space, are costly both in terms of time and resources. This study aims to develop an efficient prediction tool for Reverberation Time (RT) and Sound Pressure Level (SPL) within the atriums using deep learning, with Building Information Modelling (BIM) files as the only input. Initially, a 3-D acoustic simulation model was benchmarked against experimental measurements of RT for an existing building’s atrium, demonstrating an agreement within 15%. Subsequently, BIM files of 60 buildings were used to simulate RT and SPL at various listener locations within their atrium spaces. The BIM files provided essential geometry information, including atrium shape, dimensions, door placements, floor plans, material properties, and sound sources. This dataset was then utilized to train deep learning algorithms, enabling rapid and convenient predictions of RT and SPL for any new building's atrium space. The proposed prediction model serves as a valuable tool for architectural planning and noise regulation during the early design stages. By relying solely on basic building information from the BIM file, this tool obviates the need for time-consuming and computationally expensive simulation software typically used for acoustic evaluations in large atrium spaces.

The use of professional audio external USB sound cards for building acoustics measurements

CSIRO purchased an open application programming interface for hardware with an ethernet connection from a major acoustical instrument manufacturer. Using this interface, the author developed software for fractional octave band filtering with linear and exponential averaging. He then extended this software to automatically measure reverberation time. Fast Fourier Transform and Stepped Sine software was also developed. He then realized that CSIRO also owned two professional audio external USB sound cards. One of these had eight analogue inputs and ten analogue outputs. The other had two analogue inputs and two analogue outputs. Both could supply 48 V phantom power on their analogue inputs. It is possible to purchase phantom power preamplifiers for professional measurement microphones, but CSIRO decided to purchase phantom power to IEPE pre-amplifier adaptors. This enabled the use of professional pre-polarized microphones with IEPE microphone preamplifiers and accelerometers with built-in IEPE preamplifiers. CSIRO also had external USB hardware from another acoustical instrument manufacturer. Upon opening this instrument, it consisted of a professional audio external USB sound card and an IEPE front end for the two analogue input channels. The author decided to produce a version of his software that could be used with these three external USB sound cards.

Determination of the Reverberation Time Using the Measurement of Sound Decay Curves

In the measurements of reverberation time, measurement methods of different accuracy are used depending on the room. For ordinary rooms, the measurements are made using the interrupted noise method, which consists of determining the decay curve after switching off the excitation source of the room. The measurements are made for different source arrangements and different receiver arrangements, and at least three repetitions are made at each of such points. Due to such a realization of measurements, several dozen different reverberation curves are obtained, from which the reverberation time is read out. This article demonstrates the differences between reverberation time readouts, depending on the averaging method of reverberation curves. The first analyzed method is based on reading out the reverberation times for each obtained curve and on averaging the results obtained in this way. The second analyzed method involves averaging the reverberation curves using the linear regression method and then determining a simple regression on the basis of which the reverberation time is read out. For each method, different average reverberation time values and different standard uncertainties were obtained. The difference for the 500 Hz frequency band in a teaching room for the measurement uncertainty is 0.28 s. The results obtained in the article are extremely important when designing interiors intended for the reception of verbal sound, in particular teaching rooms.

Sustainable Technology of Wood Charcoal Diffuser for Indoor Acoustical Quality

Wood charcoal is sustainable, renewable, environmentally friendly material using which the acoustic device may be produced. Charcoal made of wood waste materials allows to improve indoor acoustical quality. The current article aims to investigate sound scattering coefficients of quadratic residue diffusers with the covering of oak (Quercus robur) wood charcoal elements. The sound scattering coefficient is calculated due to the reverberation time measurement in the reverberation chamber. The calculation results of the scattering coefficient show the growth of scattering in the frequencies – the highest value reached 0.88 (diffuser N7 with charcoal). The effectiveness of diffusers to diffuse sound waves increases as the number of wells grows. The diffuser with 80 % charcoal elements showed a higher scattering coefficient comparing to the diffuser without charcoal elements.

An overview of the proposed new ASTM standard for impulse response measurements using swept sinusoidal signals

This contribution presents the current draft of the proposed ASTM standard for impulse response measurements using swept sinusoidal signals for use in room and building acoustics. After briefly discussing the theoretical background and main body of the proposed standard, the main focus of the presentation is the validation procedure to compare reverberation time measurements between the interrupted noise method and the impulse response method. The challenges and associated uncertainties of the validation procedure are discussed, and preliminary results for rooms of different sizes are shown.