Reverberation Time Research Articles

Influence of variable sound-absorbing devices on room acoustical parameters of reverberation and intelligibility in medium-to-large multipurpose halls.

Multipurpose halls are designed to host various performances. However, achieving the ideal reverberation time (RT) for each of the different performance types can be challenging. This study investigates five halls of various sizes to determine the effects of sound-absorbing devices on variable RTs in multipurpose halls. The composition and sound absorption properties of the finishing materials were investigated in areas where sound-absorbing devices were not applied. Further, the changes in the room acoustic parameters of these medium-sized multipurpose halls were analyzed using computer-based acoustic simulations to find a suitable answer among the various solutions tested. By applying sound-absorbing devices (resonant-type) to 25% of the walls and ceilings of the target halls, the absorption and reflection modes displayed a variability range of more than 0.5 s in the bass-mid frequency (250-500 Hz). However, a variable range of 0.18 s was found in the high frequency (2000 Hz). To improve the low variable range in the high frequency, a partial application of a high-frequency high-performance sound absorption banner (porous-type) was used to secure a variable range of 0.35 s in the high frequency. Variable sound-absorbing devices should be considered to achieve effective RT variation for all frequencies.

Real and Virtual Lecture Rooms: Validation of a Virtual Reality System for the Perceptual Assessment of Room Acoustical Quality

Enhancing the acoustical quality in learning environments is necessary, especially for hearing aid (HA) users. When in-field evaluations cannot be performed, virtual reality (VR) can be adopted for acoustical quality assessments of existing and new buildings, contributing to the acquisition of subjective impressions in lab settings. To ensure an accurate spatial reproduction of the sound field in VR for HA users, multi-speaker-based systems can be employed to auralize a given environment. However, most systems require a lot of effort due to cost, size, and construction. This work deals with the validation of a VR-system based on a 16-speaker-array synced with a VR headset, arranged to be easily replicated in small non-anechoic spaces and suitable for HA users. Both objective and subjective validations are performed against a real university lecture room of 800 m3 and with 2.3 s of reverberation time at mid-frequencies. Comparisons of binaural and monoaural room acoustic parameters are performed between measurements in the real lecture room and its lab reproduction. To validate the audiovisual experience, 32 normal-hearing subjects were administered the Igroup Presence Questionnaire (IPQ) on the overall sense of perceived presence. The outcomes confirm that the system is a promising and feasible tool to predict the perceived acoustical quality of a room.

Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University

The relationship between architectural design and outdoor acoustic environments remains underexplored, particularly in educational spaces where noise levels impact comfort and usability. This study investigates the impact of building facade height on the outdoor acoustic environment in university courtyards. Acoustic measurements were conducted in two courtyards at Batna 1 University, each surrounded by buildings with distinct facade heights. Key acoustic parameters, including reverberation time (RT), early decay time (EDT), rapid speech transmission index (RaSTI), Definition (D50), and sound pressure level (SPL) attenuation were evaluated at specified source-receiver distances. The results reveal a strong correlation between RT20 and distance at higher frequencies due to building facade reflections, while lower frequencies are more influenced by geometric configuration and material absorption properties. The results demonstrate that RT and EDT increase logarithmically or polynomially with distance, especially at higher frequencies (2000–4000 Hz), due to the decrease in direct sound energy and increase in reflected sound amplitude. Taller building facades lead to longer RT and EDT values compared to lower heights. D50 and RaSTI decrease polynomially with increasing source–receiver distance, with lower values observed in the courtyard with taller facades, indicating reduced speech clarity. The SPL attenuation is influenced by surrounding geometry, with the least reduction in the courtyard with lower facade heights, followed by the taller facade courtyard, contrasting with semi-free field conditions. These findings highlight the significant role of building facade height and architectural elements in shaping the acoustic characteristics of outdoor spaces, providing valuable insights for designing acoustically comfortable urban environments, particularly in educational settings.

Acoustic Analysis of the Masjid at Necmettin Erbakan University Köyceğiz Campus in Konya

In this study, the passive acoustic performance of Necmettin Erbakan University Köyceğiz Campus Masjid was investigated. Designed as the largest masjid of the city with a capacity of 15,000 people and a volume of 43,200 m3, the masjid, which has traces of Seljuk, Ottoman and Modern architecture. is built as a complex at a location overlooking the city in the Meram District of Konya City, Turkiye. The aim of the study is to determine the acoustic comfort conditions by considering all the activities in the masjids as a whole. Within the scope of the study, the acoustic performance of the masjid was evaluated by determining different source and receiver points for each mode of activity. As a method, the chosen masjid was simulated with ODEON Room Acoustics Software Ver. 14.04 software. Objective room acoustic parameters were analysed in three groups. These are sound energy ratio parameters (reverberation time (RT), early decay time (EDT), clarity (C50, C80), lateral fraction (LF80)), speech intelligibility parameters (definition (D50), speech transmission index (STI)) and sound strength parameters (strength (G)). The results obtained were compared with precedent studies in the literature. In comparison with the acoustic values obtained in other masjid/mosque buildings, it was reported that, while the speech intelligibility of other masjids/mosques was at a satisfactory level, the masjid under consideration was at a poor level in both fully occupied and unoccupied conditions. In the analysis made for reverberation time, it was seen that the masjid discussed in this study showed similar characteristics to other masjids/mosques globally. As a result, it was determined that the dimensions of the surfaces forming the mihrab, the minbar design and the depths of the mahfil overhangs are effective regarding the acoustics of the masjid, and the design of curved surfaces should be carried out in a way that does not cause focusing problems. In addition, suggestions that can give guidelines to modern masjid designs have been put forward.

Acoustic performance prediction by means of a statistical energy analysis of two adjacent workrooms used for conferences and educational proposes

AbstractThe current design of workspaces for companies is focused on minimalistic and modern styles prioritizing the aesthetic appearance with illuminated open spaces and the use of glass walls. If the acoustic performance is not considered, inappropriate Reverberation Times (RT) might lead to difficulties for hearing a speaker or teacher during courses and conferences, causing problems in the learning process. Additionally, there might be high sound transmission between adjacent work rooms. This paper presents the development and experimental validation of numerical models using Statistical Energy Analysis (SEA) to calculate the sound reduction index through a glass wall that separates two adjacent work rooms. These structures are dedicated to conferences and educational uses but cannot be properly used due to their high sound transmission and inappropriate reverberant time. The sound insulation prediction results are validated with experimental measurements carried out in the adjacent rooms under the standard ISO-140. Afterwards, a SEA model is used to provide some acoustic correction design guidelines for these types of constructions. To improve the acoustical performance in the rooms, the acoustic effects of different absorbent panels placed on the walls and ceiling were investigated, choosing a suitable material that complies with the recommended ranges of RT and with less amount of absorption area. The SEA model is then used to understand the effects of openings size between the panels that make up the glass wall on the sound insulation capacity between rooms. Finally, a SEA model is reformulated to quantify the effect of the application of double walls for sound insulation between rooms, which implies the increase of weighted sound reduction index in 9 dB with respect to measured data.

Effect of Interpolation Artifacts on Perceived Stability of Nearby Sources in a Navigable Reverberant Virtual Environment

Convolution with spatial Room Impulse Responses can achieve realistic auralizations. When combined with interpolation between spatially distributed RIRs, this technique can be used to create navigable virtual environments. This study explores the impact of various interpolation parameters on the perceived auditory stability of a nearby static sound source with listener movements in a reverberant environment. The auditory scene was rendered via third-order Ambisonic RIR convolution combined with magnitude-least-squares binaural decoding using nonindividualized head-related transfer functions. First, the estimated direction of arrival as a function of the listener’s position within a 2D grid of RIRs under various configurations is examined as an objective metric. The perceived stability of the auditory source is then assessed through a perceptual experiment. Participants freely explored a virtual scene reproduced over headphones and a tracked head-mounted display. They were asked to rate the stability of a nonvisual source under various conditions of RIR grid density, interpolation panning method, and reverberation time. Results indicate no need to use an RIR grid size finer than 1m to optimize source stability when using a three-nearest-neighbor interpolation scheme.

Revisiting reverberation formulae

In 1992, the author proposed a generalization of Sabine's formula that develops reverberation time over a series of powers of the reflection coefficient on the boundaries. Some years later, he reduced the development to just two terms that made it possible to monitor reverberation times up to large absorptions. The present paper revisits this development and justifies it with the help of free path statistics in 3D rectangular enclosures. It proves that Kuttruff's reverberation formula is a special case of the general formula, but diverges for large absorption. Reverting to Kuttruff's original integration process leads to a formula that does not diverge. The paper further explains the difference between Eyring-type reverberation times that vanish for total absorption, and Sabine-type reverberation times that never vanish even for total absorption, and proposes a simple scheme for evaluating the asymptotic free path statistics and thus improving reverberation time prediction. Lastly, the approach is extended to non-ergodic enclosures.

Functions for noise reduction in acoustic standards: evaluating reverberation time, mean absorption coefficient, and a novel approach

Contemporary European acoustic standards utilize dimensioning functions based on either reverberation time or mean absorption coefficient. For spaces with the primary goal of noise reduction, achieving consistent overall sound pressure level (SPL) reduction, regardless of room sizes and shapes is evidently desirable. The conditions under which reverberation time and mean absorption coefficient approaches align with the objective of consistent SPL reduction are analyzed. Generally, both approaches fall short of achieving consistency in case of varying ground surface or room shape, i.e. length, width and height relations. Additionally, in large rooms, constant or room height-dependent reverberation time functions may lead to extensive absorption areas, complicating the installation of acoustical treatments and degrading their economic efficiency. A novel dimensioning function, providing consistent SPL reduction independently of room size and shape, is derived based on diffuse field theory. The novel function is multiplied by a linear function of room height and length, to alleviate accuracy limitations of diffuse field theory in case of varying room shapes. Finally, simulations systematically varying room sizes and shapes demonstrate that the novel dimensioning function significantly outperforms existing approaches in achieving consistent SPL reduction.

A tool to calculate the reverberation time of rooms with uneven absorption distribution

The reverberation time in rectangular rooms with uneven distribution of absorbing elements and surfaces, for example suspended ceilings, lead to a non-diffuse sound field. This implies longer reverberation times than predicted by sabines or eyrings reverberation formula. To enable a correct prediction of the reverberation times in such rooms, the only possibility was to apply room acoustic simulations, which require high effort to model the room and to perform the simulation. In 2021, a prediction method was published by Zhou et. al. to calculate the reverberation time in such rooms much more precisely than using sabines or eyrings formula, without the need for a room acoustic simulation. This method has been transformed into an online calculation tool, named "reverberate". In this contribution the methodology will be quickly reviewed and the online tool will be presented.

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.

Correction for reverberation time in acoustic measurements from service and equipment - Comparison between different methods

The measured values of the sound pressure level in ordinary rooms in dwellings vary depending on the combined effect of the absorption and the scattering of the furniture, or to the presence of absorbent surfaces on walls and ceilings. Regarding the measurement of noise from building services, ISO 16032 and ISO 10052 methods allow to normalize the sound pressure levels to a reference reverberation time of 0,5s. Several European countries incorporate this reverberation time correction into their regulations. However, Spanish regulations, specifically RD 1367/2007, and the majority of regional decrees and ordinances concerning the evaluation of noise from activities or equipment within buildings, do not incorporate this correction, even when measurements are performed in two scenarios: before the building's occupation when rooms are empty, or when the building is fully furnished, as may occur in the case of complaints. For that reason, this study compares the values of the reverberation time index obtained in 180 residential rooms using various methods while also evaluating the benefits and drawbacks of their application.

Objective and subjective assessment of PET felt recycled panels as an acoustic treatment in the meeting rooms

This study presents the performance of the PET-felt recycled panels for the room acoustic treatment. As the objective parameters, Reverberation Time (RT) and Speech Transmission Index (STI) were measured with B&K DIRAC, an omnidirectional loudspeaker, and power amplifier in the meeting room at the Department of Interior Design, Bilkent University, Ankara, Turkey, before the acoustic treatment. Following ISO 12913-2 Method A, Perceived Affective Quality (PQA), Appropriateness, and Assessment of the Acoustic Environment were measured. Fifteen participants who were occupants of the meeting room were selected for the questionnaire survey to be conducted using the snowball method. The findings showed that the reverberation time was five seconds at 1 kHz, which is insufficient, and the acoustic quality could have been better for the speech-purpose room. PET-felt recycled acoustic panels were applied to the ceiling and walls as a solution. After the acoustic treatments, all procedures were repeated with the same. The results showed that significant improvements have been provided in both objective and subjective assessments. RT was in the optimum range. The participants found the meeting room with PET-felt recycled panels more pleasant and eventful after the acoustic treatment.

Modal identification as a tool to increase the accuracy of acoustic absorption measurements in reverberation rooms

Measuring the acoustic properties of materials may be very challenging. At low frequencies, the sample may interfere with the room's modal behavior. When it occurs, the longest modal decays are influenced. Differences in reverberation time - empty Vs with-specimen conditions - are often emphasized by these modal effects, and, as a matter of fact, the absorption coefficient may be overestimated. The present work aims to increase the accuracy through data augmentation at low frequencies. Modal identification was performed on measured impulse responses of porous specimens, allowing the determination of modal-decay times within the third-octave bands of 80-160 Hz. A statistical analysis of natural modes highlights the modes whose decay exceeds the density function as outliers. This preliminary study ultimately provides some statistical considerations on the modal distribution of decay times and their relationship with the reverberation time measured through the Schroeder integral.

Revision of Sabine's and Eyring's reverberation theories

Sabine's and Eyring's reverberation theories are still the most important theoretical frameworks for room acoustics. The important physical quantities derived from these theories are the average sound pressure level (sound energy density) and the reverberation time (sound energy decay). Eyring revised Sabine's theory to become the reverberation time zero in the perfect absorption condition. However, Eyring's theory has contradictions regarding the sound energy density in a steady state and the energy decay from the steady state. In this study, in order to resolve these contradictions, the theoretical framework for the consistent system is presented introducing the new concept of the average impulse response in a room, and the contradictions in the current room acoustic theory are clarified. Based on the theoretical framework, Sabine's reverberation theory is revised by a different way of Eyring's theory. The reverberation time of the revised theory is shorter than that of Sabine's theory and longer than that of Eyring's theory. Finally, ray-tracing computer simulations were performed to verify the revised theory. Results of sound ray tracing simulations were in better agreement with the values calculated using the revised theory rather than those calculated using Sabine's and Eyring's theories.

Survey on acoustic preferences in Korean music practice spaces

.In 2021, the acoustic quality criteria for music practice rooms was standardized as ISO 23591. This standard will be very useful in constructing and utilizing music education facilities to train excellent music players. "Gugak," Korea's traditional music, is taught and related performers are trained at major national educational institutions. Recently, a variety of Korean music content has been introduced internationally. The Korean traditional music practice room is used similarly to a general music practice room. However, the acoustic characteristics of traditional houses where Korean traditional music is played do not have a long reverberation time and are flat across frequency bands, so we investigated what acoustic characteristics actual Korean traditional music players prefer as a practice space. The room acoustic characteristics of Korean music practice spaces at Seoul National University were measured, and a preference survey was conducted on professional performers who actually use the practice spaces. In the future, we plan to investigate preferences for Korean traditional music practice spaces using real-time sound audibility technology.

Measurement uncertainty of ISO CD 23351-2 - Preliminary results

The purpose of our study was to determine the measurement uncertainty of ISO CD 23351-2 draft standard. Two sub-studies were conducted. In study 1, an accuracy experiment was conducted where 7 participants tested in the same room (in situ) two different products (full enclosure and partial enclosure). In study 2, an experiment was conducted where a single participant tested two different products in several rooms varying in room volume and reverberation time. Study 1 yielded an in-situ reproducibility standard deviation of 0.80.9 dB. Study 2 yielded a between-site standard deviation of 0.7 dB. The small standard deviations suggest that the technical content should not be essentially changed in the next versions of the standard. The results could be considered in next version of the standard ISO DIS 23351-2.

Application of a multi-criteria decision technique to assignation of acoustic classes

Acoustic performance evaluation of buildings prior to the acoustic classification, requires consideration of a number of factors with the corresponding quantitative criteria to be satisfied. It is of question how much the deviations can be compensated and whether all performance factors are equally important. This paper presents the results of a study on assigning priorities for each acoustic factor by using the analytic hierarchy process (AHP) - a multi-criteria decision technique - based on the expert evaluations. Eleven acoustic consultants rated seven acoustic factors, i.e. regulatory criteria of sound insulation, sound level and reverberation time, in pairwise comparisons. The data were aggregated after a consistency check and weights of each acoustic factor were calculated as well as weights of six acoustic performance classes ranging from A to F. The outcomes of this study resulted in a decision matrix, named as "acoustic scorecard" which can be used to obtain a single-number rating index for acoustic quality of buildings.

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.

Effect of sound absorption on noise reduction in the automotive industry

Industrial noise is one of the most common physical factors that cause annoyance and damage workers' health in the long term. Precautions should be taken to reduce noise and improve acoustic performance in industrial working environments. This paper aims to analyze the acoustic performance of the automotive industry and contribute to the global outcomes of sustainability and the development of strategies to improve the quality of the working environment through improvement scenarios. For this purpose, the automotive industry in Türkiye was examined as a case study. In-situ acoustic measurements were made in the seat manufacturing unit of an automotive factory and the current situation was transferred to the simulation program. The effects of acoustic improvements on A-weighted sound pressure level and reverberation time at mid-frequencies (500, 1000, 2000 Hz) were investigated through three different scenarios. In the investigations, noise distributions were carried out through noise mapping. The A-weighted sound pressure levels in the automotive industry were reduced by approximately 15 dB. As a result of the study, suggestions for noise control precautions and their effects on the automotive industry seat manufacturing unit are presented.

Operating room soundscape design intervention study

Noise levels in a typical Operating Room are often well beyond the recommended 45dB for a typical OR environment. Due to stringent cleanability requirements in hospitals, there are limited finishes that can be used in these rooms. Researchers from Gresham Smith and Siebein Acoustic are currently engaged in a research project to investigate how the design of the operating room environment can be acoustically optimized for staff well-being and better patient outcomes. . The study focuses on the environmental variables that can be controlled including the effects of diffusing panels, angling of the walls, and the use of special cleanable acoustic finishes on the ceiling in 3 prototypically sized Operating Rooms using 3-dimensional acoustic models. Acoustic metrics including Reverberation Time, Speech Transmission Index and clarity were compared for 11 scenarios. Results from the acoustical modeling showed that, comparing 11 different scenarios, rooms treated with absorption performed "best" on multiple metrics (Reverberation Time, Clarity, Speech Transmission Index/Intelligibility). Future research will be implemented within existing operating rooms of our hospital partner for in situ validation. Acoustical measurements will again be taken and evaluated against the initial findings and the perceptions of the users of the spaces.