Reverberation Time Values Research Articles

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.

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.

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.

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.

Research and sound quality analysis of little theaters in Guangdong universities

Little theaters have become a new type of theater form with their low investment costs, diverse space designs, and rich performance genres. In this paper the building acoustic design is focused on as a key point to research the audience hall space, the spatial shape, stage form and audience hall seating capacity in little theaters, and it is explored that the applicability of such new audience hall spaces to domestic theater design guidelines. One of the typical little theaters is studied by ODEON simulation. The simulation results show that the mid-frequency value of reverberation time (T30) is higher, which is not meet with the mid-frequency range of the domestic theater design guidelines for reverberation time.

Case study of sound insulation performance of a first pure timber 3-story elementary school in Japan

In Japan, "the Act on Promotion of Use of Wood in Buildings to Contribute to the Realization of a Decarbonized Society" promotes the use of wood in buildings in general. The school buildings as public buildings are being promoted to be constructed with timber construction since timber school buildings effectively improve the educational environment. This paper presents the measurement results of the sound insulation performance in "the Uozu City Municipal Hoshinomori Elementary School", which is a first all-timber construction 3-story elementary school in Japan. By adopting a dry-type double floor structure for the floor finish structure and an independent ceiling for the ceiling structure, the floor impact sound insulation performances of the classrooms were Lr-60 for both heavy-weight floor impact sound and light-weight floor impact sound when evaluated using JIS A 1419-2. Moreover, the recommended reverberation time values of the Architectural Institute of Japan Environmental Standards were generally satisfied by installing dressed rockwool sound absorbing board on the ceiling.

ACOUSTICAL ANALYSIS FOR THE LECTURE ROOMS IN UNIMAP

Acoustic analysis is a measurement used to analyse the behaviour of sound waves in enclosed spaces, which influences speech intelligibility. Another parameter that influences speech intelligibility is reverberation time. Reverberation time is the time for the sound source to decay by 60 dB. A high reverberation time causes sound to dissipate more slowly, resulting in continual reflection of sound waves, which disrupts student concentration in class. Furthermore, a poor acoustic environment could have an impact on health and lecture delivery. The objective of this study is to determine and analyse the acoustic performance for five selected lecture rooms in UniMAP. The reverberation time is the parameter of this study that was obtained from the Root Mean Square (RMS) of the sound pressure by using the impulsive sound source method from a burst balloon. In this study, the "balloon pop" sound pressure was consistently recorded at approximately 105dB across all cases. This measurement indicates the peak sound intensity of the stimulus. The influence of the location of receiver, design, and space volume for the lecture room on the reverberation time was investigated. At a location of 6 m from the sound source, at the back wall of a room with chairs, BKN 5 and BPU 5 measured longer reverberation times of 1.3 s and 1.2 s, respectively. The higher value of reverberation time is caused by where the sound receiver is. If it is close to a wall, the sound receiver will be exposed to numerous reflections of sound waves, causing the room to become reverberant. Compared to other lecture rooms, the length of BKN 5 and BPU 5 is shorter. Their lengths are 11.639 m and 11.689 m, so the sound receiver is closer to the wall and makes the reverberation time higher. BKN 5 room, which has a volume of 258.3 m3, had the highest reverberation time (1.2 s on average in a room with chairs). This is because it is larger than the other rooms. So, the reverberation times get longer as the room's volume increases. BPU 5 room had the longest reverberation time for a condition room without chairs (an average of 2.4 s), but it also had the smallest space volume (242.4 m3) compared to the other rooms. Thus, it takes longer for the sound waves in the room to fade away when there are no chairs there. If the smaller room didn't have enough sound-absorbing materials, the reverberation times would be longer.

Teachers’ and Students’ Assessment of the Influence of School Rooms Acoustic Treatment on Their Performance and Wellbeing

The paper presents the latest research results concerning the correlation between changes in the room acoustics of school spaces and noticeable changes in the communication and functioning of students and teachers at school. The primary school covered by the research is the second largest school of this type in Poland. The large number of students and hard interior finishing made the acoustic conditions in the school building very unfavourable. The measurements showed that school rooms were very noisy and reverberant. The measured values of reverberation time T were in many rooms 3–4 times higher than the acceptable values specified in the mandatory Polish acoustic standard PN-B-02151-4:2015-06. Also the speech intelligibility measured by the speech transmission index was very poor, in the extreme case STI = 0:31. This situation (very characteristic for most of Polish schools) became the basis for the first such comprehensive acoustic treatment of the whole school building in Poland. This intervention allowed to meet PN-B-02151-4:2015-06 demands almost in every room accessible for students. This case gave an excellent opportunity to assess the influence of improved room acoustics on teachers’ and students’ performance and wellbeing. Measurements of the equivalent sound level LAeq, reverberation time T and STI speech transmission index were made before and after acoustic treatment. The questionnaire survey used the Acoustic Change Feelings Scale (ACFS-S, ACFS-T) for teachers and students. 378 students, and 44 teachers were included in the study. Both students’ and teachers’ answers show significant improvement of their performance and wellbeing. Positive changes were noticed in students’ level of concentration, short memory capacity and pace of work. After acoustic treatment students (both in teachers’ and their own opinion) can better hear and understand teachers’ instructions and are much more capable of task fulfilling. Both teachers and students observed clear reduction of aggression level. Teachers reported considerable drop in students’ fatigue and their own voice effort.

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.

THE ASSESSMENT OF ACOUSTICAL PERFORMANCE OF MOSQUES IN SHAH ALAM

A mosque is a place where Muslims conduct activities such as praying and listening to talks. Therefore, it is important for a mosque to have great acoustics quality as it influences the worshipper’s experience when praying or listening to Imam’s preaches. The acoustical evaluation in the mosque is rarely conducted and, in most cases, the development of good acoustic quality in the mosque is never considered at an early stage of design. In the present investigation, two large-volume mosques in Shah Alam were investigated: Section 7 Mosque (MS7) and Sultan Salahuddin Abdul Aziz Mosque (MSA). The objective of this study is to evaluate the acoustical characteristics of the mosque and its quality level. In addition, the speech intelligibility performance of each mosque was also determined using Speech Transmission Index (STI) and Rapid Transmission Index (RASTI). The experimental work found that the reverberation time of MSA has attained recommended values of reverberation time for that given size and volume while MS7 has slightly higher. The speech level attainment of both mosques is sufficient for worshippers to hear comfortably, but the background noise is high. Nevertheless, the signal-to-noise ratio (S/N ratio) indicates that both mosques have been able to obtain more than 15 dB(A) for improved speech intelligibility. The STI and RASTI prediction shows that MS7 has attained ‘fair’ and ‘bad’ ratings. In contrast, ‘poor’ and ‘bad’ ratings are achieved in the MSA.

Sound propagation and strength in small classrooms

The study involved 29 occupied primary-school classrooms in Italy where speech level (LS) and Sound Strength total (Gtot), early (G50) and late (Glate), have been acquired at 1 m from the source and in other three positions along the main axis of the room. The background noise level during the measurements was less than 56 dB(A) and the reverberation time (RT) was between 0.5 s and 1.4 s. A cut-off value of RT was assumed equal to 0.8 s for subdividing the classrooms in two groups of similar size, with lower or equal and higher RT values, respectively. The slopes per double distance of the speech level along the axis, mLS, is −2 dB/dd for both the groups. The same slope per double distance is obtained for G50 of −2.7 dB/dd, while Glate shows a slope of −0.6 dB/dd and −0.9 dB/dd for the two groups, respectively. The slope per double distance of Gtot is −2.0 dB/dd as in the case of the speech level, mLs, as expected. Optimal reverberation time range, between 0.6 s and 0.9 s, in occupied conditions, has been obtained from the difference Glate-G50 at the rear positions between 6 and 3 dB, respectively.

Objective and subjective assessment of the speech intelligibility in rooms with an Active Acoustic Enhancement Systems

Speech Transmission Index (STI), apart from the Reverberation Time (RT), remains an important parameter used for the speech intelligibility assessment indoors. As the Active Acoustic Enhancement Systems (AAES) are getting more and more commonly used, it was verified how such systems affect speech perception. In accordance with the 3382-1 and 3382-2 regulations, objective parameters were measured in two enclosures with the AAES. The RT values varied from 0.91 to 1.61 [s] for the hall in Mińsk Mazowiecki and from 0.75 to 1.52 [s] in Puławy. The STI values varied from 0.57 to 0.64 in Mińsk Mazowiecki and 0.64 to 0.69 in Puławy. Despite the significant differences in the RT values, STI coefficients remained almost unchanged. The listening test was designed to verify if the subjectively assessed intelligibility would confirm the objective measurements' results. 50-element nonsense words (logatome) lists were used for in-situ experiments and, in the second part, binaural laboratory arrangement. Both parts involved over 120 listeners. Three scenes were selected: without AAES and two different AAES setups. In summary, subjective measurements' results were compared with the obtained STI, RT and Clarity (C50, C80) values to prove their importance in indoors speech intelligibility evaluation.

Assessment of Acoustic Properties of Lecture Theatres in Ladoke Akintola University of Technology, Ogbomoso, Nigeria

Acoustic design is an important consideration in the design of lecture theaters (LTs); it has a significant impact on communication between students and instructors, their hearing comfort levels, lecture outputs and their overall performance. However, despite the significant role acoustic design plays in LTs, it has not received a significant attention both in design and literature in the developing countries especially Nigeria. This study is hence aimed at assessing the acoustic properties of lecture theaters in Ladoke Akintola University of Technology (LAUTECH) Ogbomoso with a view to establishing the design requirements for effective acoustic comfort in lecture theatres in Nigeria. Seven (58.33%) lecture theaters were randomly selected out of a total of 12 available in the study area and were objectively analysed. Shape of the lecture theaters, wall, floor, ceiling and furniture surface finishes were physically observed and their sound absorbing coefficients were evaluated and compared with data obtained from the Bureau of standards for the sound absorption coefficients, ISO 354 and literature. The total surface absorption coefficients of materials and surface area were used to calculate the Reverberation Time (RT) using Wallace Sabine model and results was compared with RT standard of between 0.5 and 1.0s recommended by scholars Odoh and Urenyang for LTs in Nigeria. The study observed that walls, floor and ceiling surfaces were treated with materials of low sound absorbing coefficients while only the ceilings were treated with high coefficient materials, the effect was observed in the RT values obtained. It was observed that only 3 (42.86%) LTs have their RTs within the recommended standards and hence are capable of providing good acoustic comfort for users while 4 (57.14%) LTs have excessive RT implying likely acoustic discomfort. The study recommended that materials with high sound absorbing coefficients be used for building component finishes and acoustic decisions should be taken right from design inception stage.

Calibrating the Sabine and Eyring formulas.

Of the many available reverberation time prediction formulas, Sabine's and Eyring's equations are still widely used. The assumptions of homogeneity and isotropy of sound energy during the decay associated with those models are usually recognized as a reason for lack of agreement between predictions and measurements. At the same time, the inaccuracy in the estimation of the sound-absorption coefficient adds to the uncertainty of calculations. This paper shows that the error of incorrectly assumed sound absorption is more detrimental to the prediction precision than the inherent error in the formulas themselves. The proposed absorption calibration procedure reduces the differences between the measured and predicted reverberation time values, showing that an accuracy within ±10% from the target reverberation time values can be achieved regardless of the absorption distribution in a room. The paper also discusses the oft neglected air absorption of sound, which may introduce considerable bias to the measurement results. The need for an air-absorption compensation procedure is highlighted, and a method for the estimation of its parameters in octave bands is proposed and compared with other approaches. The results of this study provide justification for the use of the Sabine and Eyring formulas for reverberation time predictions.

Preliminary Archaeoacoustic Study of Kanheri Caves in Mumbai (Maharashtra, India)

Here we report first ever study on acoustical evaluation of Kanheri Caves located in Sanjay Gandhi National Park, Mumbai (Maharashtra, India). These caves are dated to a period between 2nd century BCE to 7th century CE. In this study we used an ambisonic recorder to capture Impulse Response, which carries acoustic signature of the place. Out of total 109 caves 41 were surveyed in available time. Out of those reverberant environment was noted in 12 caves. Measurements were made only in 3 caves (Cave Nos. 1, 3, 11) which are important. In the beginning we carried out an FFT analysis. We then studied room acoustic parameters like Reverberation Time, Early Decay Time, Clarity, Definition, etc., based on the measurement of Impulse response. Cave No. 3 have high value of reverberation time, compared to other. Therefore it also have lower clarity compared to others. It is properties needs to be compared with similar structures (chaityas) in Maharashtra (at Ajanta, Ellora, Nashik, Junnar, etc.) and elsewhere in India. It is worthwhile to carry out further research in Cave No. 3 with more sophisticated instruments as well as 3D modeling. Since the experiment was performed with receiver at only one position, we also suggest to carry out experiment with receiver at multiple positions and then comparing them.

Subjective perceptions of students and teachers to environmental noise in public schools

Formal education takes place in the classroom, where learning involves intensive verbal communication between teachers and students. Therefore, classrooms must offer the necessary conditions for good teaching and learning activities, especially teacher-student communication. It is in this context that the importance of classroom acoustics is highlighted. Therefore, this work aimed to evaluate the acoustic quality in two classrooms. To this end, the perception of teachers and students about the noise inside and outside the classroom was investigated through questionnaires. Then, the background noise levels and the reverberation time values ​​inside the classrooms were evaluated in situ. These results were compared with the assessment of the subjective perception of the questionnaires. The levels of background noise found in classrooms were above what NBR 10152 recommends. The subjective assessment showed that students and teachers perceive the noise in classrooms and consider them bothersome. According to the teachers, noise is a factor that negatively affects teaching. In conclusion, the results obtained in this work show the lack of acoustic comfort in classrooms and highlight the need for interventions.

Experimental evaluations of acoustic properties and long-term analysis of a novel indoor living wall

Indoor living walls are important decorative elements that can recreate a natural garden in enclosed spaces. Several aspects are addressed by the scientific literature such as, for instance, classification, the effects on air quality and indoor temperature, greater aesthetic and psychological impact, and improved acoustic treatment of indoor spaces. The main objective of this study is to analyse the acoustic performances of a novel indoor living wall (LW) where plants are rooted in felt layers and grown in hydroponics with no growing medium. The sound absorption of different LW solutions was measured in laboratory (reverberation room). One of them was installed in a Test Building, where sound absorption and sound insulation were measured. The LW has been installed for about two years in the Test Building under controlled conditions of temperature and relative humidity. This gave the unique opportunity to test the sound absorption of the same living wall in the same reverberation room after about two years. The results obtained indicate that the sound absorption performance of the LW is stable, as long as the plants remain in good health. Moreover, an estimation was performed with a calculation model of the reverberation time of the room containing the living wall. Evaluation in real working conditions confirms that indoor living walls can improve the acoustic comfort of indoor spaces by increasing both the sound insulation and the sound absorption area, allowing to reach the optimal reverberation time values provided for by Italian regulations.

Environmental Acoustic Comfort in the Built Environment


 The acoustic quality of classrooms has a strong influence on the teaching and learning process. This interference assessed using the impulsive technique to measure the rate of speech transmission (STI), reverberation time (RT) and sound definition (D50). These are the most relevant acoustic descriptors in the assessment of classrooms, where verbal exposure is the means of communication between teachers and students. The evaluation took place in two buildings of the Federal University of Paraná (UFPR), built in the 1960s and another in 2016. The measured values ​​of STI, provided in the classrooms' actual acoustic conditions, were used as an adjustment parameter for simulations made with the software ODEON. After carrying out the measurements and simulations, the dimensioning of improvements was possible. The acoustic simulations presented suggestions to qualify the quality of the classrooms' acoustic comfort, ensuring that teaching and learning to do not suffer losses due to the physical structure of the classrooms. The measured values ​​of STI, RT and D50 show that, in the old building, except for a single classroom that preserves the original ceiling that had a high sound absorption coefficient, it has reasonable values, below the ideal for classrooms, according to the IEC 60268-16 (2011) standard. The investigation showed that the rooms with a roof replaced by a PVC covering had a sharp drop in acoustic quality. The newest building has classrooms with proper acoustic comfort conditions.

Identification of Acoustic Comfort in Classroom of Gedung Kuliah Bersama V of Bengkulu University

Classroom is a place for a group of people to study and improve their knowledge. To make learning activities becomes more effective, a classroom is expected to have comfort in terms of thermal, lighting, ventilation, and acoustic. This study focused only on the acoustic comfort of the classrooms at Gedung Kuliah Bersama V (GB V), University of Bengkulu (UNIB). Acoustic condition in the classroom has a crucial role for student concentration that affect to student’s achievement. Reverberation time (RT), initial time delay gap (ITDG), and the possibility of echo phenomenon in the classroom will be identified in this study as the acoustic parameters. Ecotect v5.5 software is being used to simulate three types of a classroom that represent classroom types at GB V. This simulation is used to identify RT, ITDG, and echo occurs in the classroom. Results of this study showed that ITDG values of these three classrooms had met the standard the classroom that used for conversation, which is under 20 m/s. Echo phenomenon is also relatively not seen in the simulation. It means that these three classrooms were also comfortable enough in terms of acoustic. However, the RT values in the three rooms still have not reached the optimum condition, 1.40 m/s for RK 2.02, which is higher than standard and lower than standard for the first room and 0.23 m/s and 0.10 m/s for the second and third room. So thus, some design recommendations are needed to get an optimum RT value.

Split Bregman Approach to Linear Prediction Based Dereverberation With Enforced Speech Sparsity

The recordings of speech in enclosures are corrupted by reverberation caused by multipath wave propagation from the speaker to the distant microphones. In this letter, we address the problem of reducing the late part of room reverberation. The presented blind dereverberation method consists in multichannel linear prediction (MCLP) and enforces sparsity of the dereverberated speech by adopting the split Bregman approach. The proposed algorithm alternately solves two optimization problems, where the former cost function is derived by assuming that speech is modelled using a sparse prior distribution, while the latter optimization emphasizes speech sparsity by an additional incorporation of a weighted $\ell _1$ -norm of the output signal to the standard linear prediction based cost function. The results of experiments performed using simulated and measured room impulse responses for various reverberation time values indicate superior performance of the proposed sparse split Bregman (SSB) method over state-of-the-art non-sparse and sparse MCLP-based dereverberation methods in terms of standard evaluation measures and as pre-processsing to the automatic speech recognition.