Flutter Echo Research Articles

Assessment of modal density and free path distribution in central-planned halls.

Central-planned halls are highly widespread in the historical architectures of the Western world, such as rotundae, Christian baptisteries, and Roman tombs. In such halls, whispering galleries, flutter echoes, and sound focusing are the acoustic phenomena mainly investigated by scholars. Instead, modal behaviour and free path distribution are generally less treated in literature. The present study explores the modal density at low frequencies and the relationship with the most recurrent free path lengths in three historical nearly circular spaces, here assessed as case studies. Acoustic measurements allowed the collection of objective experimental data, i.e., room impulse responses and the resulting room acoustics criteria. Wave-based numerical models allowed for the investigation of the eigenfrequencies distribution, while the free paths trend has been experienced through ray-based models. The main outcomes of both analyses show the prominence of the circular modes, rather than the diametral and the elevation ones. Moreover, the mean free path calculated using ray-tracing proves to be higher than the theoretical value commonly assumed for any kind of shape. The consequent longer reverberations compared to halls with other shapes and the same volume justify the significant support historically provided to sound signals by circular halls.

Architectural improvements to increase reverberation and reduce flutter echo in two music rehearsal rooms

New music rehearsal rooms at a small college suffered from flutter echo and very short reverberation times. The architecture of these rooms included visually elegant designs which the college wanted to retain if feasible. We coordinated with the college, the architect, and a wood worker to develop concept ideas for improvement which would be cost effective and retain the elegant architectural. By using finite difference time domain (FDTD) simulation and mock-up listening tests, we evaluated the acoustic properties of these concept options. After completion of these tests and simulations, a design was implemented in each room based on acoustic performance and visual impact. We will present measurement results pre- and post-renovation, along with our methodology used for the FDTD analysis and mockup testing.

Loudness control in a small volume Recital Hall

Mississippi State University’s new music building features a 173-seat Lecture Hall that also serves as a recital hall. The limited volume presents an acoustical challenge to accommodate a variety of uses—most concerning are the louder and larger ensembles with up to 25 musicians and 50 singers sharing the stage. Due to project-specific constraints, the 24 foot stage-to-ceiling height and the 60,000 cubic foot volume are less than the early design-phase recommendations for the expected uses, which raises concerns about loudness buildup and inability to generate desired reverberation. Our goal is to implement variable loudness control systems that provide a useful reverberation range with good clarity across different settings. We identify and justify four loudness control strategies—maintain early, supportive reflections; shape the room to form patterns of sustained sound in areas with adjustable absorption; reduce high-frequency sustain in areas near audiences; avoid flutter echoes—and illustrate how those strategies were integrated into the architectural design through room shaping, fixed wall treatments, and deployable curtains and banners. We discuss subjective and instrumented observations about the loudness (measured in Sound Strength G), reverberation time (measured in T30), clarity (measured in Clarity Index C80), and overall character of the room response.

Measuring decay time in non-diffuse rooms

Measurements in non-diffuse rooms present challenges in determining decay time. A room with a severe flutter echo may exhibit a double slope decay without the incorporation of a coupled chamber. The measured decay time may differ greatly depending on the software used to measure, the choice of Schroeder integral sampling range, and how noise correction is implemented. A standardized approach to characterizing decay time under non-diffuse conditions is needed. In this talk, the authors discuss possible methods for determining the optimal noise correction and sampling of the Schroeder integral, including varying the IR truncation and sampling start and stop range. Computational optimization methods are employed to objectively determine the most reasonable settings for interpreting a particular impulse response.

Calibrating geometrical acoustics models of non-diffuse rooms

A case study is presented for the challenging case in which a conference room with a strongly non-diffuse impulse response is measured, and geometrical acoustics models are built and calibrated using two separate software packages simultaneously. Both software packages are able to reproduce the measured flutter echoes, although with surprising input scattering coefficients, particularly for parallel stainless steel wall finishes. Caution is advised concerning common assumptions about scattering coefficients for operable partitions. Furthermore, the authors suggest that benchmarking for acoustics simulation software be performed in strongly non-diffuse rooms, in addition to the existing round robin studies that have traditionally been held using more normal or diffuse rooms.

Pitch strength and annoyance of acoustic analogs of flutter echo - a pilot study

The impetus for this pilot study was the observation of flutter echoes on the aisle of a church with a barrel-vaulted ceiling. When source and receiver height were comparable, the flutter echoes consisted of a 39-msec repeating pattern of three short pulses that persisted for reverberation times of up to 5 sec. The disruptive quality of these echoes perceptually was striking. It was hypothesized that the perception of a sequence of rapidly alternating periodicity pitches might be the source of this disruptive quality. A pilot study was conducted to assess the perceived pitch, pitch strength, and annoyance of isochronous and anisochronous synthetic pulse trains involving up to three different inter-pulse intervals per pattern. Intervals of the anisochronous pulse trains were controlled to create harmonic and inharmonic relationships among the intervals, which ranged from 5-20 msec. Twelve adult college students participated in the study remotely via videoconferencing due to social distancing requirements. A modified category scaling method was used. Participants positioned a slider on a graphical user interface to reflect their ratings of pitch strength and annoyance and used a slider to adjust the frequency of a reference tone for pitch matching. Results and implications for further research will be presented.

Room Acoustical Parameters as Predictors of Acoustic Comfort in Outdoor Spaces of Housing Complexes.

Room acoustical parameters have frequently been used to evaluate or predict the acoustical performance in rooms. For housing complexes in urban areas with high population density, it is important to improve acoustic performance not solely indoors, but outdoors as well; for example on the balconies or in the yards. This paper investigates to what extent classic room acoustical parameters would be able to predict the perceived acoustic comfort in outdoor spaces (i.e., courtyards) of virtual housing complexes. Individual and combined effects of a series of independent variables (such as facade absorption, sound source, and observer position) on short-term acoustic comfort were investigated in three laboratory experiments. ODEON software was used for virtual inner yard simulation, whereby 2D spatialization was carried out for a playback over five loudspeakers. Moderate facade absorption was found to increase acoustic comfort. Relatively pleasant and relatively unpleasant sounds were associated with comfort and discomfort, respectively. Lower acoustic comfort ratings were observed at receiver positions with high sound pressure levels and/or strong flutter echoes. A further analysis of the results is carried out here with respect to the room acoustical parameters and their ability to predict the acoustic comfort ratings. Speech transmission index (STI), definition (D50), clarity of speech (C50) and music (C80), early decay time (EDT), and lateral energy fraction (LF80) were found to be significantly correlated with acoustic comfort. They were found to be significant predictors of acoustic comfort in a series of linear mixed-effect models. Furthermore, linear mixed-effect models were established with the A-weighted equivalent continuous sound level, LAeq, as a significant predictor of acoustic comfort.

Computer Modeling of Barrel-Vaulted Sanctuary Exhibiting Flutter Echo with Comparison to Measurements

Computer modeling in acoustics allows for the prediction of acoustical defects and the evaluation of potential remediations. In this article, computer modeling is applied to the case of a barrel-vaulted sanctuary whose architectural design and construction led to severe flutter echoes along the main aisle, which was later mitigated through acoustical remediations. State-of-the-art geometrical acoustics and wave-based simulations are carried out to analyze the acoustics of this space, with a particular focus on the flutter echoes along the main aisle, before and after remediations. Multi-resolution wavelet and spectrogram analyses are carried out to isolate and characterize flutter echoes within measurements and computer-simulated room impulse responses. Comparisons of simulated responses to measurements are also made in terms of decay times and curves. Simulated room impulse responses from both geometrical acoustics and wave-based methods show evidence of flutter echoes matching measurements, to varying degrees. Time-frequency analyses isolating flutter echoes demonstrate better matches to measurements from wave-based simulated responses, at the cost of longer simulation times than geometrical acoustics simulations. This case study highlights the importance of computer modeling of acoustics in early design phases of architectural planning of worship spaces.

The effect of acoustical remedies changing the reverberation time for different frequencies in a dome used for worship: A case study

Acoustic studies were carried out in a dome-shaped sacral room of the Pentecostal Congregation in Katowice in Silesia, Poland. The investigated room is used for religious purposes, and in terms of acoustics it facilitates the propagation of speech and musical sounds. In the study, acoustic measurement methods were adopted to assess sound reverberation. The “in situ” method was used to determine sound decay curves, and reverberation time was determined in line with EN ISO 3382-1: 2009, EN ISO 3382-2: 2010. Participants of the services complained about poor reception conditions of verbal and musical sounds and flutter echo in some places of the room. Acoustic studies confirmed the listeners' impressions. The room had long reverberation time. To improve the acoustic parameters of the interior, the application of spray-deposited insulating material was proposed. The selected material was subjected to laboratory tests on sound-absorbing properties. The sound absorption coefficient of the material was determined as a frequency function. Then the material was applied on the inner surface of the dome. Acoustic measurements of the adapted interior were repeated. Based on the measurements, the STI was calculated. In effect, a significant improvement in reverberation conditions of the investigated interior was obtained.

Case study: Measurement and treatment of severe flutter echo caused by a barrel-vaulted sanctuary ceiling

This case study centers on a recently constructed church with a barrel-vaulted sanctuary ceiling exhibiting excessive reverberation times and flutter echo. Reverberation times in excess of 5 s in the speech frequency range were observed at locations along the tiled center aisle, along with correspondingly elevated sound pressure levels and a highly non-diffuse sound field. Preliminary measurements, which included covering the reflective center aisle, substantiated the hypothesis that the acoustical behavior was caused by the interaction between the center aisle and the barrel-vaulted ceiling. The proposed recommendations for retrofitting the space to improve intelligibility along the aisle were modified with a view toward attaining an acceptable reduction in the reverberation times while retaining the spacious acoustical quality of the sanctuary. Additional acoustical measurements were performed prior to and following the completion of the recommended treatment. This case study describes the series of measurements and presents results of the treatment, in terms of reverberation time, impulse response decay curves, impulse response time waveforms and spectrograms, and sound pressure level measures. The unique characteristics of flutter echo due to the barrel-vaulted ceiling before and after the successful acoustical treatment are described.

The spectrum of concrete: Material research and experimentation expanding previous sound lab results for various acoustical environments

This paper presentation continues progress of an active multiphase acoustical research project emphasizing implementation of prototypical concrete systems in multiuse spaces to transform unhealthy learning environments. Building upon research results shared during the December 2017 and November 2018 ASA paper presentations, various admixtures/additives are being explored as viable ingredients to develop concrete mixtures capable of decreasing excessive reverberation and flutter echo while increasing clarity and speech intelligibility. Faculty and students, working in both physical and digital realms with industry partners, are analyzing and validating consequences of manipulating materials for resultant acoustical criteria. The work presented here showcases research and development of how concrete can be processed, shaped, surfaced, and implemented into full-scale built form to provide scholarly merit applicable in/with similar spaces and/or functions.

Forecasting flutter echo: The role of largest local Lyapunov exponents

In this article, we theoretically established a method of mapping ray movement in three-dimensional (3D) enclosures and used a ray-tracing algorithm to describe the paths along which a ray propagates in a space with a slope or Sinai geometry. Time series points extracted from the paths were used to calculate the largest local Lyapunov exponents (LLLEs). We investigated the LLLE distributions for ray systems in the two spaces. The distributions contained considerable information on ray system dynamics; this information can be used to study the evolution of ergodic properties. A comparison of the LLLEs of the ray systems in the two spaces showed that the Sinai geometry produced a diffusive characteristic, whereas the slope geometry produced flutter echo. By applying the method to building acoustics, the acoustic fields in different rooms were described. A diffuse acoustic performance was indicated by the uniformity of the sound pressure level (SPL) by simulations and experiment. The results show that LLLEs have potential for use in acoustic performance description.

Continued Summer Sound Lab Academic Research: Developing thin shell precast systems for acoustical environments

This paper presentation continues progress of an active multiphase acoustical research project emphasizing implementation of adaptive interactive acoustical responses in multiuse spaces to transform unhealthy learning environments. Building upon the December 2017 paper presentation, slotted and pocketed thin shell precast systems are being explored as a viable option to decrease excessive reverberation and flutter echo while increasing clarity and speech intelligibility within a multiuse space on campus. The project, launched Spring 2017, is spanning multiple phases and disciplines as faculty and students are analyzing and validating findings in both computer and physical form. Collected data will lead researchers to design applications for adaptive and resilient learning environments with minimized acoustical distractions and improved speech intelligibility. The work presented here defines prototypical concepts to inform final full-scale built form, providing scholarly merit applicable to similar spaces and/or functions. This paper presentation continues progress of an active multiphase acoustical research project emphasizing implementation of adaptive interactive acoustical responses in multiuse spaces to transform unhealthy learning environments. Building upon the December 2017 paper presentation, slotted and pocketed thin shell precast systems are being explored as a viable option to decrease excessive reverberation and flutter echo while increasing clarity and speech intelligibility within a multiuse space on campus. The project, launched Spring 2017, is spanning multiple phases and disciplines as faculty and students are analyzing and validating findings in both computer and physical form. Collected data will lead researchers to design applications for adaptive and resilient learning environments with minimized acoustical distractions and improved speech intelligibility. The work presented here defines prototypical concepts to inform final full-scale built form, providing scholarly merit applicable to similar spaces ...

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.

Acoustical renovation and sound system design in a church in buenos aires, argentina

Church “Nuestra Señora de la Paz” suffered from an excessive reverberation (4 seconds at mid-frequencies) and an ill-suited sound reinforcement system. Measurements of the Speech Transmission Index (STI) in the congregation area indicated that speech intelligibility was greatly impaired. Thermal insulation on the roof was insufficient and impacted in air conditioning costs. Renovations included the design of a sound-absorbing and thermally insulating covering in the roof area. Subsequent measurement of room acoustic parameters showed that the proposed goals were met. A new sound system, which increased speech intelligibility and audio quality substantially, was designed and tested. The possible appearance of acoustical defects after the renovation, such as echoes and flutter echoes, was studied using objective tests on the measured Room Impulse Responses (RIRs).

Theoretical model of reverberation decay in a rectangular room—Potential for predicting flutter echo

Reverberation is the most important factor governing the acoustic design of a room. It is known that non-exponential reverberation decay may occur in a room with unevenly distributed sound absorption, namely, with non-diffuse sound field. We propose the theoretical model of reverberation decay in non-diffuse sound fields. The reverberation decay is characterized by absorption and scattering properties of the surfaces of a room. As a first step for developing reverberation theory of non-diffuse sound field, the proposed model deals with rectangular rooms, because rectangular rooms are very common in any kind of buildings and architecture. Spatial absorption coefficients for three orthogonal directions are defined as the parameters which characterize the energy decay curve in the rectangular room. Within this model, the reverberation in such rectangular room becomes a combination of exponential and power law decay. The power law dependence as a function of time has a singularity. This issue is resolved within the model. The model was verified by comparison with the results of computer simulation using sound ray tracing method. Theoretical solutions of the proposed model agree well with the results of the computer simulation. Potential for predicting flutter echo by this model is also discussed.

Results from the summer sound lab: Exploring acoustical options for a multiuse space on campus

This paper summarizes the progress of an active multiphase acoustical research project emphasizing implementation of adaptive interactive acoustical responses in multiuse spaces to transform unhealthy learning environments. A multiuse space on campus, which suffers from poor speech intelligibility as a result of excessive reverb and flutter echo revealed through finish materials, room volume, and shape, functions as an architectural sound lab and location for data collection. The project, launched Spring 2017, will span multiple phases and disciplines for data collection, design, prototyping, validation, and construction. Faculty and students are collecting and analyzing acoustical data, adding to a literature review and database of comparable acoustical challenges and results, and validating findings in both computer and physical model form. Collected data will lead researchers to design applications for adaptive and resilient learning environments with minimized acoustical distractions and improved speech intelligibility. Their work will culminate in a final full-scale built form, providing scholarly merit applicable to similar spaces and/or functions. Established industry partnerships will provide development of physical panel-based prototyping, followed by acoustical testing, full-size fabrication, and installation for public interaction and academic validation.

Acoustic Comfort in Atria Covered by Novel Structural Skins

In large rooms, where many people gather such as atria, food courts or restaurants, relatively high noise levels can occur, due to so called “Lombard effect”. The “Lombard effect” is a reflex of vocal chords that relates to changes of the voice characteristics in noisy background. Typically an increase of the vocal intensity, the fundamental voice frequency, and the word duration will occur. Noise levels in an enclosed place are influenced not only by the amount and character of sound sources present, but also by the overall reverberation of sound in the space.This article deals with the investigation of the acoustic conditions in three large gathering places covered by glass and foils. We focus on the investigation of the influence of the amount and position of sound absorption in atria on their reverberation time and on the occurrence of various sound phenomena, such as flutter echoes, focusing of sound etc. Results of the measurements are used for calibration of acoustical models. Different architectural solutions simulated in ray-based software are compared.

Analysis and Acoustic Correction of a Contemporary Italian Church

Nowadays, architects and designers build new churches primarily considering the architectural shape to emphasize the iconographic message without sufficiently investigating the acoustic climate. In many cases, designers propose elliptical or irregular geometries for the shapes of these buildings and, in addition, the furniture of indoor surfaces have unsuitable absorption and diffusivity coefficients. Thereby, it can be observed the arise of serious acoustical problems such as standing waves, flutter echo, sound focusing and intensive late reflections (greater than 100ms) which seriously decrease speech intelligibility and diminish the effectiveness of the early sound energy. This paper presents the results of an acoustic survey on the Catholic Church “Invaluable Blood of Jesus”, situated in Ragusa (Italy), which is characterized by many problems mentioned above. During last years, some refurbishment interventions have attempted to reduce the acoustic discomfort, but an uncorrected approach has completely compromised the speech intelligibility, especially during liturgical functions. Recently, the authors have been involved to propose suitable interventions for improving the acoustic quality of this environment. Preliminarily, a measurement survey was conducted to evaluate the main acoustic indices (RT60, STI, EDT, C80, D50) and portray the current acoustic climate. After that, it was developed an acoustic computer simulation on a 3D model of the church, in order to calibrate the model comparing measured and simulated data. This procedure allowed testing the reliability and accuracy of the model.Finally, We propose two different interventions of acoustic correction. Globally it is possible to obtain an improvement of RT60 from 7.3 to 2.5 s at 1kHz and STI increases from 33% to 40%, at 1000Hz.

Recent Topics in Acoustic Scattering Coefficient Determination for Wall Surfaces

Three recent topics in acoustic scattering coefficient determination for wall surfaces are briefly presented: 1) validation of the reverberation room method of ISO 17497-1, 2) a new method of measuring normal-incidence coefficients, and 3) alternative determination by numerical simulation. First, the ISO method is reconsidered regarding unclear requirements of two approaches with stepwise and continuous rotation. It is theoretically and experimentally verified that the stepwise approach has a minimum number of angular steps, whereas the continuous approach has a minimum revolution period if using MLS signals, however if using swept sine signals, in the same way as the stepwise approach. Furthermore, the minimum requirements increase as the sample's scattering coefficient is higher, and unexpectedly, as the room's absorption area is smaller. Second, a new laboratory method is introduced for measuring normal-incidence coefficients, which would be useful to evaluate suppression of flutter echoes. The measurement is done in a rectangular room where installing highly absorbent materials on all vertical walls, and a test sample on the floor. In the one-dimensional field, normal-incidence coefficients of the sample can be determined by measuring the change in reverberation time with and without it. Third, as the alternative to measurement, numerical determination of scattering coefficients is demonstrated and further practical applications are discussed.