Including Reverberation Time Research Articles

The Acoustical Characteristics of the Kocatepe Mosque in Ankara, Turkey

This research investigates the acoustical characteristics of mosques of the classical Ottoman period and the contemporary period. Kocatepe Mosque, the major mosque in Ankara, Turkey, as a case from the contemporary era is a unique combination of 16th century Ottoman aesthetics and 20th century technology. Although previous and recent mosques have been inspired by this combination, the use of a reinforced concrete dome without cavity resonators could cause long reverberation times especially in low frequencies. The purpose of this study was to investigate this condition by studying Kocatepe Mosque. The acoustical characteristics of the mosque were analyzed by computer simulation. Objective room-acoustic indicators including reverberation time (RT), early decay time (EDT), clarity (C80), definition (D50), lateral fraction (LF), speech transmission index (STI) and strength (G) are presented. The results show that the acoustical quality of Kocatepe Mosque is not optimal when it is empty, and closest to optimal conditions when fully occupied. The new techniques to overcome excessive low frequency attenuation caused by huge concrete shells of today's mosques are discussed in conclusion.

Remedial acoustical design of a college band instrument rehearsal room modified because of asbestos treatment

Removal of asbestos material from an instrument rehearsal room at CSU, Sacramento, including acoustical treatment led to a new design. The fix dramatically altered the characteristics of the room and resulted in unacceptable conditions. Temporary installation of heavy velour curtains made the room tolerable, but a permanent fix was sought. The room has nonparallel surfaces, large and long, barrel-shaped diffuser panels, hard walls and floor, and a mostly hard ceiling. Field measurements were made under three conditions to quantify the room’s acoustical conditions, including reverberation time and background sound levels. One-third octave band measurements were made from 12.5 to 10,000 Hz to cover all instruments and background sources. The 1-inch acoustical treatment first added to the room was not adequate to attenuate sound below 500 Hz, and insufficient to handle the high acoustical energy produced by the band. The echo between the floor and ceiling was very distinct, as neither surface had any treatment. Calculations using two different methods showed that substantial low-frequency sound absorption was required to reduce the sound at all frequencies to the design goal. The room is currently undergoing the modification, and results may be available at the time of the presentation.

An investigation of Columbia College Chicago reverberation chamber

The Audio Arts and Acoustics department at Columbia College in Chicago acquired a new building in 2003. The facility, a former bank, contained an old steel and concrete vault that was converted to a reverberation chamber. The acoustic properties of the space, including reverberation time, modal density, and early reflection maps were analyzed and compared to a computer model. Reflectograms were predicted at various locations and compared to test data acquired with Time Delay Spectrometry (TDS). Polar Energy Time (PET) techniques were also used to identify the location of a single 4×4 sample of foam absorber and the results of the test were compared to the predicted value from the computer model. The results of the tests show that, under its current configuration, the room is usable as a reverberation chamber down to 300 Hz, and that the computer model was able to accurately predict the results from the PET methodology.

Measurement surveys of eight British concert halls

Objective measurement surveys and subjective surveys have been carried out in eight leading concert venues in the United Kingdom. In each hall a questionnaire survey of the whole audience at a concert of symphonic music was used to elicit the opinions of nonexpert listeners. Altogether over 3000 responses have been received. An objective survey of each (empty) hall has also been carried out, the parameters measured including reverberation time, early decay time, clarity index and definition. The number of measurement positions in each hall varied from 36 to 92. The questionnaire response data is to be correlated with the results of the objective measurements to investigate the relationship between objective data and the subjective impressions of nonexpert listeners. This paper will present summary data of the measurements in the eight halls and examine the variation in the values of the different parameters across each hall. The differences between the halls will also be considered. 2284,2284

Performance of an adaptive beamforming noise reduction scheme for hearing aid applications. I. Prediction of the signal-to-noise-ratio improvement.

Adaptive beamformers have been proposed as noise reduction schemes for conventional hearing aids and cochlear implants. A method to predict the amount of noise reduction that can be achieved by a two-microphone adaptive beamformer is presented. The prediction is based on a model of the acoustic environment in which the presence of one acoustic target-signal source and one acoustic noise source in a reverberant enclosure is assumed. The acoustic field is sampled using two omnidirectional microphones mounted close to the ears of a user. The model takes eleven different parameters into account, including reverberation time and size of the room, directionality of the acoustic sources, and design parameters of the beamformer itself, including length of the adaptive filter and delay in the target signal path. An approximation to predict the achievable signal-to-noise improvement based on the model is presented. Potential applications as well as limitations of the proposed prediction method are discussed and a FORTRAN subroutine to predict the achievable signal-to-noise improvement is provided. Experimental verification of the predictions is provided in a companion paper [J. Acoust. Soc. Am. 109, 1134 (2001)].

Effects of an audience and variable architectural features on the acoustics of Boettcher Concert Hall

Acoustic parameters (including reverberation time, early decay time, relative sound level, early to late sound ratios, lateral energy fraction, and interaural cross correlation) have been measured in Boettcher Concert Hall. All parameters were measured using different settings of variable architectural features such as acoustic drapes, variable height overhead stage reflectors, and movable stage towers. The monaural parameters were also measured with and without an audience. The effects of the variable features on the acoustic parameters are assessed. The effects of the audience on some of the acoustic parameters have been estimated using standard calculation methods. Calculated differences are compared to those actually measured in the hall. [Work supported by the Concert Hall Research Group.]

Cost effective variable acoustics for multipurpose halls: 51 Walden Street.

51 Walden Street is a small multipurpose hall in Concord, Massachussets that serves for drama, symphony orchestra, concert band, and various other performance and rehearsal activities. As in many halls designed for a wide range of uses, limited passive variable acoustic provisions had been implemented with qualified success. However, recent measurements of the resulting acoustical conditions indicated that better acoustics required considerably more variability than possible using these simple techniques, which included varying the amount and positioning of sound absorption and reflecting surfaces within the hall. More complex passive variability, such as varying the room volume, was judged as not feasible because of the obvious architectural and cost limitations typically faced by community arts organizations. An experiment was carried out using a novel digitally controlled electroacoustic system to vary selected acoustical parameters, including reverberation time, early decay time, and clarity. Working with the major tenant groups during a season of rehearsals and performances, it was possible to correlate their judgments with objective acoustical measures. The flexibility and ease of electronically adjusting these parameters, as well as several new measures described, offered the opportunity to study their subjective significance efficiently.

Acoustical measurements in rooms with variable acoustics.

A dedicated, computer-based system was developed to perform a complete set of acoustical measurements of recent interest in full-sized rooms and in scale models of rooms. The measurements included reverberation time, early reverberation time, loudness, early-to-late temporal energy ratios, lateral energy fractions, interaural cross correlation, and speech transmission index, among others. Measurements were made at multiple locations in four large multiuse halls with different variable acoustics features. Measurements were made with and without an orchestral shell in place, with a concert wall in use, with a ceiling lowered to close off the upper part of a balcony, with curtains raised and lowered on the walls of a room, and with a curtain used to close off the rear position of a large room. Changes in the various measurements at equivalent seating locations will be presented. The magnitude of the changes are compared to changes perceived by listeners of various acoustical qualities in a recent study [R. P. Cervone etal., J. Acoust. Soc. Am. 89, 1857 (A) (1991)]. [Work supported by NSF.]

Subjective and objective evaluations of classrooms and lecture halls

Speech intelligibility tests were given to groups of college students in ten classrooms and lecture halls of various sizes on the University of Florida campus. A series of acoustical measurements including reverberation time, early reverberation time, loudness, early to late temporal energy ratios, lateral energy fractions, interaural cross correlation, and speech transmission index were made at multiple locations in each of the rooms. Correlation analysis and statistical modeling identified significant relationships among intelligibility scores of the listeners with the physical measurements made in the room. Special attention was given to the architectural characteristics of the rooms in the analysis. The variations in intelligibility scores and the physical measurements among the different rooms and within each of the rooms are presented. [Work supported by NSF.]

Subjective and objective evaluations of rooms for music

A dedicated, computer-based analysis system was developed to perform a complete set of acoustical measurements of recent interest in full-size rooms and in scale models of rooms. The measurements included reverberation time, early reverberation time, loudness, early to late temporal energy ratios, lateral energy fractions, interaural cross correlation, and speech transmission index among others. Measurements were made at multiple locations in ten large concert halls. Groups of listeners evaluated live music performances at three locations in each of the rooms using a seven point semantic differential rating scale. Correlation analysis and statistical modeling identified significant relationships among the qualities of the music in the room rated by the listeners with the physical measurements made in the rooms. Variations of subjective qualities were identified among the different rooms and within each of the rooms as well. The subjective qualities that contributed to overall acoustical impression were also identified. [Work supported by NSF.]

A comparison of three classical concert halls

Values of newer auditorium acoustics quantities are presented for measurements in three well-known classical concert halls: the Amsterdam Concertgebouw, the Vienna Grosser Musikvereinssaal, and the Boston Symphony Hall. The measured octave band quantities included reverberation time, early decay time, sound strength or level, early/late sound ratios, and lateral energy fractions. Hall average values from the measured unoccupied conditions are presented as well as estimated occupied values. The variation of parameters with both source and receiver position is examined in detail. The results help to define the range of conditions that are to be expected in good concert halls, and reveal some of the detailed differences among these halls.

Acoustic Measurements in School Classrooms

Acoustic measurements were made in the classrooms of twenty-two different school buildings in a study sponsored by the Acoustical Materials Association. Interior acoustical treatments in the rooms included full ceiling treatment, peripheral treatment, and no treatment. Measurements included reverberation time, definition, percentage articulation, transmission loss and noise reduction between rooms, followed by a questionnaire survey of the teachers who use the classrooms. The questionnaire returns indicated some interesting teacher impressions which will be reported, although not enough teachers were polled to treat their answers in a statistical manner.

A Tone Burst Generator

A tone burst generator is described for the production of electronically keyed oscillations free of transients. The unit consists of an oscillator, a keying circuit, and a negative pulse generator with variable duration and repetition rate. The advantages of the tone burst method in testing the frequency and transient response of amplifiers and transducers are pointed out. Some applications including reverberation time measurements are also mentioned. The almost instant rise and decay of the tone bursts was achieved by providing practically zero impedance path to ground for the keying pulses.