Acoustic Scale Model Research Articles

Determination of The Scattering Parameters of Fittings in Industrial Buildings for Use in Computer Based Factory Noise Prediction Models: Part 1 – Theoretical Background

In this the first of two companion papers, it is established that the accepted view amongst researchers is that models in which the scatterers of sound in industrial buildings are defined in terms of a mean scattering cross section and absorption coefficient are the most accurate. At the present time there exists no method of determining these parameters. From consideration of the behaviour of sound in a proportionate live room (reverberation chamber) it is shown that the scatterers with a low mean coefficient of absorption the product of mean scattering cross section and absorption coefficient is equal to one quarter of the effective area of absorption of the fitting. The results of computer simulation employing the RAYSCAT model suggest that, although individual values of the two parameters are required as input data, it is actually the product of the two parameters which determines the sound propagation characteristics of a room containing scattering fittings. The results obtained from further simulations suggest that if the equivalent area of absorption of fittings is known, then the sound propagation characteristics of a room containing those fittings can be predicted to a high degree of accuracy by employing an estimated a value of mean absorption coefficient and a value of mean scattering cross section calculated as equal to the effective area of absorption divided by four times the estimated value of the absorption coefficient. In the companion paper which is to follow, the validity of these hypotheses is examined by means of measurements made on acoustic scale models.

The acoustic survey of British auditoria

By 1980 the understanding of acoustics for concert halls had reached an interesting stage. Two German studies at Göttingen and Berlin with dummy head recordings had gone a long way to determining the important subjective and objective dimensions. However the influence of architectural form on subjective acoustic conditions was much less well established. There were two possible routes: testing acoustic scale models or full-size auditoria. With access to sufficient full-size auditoria, the latter was the obvious choice since it also allowed true subjective assessment. The acoustic survey of British auditoria involved both objective measurements and subjective assessments in over 40 auditoria including concert halls, drama theaters, opera houses, and multi-purpose spaces. The whole exercise has now been published [M. Barron, AuditoriumAcousticsandArchitecturalDesign (Chapman & Hall, London, 1993)]. In concert halls a major conclusion of the subjective exercise was that halls with good reputations score well with regard to several attributes, such as reverberance, envelopment, and intimacy. Several novel observations on the effect of design details will also be discussed.

The use of an acoustic scale model for the design of the Escondido Civic Center 1500-seat multi-use theatre

The use of a 1/24th scale physical model will be discussed regarding how it was used to aid the acoustic design of the new 1500-seat Lyric Theatre for the Escondido Civic Center of North San Diego County. The discussion will focus on the acoustic goals of the project, how the model was tested to determine if goals were accomplished, the modifications that were performed, and final results. Impulse responses along with energy ratios were the primary measurement performed. During model testing, the emphasis was to determine how the shape of the room could be optimized within a schematic design scheme developed in concert with the project architect and theatre consultant. Numerous ceiling designs and subtle changes to the shape of the room in plan were tested for their acoustic merit. The evaluation of flat and diffuse surfaces in specific locations was evaluated to determine their influence on achieving the desired impulse response.

Scale modelling for predicting noise propagation in factories

There is a recognised need for an accurate method of predicting noise levels in factories to enable exposure levels to be evaluated and the effect of noise control measures to be assesed. Research in this field has been given added impetus in the European Community owing to legislation introduced in January 1990 limiting the exposure of workpeople to noise. Theoretical prediction methods exist and work is in progress to develop and refine them. The alternative method of physical scale modelling also has a valuable contribution to make both as a research tool and a prediction method. An account is given of an investigation to determine how accurately the sound field in a factory can be reproduced at model scale. A model of an existing factory was built at a scale factor of 1:16 and comparative measurements in the factory and its scale-model showed good agreement. The reverberation times generally correlated within 10% and the values for sound propagation with distance generally within 2 dB. The model was subsequently used to evaluate the potential of noise control measures, in particular suspended absorbers and barriers. In the case of barriers, the model results agree well with published data on the performance of barriers at full scale. This lends support, as do the other results of this investigation, to the premise that acoustic scale modelling of factories is a viable technique for investigating noise in factories and predicting the effect of noise control measures.

Videotape demonstrations of auditorium design principles using acoustic scale models

A videotape demonstration was developed to illustrate acoustical design principles for auditoria to undergraduate architecture students. An acoustical scale model of an auditorium was constructed with removable walls, floor, and ceiling. Starting with a microphone in free space, an enclosure is built in steps around the source and receiver. An echogram is recorded at each step so students can see the effect of increased enclosure on the sound that arrives at the listener's position. Theater seating, various quantities of absorbent material, acoustically shaped ceiling panels, and diffusing elements are added to the enclosure as the demonstration progresses. Echoes, excessive loudness, flutter, and other basic acoustical problems and their solutions are presented. The demonstrations show changes in the impulse response of the room as changes are made in the shape and volume of the room. It serves as an introduction to acoustical modeling and auditorium design for the students.

The graduate program in architectural acoustics at the University of Florida

The graduate program in architectural acoustics at the University of Florida offers a course of study that leads to Master of Architecture and Ph.D. degrees. The focus of the program is to address the theories and practice of acoustics in architectural settings. Core courses are offered in basic acoustical principles and the acoustical design of buildings. Students complete a major design project assuming the role of an acoustical consultant. They provide comprehensive acoustical recommendations for all aspects of a major building including room-shaping and finishes, mechanical system noise control, and the design of a speech reinforcement system. They gain experience in field measurement techniques, computer calculations, ray diagramming, and scale modeling. Additional study is pursued through individual research and thesis projects. Examples of previous projects include acoustical scale modeling, the acoustics of the case of an organ, measurements of new indices of acoustical quality in various auditoria, and studies of subjective musical quality. The program is supported by a dedicated laboratory and studio space and three faculty.

Acoustic scale modelling of factories, part III: The potential of scale models for predicting the effect of noise control measures

In this third and final part of the report on factory scale modelling, an account is given of an investigation to determine how accurately the sound field in a factory can be reproduced at model scale. A model of an existing factory was built at a scale factor of 1:16 and comparative measurements in the factory and its scale model showed good agreement. The reverberation times were generally within 10% and the values for sound propagation with distance generally within 2 dB. The model was subsequently used to evaluate the potential of noise control measures, in particular suspended absorbers and barriers. In the case of barriers, the model results agree well with published data on the performance of barriers at full scale. This lends support, as do the other results of this investigation, to the premise that acoustic scale modelling of factories is a viable technique for predicting the efficacy of noise control measures.

Acoustical scale modeling in the classroom at MIT

This is an actual demonstration of the acoustical scale modeling methods that have been used by the author for several years to help teach some basic acoustics to students in two classes at the Department of Architecture. One class in the curriculum of Building Technology, instructed by Cad Rosenberg of BBN, uses scale modeling to show how the reverberant sound within an interior space forms from numerous wall reflections. Echograms from various model room configurations illustrate simple defects that may occur in the quality of the interior sound. Also, students see demonstrated clearly the difference between the reduction of noise by surface absorption and by transmission loss. Another course attended by equal numbers of undergraduate and graduate students is instructed by Tim Johnson and John Furlong. It explores various aspects of the microclimate. One part, acoustics, is presented in the form of one lecture and a brief acoustical scale modeling demonstration immediately followed by a laboratory activity in which students investigate the control of sound from a highway into a nearby community. Students use the scale modeling instrumentation to help select the appropriate noise barrier that will give an acceptable community noise level, or/and noise reduction, at the lowest cost. There is no one solution to the problem, but students must show their understanding of acoustics by justifying their selection quantitatively.

Acoustic scale modelling of factories, part II: 1-50 Cale model investigations of factory sound fields

In this second part of a report on factory scale modelling use of a 1:50 scale variable model as a research tool is described. Details of the model are presented. The results of measurements of reverberation time and sound propagation, made in various model configurations, are used to investigate the main factors influencing factory sound fields, and the applicability of the Sabine theory to factories. The parameters investigated are the enclosure geometry (aspect ratio, volume and roof pitch), surface absorption and fittings (density, size, surface area, vertical distribution and specific types). Despite certain limitations and uncertainties resulting, for example, from surprising results associated with surface absorption, models are shown to be effective research tools. The inapplicability of the Sabine theory is confirmed and elucidated.

Acoustic scale modelling of factories, part I: Principles, instrumentation and techniques

This paper introduces a three-part report describing research into the acoustic scale modelling of factories. In it, the background to the research is presented and measures to characterize factory sound fields are chosen. The principles of factory scale modelling and its practical application are discussed. Details of transducers for use in factory models are given; in particular, an investigation of the performance of crossing air-jet noise sources is reported. The instrumentation, experimental arrangements and procedures for the modelling work are described. Finally, the experimental accuracy of the factory modelling techniques is evaluated.

Measurement of panel reflection using acoustical scale modeling techniques—Part II: Measured results

The importance of early reflected sound from side wall and ceiling reflecting panels in auditoria and other spaces used for musical presentation has been recognized for some 25 years. Quantitative data on the size, shape, and orientation of reflecting panels to achieve wideband spectral reflections have not been widely available. This paper presents some results on the effect of size and aspect ratio on the reflection characteristics of hard panels measured using scale modeling techniques.

Measurement of panel reflection using acoustical scale modeling techniques—Part I: Instrumentation and procedures

It has been recognized for some time that the size, shape, and orientation of reflecting panels used in enclosed spaces affect their reflecting characteristics. This paper describes the test chamber, instrumentation, and procedures that have been developed to measure reflecting characteristics of scale model panels.

Studies of parallel barrier performance by acoustical modeling.

An investigation is presented into the performance of parallel barrier configurations, using acoustical scale modeling. A realistic geometry is investigated, with the source being positioned over a paved roadway and the receiver over grass-covered ground. The grass-covered ground surface was properly modeled in terms of its impedance. Results were obtained for a range of barrier types, and demonstrate that frequency dependent effects are evident in barrier insertion loss data. In most cases, the barrier on the far side of the source did not significantly affect sound levels at the receiver. The most effective barrier design was found to be that of a gradual grass-covered slope up to an upright, thin barrier.

A scale model investigation of sound radiation from building elements

The directionality of sound radiated by typical building elements was investigated by using the techniques of acoustic scale modelling. The governing parameters of the model were first established and suitable modelling materials were then selected. The radiation patterns of a number of typical building elements were recorded and from a systematic examination of these patterns a predictive method suitable for implementation on a small computer was established.

Noise control for rapid transit cars on elevated structures

Noise control treatments for the propulsion motor noise of rapid transit cars on concrete elevated structures and the noise reduction from barrier walls were investigated by using acoustical scale models and supplemented by field measurements of noise from trains operated by the Port Authority Transportation Corporation (PATCO) in New Jersey. The results show that vehicle skirts and undercar sound absorption can provide substantial cost-effective reductions in propulsion noise at the wayside of transit systems with concrete elevated guideways. The acoustical scale model noise reductions applied to PATCO vehicles on concrete elevated structures show reductions in the A-weighted noise levels of 5 dB for undercar sound absorption, 5 dB for vehicle skirts, and 10 dB for combined undercar absorption and vehicle skirts. Acoustical scale model results for sound barrier walls lined with absorptive treatment showed reductions from 7 dB to 12 dB of noise from vehicles in the far track, depending on the height of the wall, and reductions from 12 dB to 20 dB of noise from vehicles on the near track. Transit vehicles at high speeds where propulsion system noise dominates are 7 dB(A) noisier at 50 ft on concrete elevated structures than on at-grade on tie and ballast. Of this amount, 3 dB is due to loss of ground effect, and 4 dB is due to the absence of undercar absorption provided by ballast.

Auditorium acoustic modelling now

Acoustic scale modelling of auditoria now has a history extending back half a century. Although progress has been intermittent, the essential viability of the modelling process has guaranteed continued elaboration of testing techniques. Modelling techniques have now achieved a certain sophistication and auditorium models have proved very valuable as consultancy and research aids. The work of Jordan and other groups involved in acoustic modelling of auditoria is reviewed briefly with a discussion of the scale factors used and the nature of testing. The current techniques for both objective and subjective model testing are discussed, concluding in each case with an assessment of the level of contribution each can make to auditorium design.

Acoustic Scale Model Study in Tunnel

Acoustic Scale Model Study in Tunnel

A theoretical analysis and experimental study of the behaviour of sound in corridors

The acoustic ray image theory is developed to predict relative sound levels along a corridor or in adjacent rooms caused by a sound source in the corridor. From experimental results in full-size corridors and in an acoustic scale model, general comments about the behaviour of sound in corridors—and the effect of small intruding walls and the position of absorbent material on sound levels in the corridor—are made.

A method to predict the transmission of sound through corridors

This paper describes research carried out to provide a method by means of which sound levels in rooms adjacent to a corridor, produced by the transmission of sound through the corridor from another room, can be predicted. This has been done by using an acoustic scale model to show how the physical parameters of the corridor affect the transmission of sound into, out of and along the corridor. From analysis of the experimental results, empirical formulae are obtained to predict: (1) sound levels along a corridor produced by a sound source in the corridor or an adjacent room, (2) sound levels along a side corridor produced by a sound source in the main corridor and (3) sound levels in rooms adjacent to the corridor produced by a sound source in another room.

The Measurement of Airborne Sound Insulation Using Acoustic Scale Models

In this paper a preliminary study of the use of acoustic scale models for the investigation of the airborne sound insulation of glazed windows is described. The techniques employed are explained and an account is given of some interesting experimental results. The techniques used allow investigation of the effects on sound transmission of building facade geometry and form of window openings, as well as the dimensions and properties of glazing materials. It is felt that the techniques described could form a useful aid for those engaged in teaching architectural/building acoustics.