Noise Control Measures Research Articles

Sound attenuation in forests

Many measurements of sound attenuation rates in forests have been made but there is little in common in the measuring procedures used or the results obtained. Consequently there is a considerable divergence of opinion on the effectiveness of vegetation as a noise control measure. In this paper the factors controlling the transmission of sound through vegetation are examined and the attenuation rates achieved in pine plantations are presented.

Helicopter airframe vibration transmission modeling using statistical energy analysis (SEA)

Predicting helicopter cabin noise is a complex issue. It involves sources which are both acoustic/aerodynamic and vibratory in nature as well as a compact, highly interconnected airframe structure. Energy transmits to the cabin via both airframe structureborne and airborne paths. Since a large number of resonant structural and acoustic modes are excited, a statistical approach, which treats the response of the modes collectively in groups, is desirable. The method described for modeling the helicopter airframe and cabin interior is Statistical Energy Analysis (SEA). An SEA model, funded by a NASA Langley Research Center contract, is presented for the Sikorsky S‐76 (a4‐13 passenger commercial helicopter). This overall source‐to‐receiver modeling approach allows for evaluation of various noise control measures in the conceptual design stage. Expressions have been developed for SEA parameters and the coupling loss factor has been related to the more familiar vibration transmission coefficient. A full scale measurement program was conducted on the Sikorsky S‐76. Data are presented showing the SEA parameters and comparisons with the analytic model. These data include vibration transmission along the beam/panel structure and transmission through intermediate acoustic spaces connected by both resonant and nonresonant conditions. In addition, data are presented concerning the effects of coherent and noncoherent source excitations at distant response points. [Work supported by NASA Langley.]

Noise Control on Diesel Tugs—A Sequel

This paper is a sequel to "Noise Control on Diesel Tugs" published in the October 1973 Marine Technology. It reviews fundamental principles of tugboat noise control, including new construction specifications. Noise levels on four classes of new or repowered tugs are presented, including comparison of vessels with and without main engine isolation. The problem of backfitting noise control measures to existing vessels is discussed in detail. An experiment is reported in which consecutive measures were fitted in six steps to evaluate the effectiveness of each measure. The problem of deck noise and noise standards being considered by the U. S. Coast Guard is discussed. The paper is illustrated with tables of noise levels and shows typical vessel arrangements. A short bibliography is included.

Noise control measures in the new Singapore International Airport

This paper describes some of the noise control measures incorporated in the passenger terminal building at the new Singapore International Airport at Changi. As the acoustics adviser to the Changi Airport Development Division of the Public Works Department, the overall consultant of this multi-billion dollar project, the author reviews some of the special steps taken to incorporate some of the latest acoustics techniques in this airport. These include the establishment of norms for various factors such as permissible background noise levels to each of the rooms, offices, arrival and departure halls, etc. Specifications called for include optimum sound absorption and reverberations times, airborne sound insulation for internal and external building partitions and structureborne sound insulation, as well as a sophisticated sound reinforcement system. Because of the stringent requirements following some of these specifications, the factor of the skill and workmanship of the contractors involved in the erection of the partitions had to be allowed for. As a result, all suppliers and contractors were requested to send their building proposals for testing. This was done in the newly established Acoustics Laboratory of the National University of Singapore at Kent Ridge. Some of the tests carried out, based on International Standards ISO R140 and ASTM E90 on sound transmission loss and ISO R354 and ASTM C423 on sound absorption, are evaluated.

The acoustics of the new Changi International Airport in Singapore

A previous paper has described the general acoustic design of the new international airport in Singapore and some of the noise control measures adopted. This paper reports on some of the subsequent field tests and measurements carried out in the main areas of the airport since the completion of the actual construction works and prior to the opening of the airport to the public. Sound transmission measurements were made following ISO/R140 involving field tests of both the facade as well as the internal partitions. Special sound generating equipment providing up to 110 dB of white noise was used in some of the measurements. This was necessary in view of the high sound insulation values designed in some parts of the building facade. Reverberation time measurements were obtained following the recommendations of ISO/R354 and room absorption values obtained using Sabines' formula. Background noise levels were measured both prior to the opening of the airport to the general public as well as afterwards to study ...

Methods for state and local governments to use to control noise from construction sites

State and local noise control officials, citizens' groups, construction equipment manufacturers, construction contractors, and unions that represent construction workers were consulted nationally. The investigation was undertaken after it was reasonably clear that in the near future the Federal government is unlikely to issue new regulations controlling the noise levels from construction equipment. The manufacturers reported that the primary impetus for noise reductions came from the need to reduce exposure to occupational noise and from the need for quiet equipment to compete successfully in Europe. The most effective noise control measures are in-use controls. These include hours of operation, operation of equipment with all noise control components in place, location of equipment on the construction site, use of alternate equipment, rerouting of vehicles, property line standards, and operator concern. The marginal cost of adding a construction component to an existing municipal noise control program was calculated. [Work supported in part by U.S. Environmental Protection Agency.]

Mechanical mobility methods for ranking structural transmission path strengths

This paper examines the utility of mechanical mobility methods for ranking the vibration transmission path strengths in complex fluid-structure radiation problems. Such path-strength determinations are an important factor for selecting appropriate noise control measures and in minimizing the economic impact of corrective action. Emphasis is placed on the direct equivalent force-moment method which is based on the identification of multiple source inputs (i.e., level, location, and degree of freedom), and the characterization of radiating surfaces (i.e., level, location, degree of freedom, type, and coherence with other outputs). This method is shown to serve as a means of experimentally verifying finite element models and as an aid in modeling source-forcing functions.

Spatial statistics of transformer core noise

The tonal nature of the core noise, the difference in pathlength and the different phasing of the sound radiated from the various tank surfaces result in complex spatial interference patterns which characterize the farfield which can be described only in statistical terms. This paper deals with the consequences of the statistical nature of sound field in regard to: (1) Developing meaningful techniques to measure farfield transformer core noise, (2) predicting the expected value and spatial statistics of the farfield noise on the basis of near/field measurements, and (3) defining meaningful performance evaluation measures to determine the effectiveness of such retrofit noise control measures as barriers and enclosures.

Assessment of noise annoyance: The introduction of a threshold level in Leq calculations

A modification of L eq is suggested. The modification is based on the simple fact that noise below a certain threshold may not be heard, and consequently cannot contribute to the annoyance. Results from laboratory experiments show a reasonably good correlation with subjective evaluation. The proposed index may give a new basis for the understanding of the effects of noise control measures.

Utilization of community noise attitudinal survey data to effect program policy changes

A community noise assessment survey was conducted in Salt Lake City, Utah to estimate individual and group variations in attitudes and responses toward noise. The data generated from the survey provided local noise control officials with an evaluation of the existing noise program, identification of specific problem sources of noise in the city, identification of noise control measures supported by community members, an assessment of the health and welfare effects of noise on residents, and perceptions of noise as an environmental hazard in the community. By stratifying the population of the city according to 14 homogeneous noise zories and eight geographical areas, the needs of various sections of the community became apparent. An analysis of the public's awareness of the existing noise control program and the effectiveness of that program was used to influence program policy changes. Based on survey data, certain geographical locations and noise zones were identified as in need of program strategy changes in the areas of public awareness, land use planning, and enforcement. The survey allowed local noise control officials to establish a comprehensive noise abatement plan addressing critical areas in need of immediate attention and long‐range program goals.

The effects of external lagging on low frequency sound transmission through the walls of rectangular ducts

Previous work by the author [1], on the transmission of internally propagated acoustic noise through the walls of rectangular ducts, is extended here in an investigation of the effects of external “lagging” (consisting of a layer of porous sound-absorbing material, and an impervious external covering) on the duct walls; this type of treatment is commonly applied as a noise control measure. A simple theoretical model, based, as before, on a coupled acoustic/structural wave system, is devised and shown to give reasonably accurate predictions in comparison with measurements of the wall transmission loss (though not in the case of lagging in which an external covering of very non-uniform thickness is incorporated). The conclusion is reached that external lagging used as an acoustic treatment is not, in general, particularly satisfactory.

History of architectural acoustics and noise control in Hawaii

This paper traces the history of architectural acoustics and noise control from ancient to modern Hawaii. It begins with Hawaiian folklores on noise and concludes with suggestions on future studies on natural ventilation system with high noise transmission loss, building design and community planning for noise control. Between these limits, the paper describes an indoctrination program which eventually led the City and County of Honolulu Building Superintendent to require architects to include acoustic and noise control measures in their design of public schools and municipal buildings. Some of the changes in architectural design which followed are described. The paper also considers laws and ordinances regulating noise at the state and municipal levels, and related problems involving enforcement and conviction in the court.

Noise control approaches in the design of thermal power plants

From the working environment and community nuisance aspects, the sound levels in power plants are high enough to warrant noise control measures. In many cases they are required by law. Since retrofit measures are often expensive, inconvenient, and sometimes impractical, noise control features should be incorporated in the initial design. Features such as topography of the plant site, general arrangement of equipment and plant architecture could be effectively used for control of noise in addition to specifying low noise machinery.

Noise exposure and control on fixed marine structures

Machinery noise sources on drilling and producing platforms include engines, turbines, gears, generators, pumps, and compressors. Noise transmission is both structure borne and airborne. Exposure evaluation requires consideration of work shifts ranging up to 12 hours per day, seven days in a row, as contrasted to the typical on-shore 40-hour work week. Exposure time is not limited to the work shift for those who must live on the structure. The design problem is to avoid or eliminate excessive noise levels. If this is not feasible, noise levels are minimized both as to intensity and the physical area affected by use of quiet machinery. Enclosure and/or other acoustical treatment is then used to bring exposure within acceptable limits. Off-duty areas and crew quarters are placed in the quietest available location, and isolated from structure-borne and airborne noise. Examples of noise sources and control measures are illustrated by case histories.

Computer predictions of refinery noise levels

The predictions of a computer model [S. H. Judd, S. L. Dryden, and L. Tornheim, “Development of a community noise prediction model” and S. H. Judd and S. L. Dryden, “Application of a computer model for predicting remote noise levels”, J. Acoust. Soc. Am. 58, S16(A)(1975)] agree closely with the measured noise levels around a major refinery. This is illustrated by octave band and A-weighted results. The construction of major new facilities which partially shield the existing plants has provided attenuation consistent with the algorithm used in the model as shown by before and after data. The model is being used to verify design predictions of source sound power levels and to assist in the selection of noise control measures.

Noise control in new plant design

A noise control program for new plant design has been implemented by an engineering‐construction firm. The results of the program indicates that when designers and specifiers of equipment are aware of noise objectives, noise control measures are implemented more economically. In the design of a 100‐million‐dollar industrial complex, corrective measures were taken during the feasibility stage. During the procurement phase, competitive equipment was evaluated based on its effect on the acoustical environment. In several instances, the specifier of the equipment worked closely with the equipment manufacturer. This relationship, in the selection of fans, led to the use of slower speed fans with different blade types. The emphasis placed on motor noise paved the way for one manufacturer to offer a “quiet line motor.” By changing the motor insulation and redesigning the motor cooling fan, a significant noise reduction was achieved. A comprehensive computer program system was developed to help the design engineer evaluate several alternatives. The results of the program and its effect on plant costs will be presented.

Application of a computer model for predicting remote noise levels

The prediction of noise levels at selected remote locations is an integral part of estimating the environmental impact of new stationary sources or of noise reduction for existing facilities. A three-dimensional computer model, with provisions for temperature and humidity variation and for solid barriers and semitransparent arrays such as process plants or forests, predicts the composite noise level produced at remote locations by many sources. The output also shows the individual octave band contributions of the more important sources, ranked by their A-weighted level at each observer location. These features enable the model to be used for a variety of purposes, including Environmental Impact Statements, site selection and layout of process plants, determination of required noise limits on new equipment, estimation of the effect of major noise barriers, determination of the feasibility of meeting noise ordinances, and optimization of cost-effectiveness of noise control measures.

Case study of single industry in a residential neighborhood

A cardboard carton manufacturer attempts to be a good neighbor in an otherwise residential section of a large city. The city's environmental noise department establishes that the lot line noise levels produced exceed the noise ordinance by as much as 25 dB in some actaves, takes the company to court, and obtains an injunction. A noise control engineer is hired to help the company reduce the noise levels in a manner that will please the city, the neighbors, and the company's owners. This case study describes the noise control measures and the legal steps that ultimately led to a satisfactory solution of the problem without closing down the plant.

Noise control design guide for the food‐processing industry

Noise problems in food‐processing plants have evolved naturally, chiefly due to progressive mechanization and automation in the industry. As a result, plant operators have, for a long time, been concerned with excessive personnel noise exposure and individually have attempted to introduce noise control wherever possible. However, no industry‐wide effort to the solution of common noise problems was attempted in the past. The control of equipment noise in this industry is made especially difficult due to the strict standards of sanitation that must be preserved throughout each plant. This paper presents a brief overview of an industry‐wide program, sponsored by the Northwest Food Processors Association, which resulted in the development of controls for six major noise sources in the industry. The results are contained in a design guide which takes the process engineer through the basic steps necessary to measure, evaluate and incorporate the proper noise control measures that will provide compliance with the OSHA regulations. The solutions presented include the following equipment or processes: (1) vibrating conveyors, (2) blanchers, (3) positive and negative air systems, (4) high pressure air and steam, (5) can handling systems, and (6) seamers. Typical solutions are presented and described.

Isosonic Templates for Plant Layout

Abstract Using critical noise lines of equal loudness surrounding production equipment, the best relative position of such equipment can be determined to ensure an acceptable noise level environment. In the facilities design stage, noise levels can be optimized along with all the other plant layout objectives. As a corrective measure, isosonic templates can indicate the minimal relocation necessary, and identify the points at which further noise control measures must be taken. By recording the percentage of time an operator spends in each noise zone, and considering the additive effect of adjacent noise sources, the total cumulative operator noise exposure can be determined.