Van Biesel

Stiffener Shape Design to Minimize Interior Noise

Results of a research program to develop computational methods to minimize noise transmission into aircraft fuselage interiors are discussed. A design tool to perform a constrained optimization of the acoustic environment within a vibrating structure is developed utilizing e nite element methods and boundary element methods (FEM/BEM), and its application to aircraft cabin noise problems is studied. The results of a study to optimize the cross section shapes of frames and stringers of an idealized aircraftlike stiffened cylinder are reviewed. The structure is optimized for minimum noise at specie ed points in the interior, as a result of a single frequency (tonal) exterior acoustic disturbance. For the cylinder and excitation frequency studied, it has been found that spatially varying the stiffener sizes over the cylinder is more important than optimizing the shape of the cross sections. Because FEM/BEM methods are only reliable for lower frequencies, the problems studied are applicable to low-frequency tonal noise such as seen in turboprop aircraft.

Anechoic chamber qualification: traverse method, inverse square law analysis method, and nature of test signal.

Qualification of anechoic chambers is intended to demonstrate that the chamber supports the intended free-field environment within some permissible tolerance bounds. Key qualification issues include the method used to obtain traverse data, the analysis method for the data, and the use of pure tone or broadband noise as the chamber excitation signal. This paper evaluates the relative merits of continuous versus discrete traverses, of fixed versus optimal reference analysis of the traverse data, and of the use of pure tone versus broadband signals. The current practice of using widely space discrete sampling along a traverse is shown to inadequately sample the complexity of the sound field extant with pure tone traverses, but is suitable for broadband traverses. Continuous traverses, with spatial resolution on the order of 15% of the wavelength at the frequency of interest, are shown to be necessary to fully resolve the spatial complexity of pure tone qualifications. The use of an optimal reference method for computing the deviations from inverse square law is shown to significantly improve the apparent performance of the chamber for pure tone qualifications. Finally, the use of broadband noise as the test signal, as compared to pure tone traverses over the same span, is demonstrated to be a marginal indicator of chamber performance.

Design features for free‐field qualification of a new semi‐anechoic room, and qualification performance

Precision qualification of a semi‐anechoic room requires careful attention to the sound source and traversing method. Prior work, with test sources mounted above the reflecting floor of such a room, has indicated the potential for image source problems in the resulting field. To address such shortcomings, the new Georgia Tech semi‐anechoic room was constructed with a recessed enclosure in the center of the floor. This enclosure permits the implementation of test sources coincident with the reflecting plane of the floor. In addition, prior work in an anechoic room has indicated the inadequacy of qualification traverses implemented at large spacings. To address this issue, hard‐points were designed and implemented within the room to permit installation of traverse cables extending radially from the in‐floor source enclosure out to the walls and corners. These traverse cables are an integral component of a custom continuous traverse system. The design features of the chamber which facilitate chamber qualific...

Insertion loss of personal protective clothing

Abstract The use of personal protective clothing that covers the head is a common practice in many industries. Such personal protective clothing will impact the sound pressure level and the frequency content of sounds to which the wearer will be exposed. The use of such clothing, then, may impact speech and alarm audibility. A measure of the impact of such clothing is its insertion loss. Insertion loss measurements were performed on four types of personal protective clothing which utilize cloth and plastic hood configurations to protect the head. All clothing configurations tested at least partially cover the ears. The measurements revealed that insertion loss of the items tested was notable at frequencies above 1000 Hz only and was a function of material stiffness and acoustic flanking paths to the ear. Further, an estimate of the clothings noise reduction rating reveals poor performance in that regard, even though the insertion loss of the test articles was significant at frequencies at and above 1000 Hz.

The impact of sampling location on the minimization of noise in a cavity with flexible walls

Many transportation systems, e.g., aircraft and automobiles, have significant interior noise levels. There is substantial interest to reduce such interior noise levels, while minimizing weight penalties. Since the noise field inside such vehicles cannot, in general, be determined analytically, numerical techniques are commonly used to model the structural response of the cavity walls and the accompanying coupling to the interior space. These approaches may then be coupled to an optimization algorithm to determine how the walls should be designed so as to minimize the interior noise. However, a significant feature of such an approach is the need to sample the interior field at a number of points within the volume. This presentation will evaluate how the distribution of the sampling points within a volume may influence the efficiency of an optimization algorithm. The relative merits of sampling points distributed throughout the volume will be compared to points restricted to a layer near the cavity boundari...

Perspectives on anechoic chamber qualification

The qualification of a new anechoic chamber requires demonstration that the chamber produces a free‐field environment within some tolerance bounds and over some acceptable volume. At the most basic level, qualification requires measurement of sound levels at increasing distances from a test source, and then comparing the levels to a theoretical free‐field decay. While simple in concept, the actual performance of a qualification test is problematic in implementation, with troublesome issues relevant to the nature of the sound source, test signal (broadband or pure tone), spatial resolution of measurements (e.g., measurements at discrete locations or spatially continuous), and comparison of the data to a theoretical decay. This presentation will provide a brief historical perspective on chamber qualification and review current practice. It will demonstrate the inadequacy of broadband noise and widely spaced discrete measurements for qualification purposes. It will demonstrate that pure tone signals and spat...

New acoustic test facility at Georgia Tech

Georgia Tech’s Integrated Acoustics Laboratory (IAL) is a state of the art research facility dedicated to the study of acoustics and vibration. The centerpiece of the laboratory is a 24 ft×24 ft×20 ft full anechoic chamber, which has been in operation since 1998. The IAL is currently expanding to include a reverberation room and hemi‐anechoic chamber, designed and built by Acoustic Systems. These two chambers will be joined by an 8 ft×8 ft transmission loss opening, allowing for a detailed measurement and analysis of complex barriers. Both chambers will accommodate vehicles and similarly large structures. The reverberation room will have adequate volume for standardized absorption measurements. Each chamber will be equipped with dedicated multichannel data acquisition systems and instrumentation for the support of simultaneous research in all areas of the laboratory. The new test chambers are funded by a grant from the Ford Motor Company and are planned to be completed and fully functional by 1 January 2003.

New underwater acoustic tank facility at Georgia Tech

A large underwater acoustic tank facility located in the Woodruff School of Mechanical Engineering at Georgia Tech has recently been completed. The facility includes a rectangular concrete water tank 25 feet deep, 25 feet wide, and 34 feet long containing around 160,000 gallons of water. There are three computer‐controlled positioners: an x‐y‐z‐θ positioner and a z‐θ positioner mounted on carriages and a bottom mounted rotator. The facility has a large rectangular nearfield array which can be used either as a receiver or a transmitter. A single vertical nearfield line array can be translated by the x‐y positioner to synthesize a cylindrical nearfield receiving array. The rectangular nearfield transmitting array and the synthesized cylindrical receiving array were designed to be used with the bottom mounted rotator to measure the true farfield bistatic target strength of any target up to one meter in length as a function of the target aspect angle. Such measurements can be done from 2 kHz to over 10 kHz. The tank is being used for transducer development, materials, and flow noise studies in addition to structural acoustics. Several available multichannel data acquisition systems will be described. [Work supported, in part, by a DURIP grant from ONR.]

Georgia Institute of Technology—Integrated Acoustics Laboratory

The Integrated Acoustics Laboratory of Georgia Tech’s School of Mechanical Engineering is a state‐of‐the‐art facility dedicated to acoustic and vibration research. The current centerpiece of the lab is an anechoic chamber with interior dimensions of 17 ft × 17 ft × 12 ft. Laboratory instrumentation includes a 32‐channel HP VXI data acquisition system, full LMS CADA‐X data acquisition and analysis package, HP‐VEE, a two‐channel Siglab analyzer, Polytec PSV‐200 scanning laser Doppler vibrometer, and a host of microphones, accelerometers, shakers, and load cells. The laboratories also feature a brake dynamometer. Two finite‐element modeling stations are equipped with MSC/NASTRAN and COMET/Acoustics software for the complete modeling of vibration and acoustics. The laboratory will be expanded this year to include a reverberation room, which will be attached to the existing anechoic chamber by a hatch for transmission loss testing. The expansion will also add a semi‐anechoic chamber to the lab.

Source dependency on anechoic chamber validation

The ANSI S12.35‐1990 and similar ISO standard establish procedures for measuring sound power in anechoic and hemi‐anechoic chambers. In addition, the standards outline procedures for use in the qualification of the test chambers that are used for such measurements. The basic qualification procedure requires a draw‐away test from a sound source. Different sources are recommended to generate test signals over low‐, medium‐, and high‐frequency ranges. These ranges overlap at crossover frequencies, where two different sources may be used to test the chamber. For example, the mid‐range source is constructed by bolting together two 10‐cm speakers face‐to‐face. The recommended high‐frequency source is a baffled speaker radiating through a narrow cylindrical tube. The work presented here investigates how the qualification tests at mid and high frequencies are impacted by the use of sound sources fabricated to the recommended configurations, but using speakers from different manufacturers, and, for the high‐freque...