Jacob Klos

Volumetric acoustic vector intensity imager

A new measurement system, consisting of a mobile array of 50 microphones that form a spherical surface of radius 0.2m, that images the acoustic intensity vector throughout a large volume is discussed. A simultaneous measurement of the pressure field across all the microphones provides time-domain holograms. Spherical harmonic expansions are used to convert the measured pressure into a volumetric vector intensity field on a grid of points ranging from the origin to a maximum radius of 0.4m. Displays of the volumetric intensity image are used to locate noise sources outside the volume. There is no restriction on the type of noise source that can be studied. An experiment inside a Boeing 757 aircraft in flight successfully tested the ability of the array to locate flow-noise-excited sources on the fuselage. Reference transducers located on suspected noise source locations can also be used to increase the ability of this device to separate and identify multiple noise sources at a given frequency by using the ...

Human response to low‐intensity sonic booms heard indoors and outdoors

A house on Edwards Air Force Base, CA, was exposed to low‐intensity N‐wave sonic booms during a 3‐week test period in June 2006. The house was instrumented to measure the booms both inside and out. F‐18 aircraft were flown to achieve a variety of boom overpressures from approximately 0.01 to 0.06 psf. During 4 test days, 77 test subjects heard the booms while seated inside and outside the house. Using the Magnitude Estimation methodology and artificial reference sounds, the subjects rated the annoyance of the booms. Since the same subjects heard similar booms both inside and outside the house, comparative ratings of indoor and outdoor annoyance were obtained. Preliminary results from this test will be presented.

Energy finite‐element analysis of the NASA aluminum test‐bed cylinder

A formulation was developed in the energy finite‐element analysis (EFEA) for modeling the vibration of cylindrical structures with periodic axial and circumferential stiffeners. Appropriate power transfer coefficients are computed from the values of the propagation constants. The joint matrices of the EFEA formulation are computed based on the power transfer coefficients derived from periodic structure theory. EFEA analyses are performed for the NASA aluminum test‐bed cylinder and simulations are compared to experiments. Excitation is applied on the test‐bed cylinder by four shakers and the vibration is measured at 40 bays formed by axial and circumferential stiffeners. The EFEA results are compared successfully to test data between the 1/3 octave bands of 315 and 6300 Hz. [Work sponsored by NASA Langley Structural Acoustics Branch.]

Sound Transmission Through a Curved Honeycomb Composite Panel

Composite structures are often used in aircraft because of the advantages offered by a high strength to weight ratio. However, the acoustical properties of these light and stiff structures can often be less than desirable resulting in high aircraft interior noise levels. In this paper, measurements and predictions of the transmission loss of a curved honeycomb composite panel are presented. The transmission loss predictions are validated by comparisons to measurements. An assessment of the behavior of the panel is made from the dispersion characteristics of transverse waves propagating in the panel. The speed of transverse waves propagating in the panel is found to be sonic or supersonic over the frequency range from 100 to 5000 Hz. The acoustical benefit of reducing the wave speed for transverse vibration is demonstrated.

The effects of voids and recesses on the transmission loss of honeycomb sandwich panels

Sandwich honeycomb composite panels are lightweight and strong, and, therefore, provide a reasonable alternative to the aluminum ring frame/stringer architecture currently used for most aircraft airframes. One drawback to honeycomb panels is that they radiate noise into the aircraft cabin very efficiently provoking the need for additional sound treatment which adds weight and reduces the material�s cost advantage. A series of honeycomb panels was made which incorporated different design strategies aimed at reducing the honeycomb panels� radiation efficiency while at the same time maintaining their strength. The majority of the designs were centered on the concept of creating areas of reduced stiffness in the panel by adding voids and recesses to the core. The effort culminated with a reinforced/recessed panel which had 6 dB higher transmission loss than the baseline solid core panel while maintaining comparable strength.

Overview of an indoor sonic boom simulator at NASA Langley Research Center.

A facility has been constructed at NASA Langley Research Center to simulate the soundscape inside residential houses that are ensonified by environmental noise from aircraft. The purpose of this facility, the interior effect room, is to examine parameters that affect psychoacoustic response in a controllable indoor listening environment. The single room facility, built using typical residential construction methods and materials, is surrounded on two sides by arrays of loudspeakers. These exterior arrays are used to simulate aircraft noise sources that transmit into a room of a typical house. The exterior sound reproduction system, which consists of 52 subwoofers and 52 mid‐ranges in close proximity to the walls of the room, has been designed to enable study of sonic booms transmitted into residential structures and has a usable bandwidth of 3 Hz–6 kHz. In addition to these exterior arrays, satellite speakers placed inside the room are used to simulate rattle and other audible contact‐induced noise that can result from low frequency excitation of a residential house. The layout of the facility, operational characteristics, acoustical characteristics, and equalization approaches are summarized. Current research efforts utilizing the facility are described in two companion papers.

Building structural acoustic response to aircraft sonic booms

As part of the NASA Low Boom/No Boom flight test project, a series of low‐amplitude sonic‐boom tests was completed over a 3‐week period in June of 2006. This series of flight tests was designed to evaluate indoor/outdoor human subjective response, structural acoustic building response, and the effects of atmospheric turbulence for low‐amplitude sonic booms characterized by overpressures in the nominal range of 0.1 to 0.6 pounds per square foot (psf). Low‐amplitude sonic booms were generated by F‐18 aircraft using dive trajectories to produce a range of overpressures at the Edwards Air Force Base housing area. In addition, straight and level supersonic flights were used to generate normal level (nominally 1.4 psf) sonic‐boom overpressures at the housing area. This report will describe the structural acoustic building response measurements obtained during this flight test project. A single‐family ranch‐style home was instrumented with nearly 300 microphone and accelerometer sensors to determine the incident...

Radiated Sound Power from a Curved Honeycomb Panel

The validation of finite element and boundary element model for the vibro-acoustic response of a curved honeycomb core composite aircraft panel is completed. The finite element and boundary element models were previously validated separately. This validation process was hampered significantly by the method in which the panel was installed in the test facility. The fixture used was made primarily of fiberboard and the panel was held in a groove in the fiberboard by a compression fitting made of plastic tubing. The validated model is intended to be used to evaluate noise reduction concepts from both an experimental and analytic basis simultaneously. An initial parametric study of the influence of core thickness on the radiated sound power from this panel, using this numerical model was subsequently conducted. This study was significantly influenced by the presence of strong boundary condition effects but indicated that the radiated sound power from this panel was insensitive to core thickness primarily due to the offsetting effects of added mass and added stiffness in the frequency range investigated.

Modeling and Validation of Damped Plexiglas Windows for Noise Control

Windows are a significant path for structure-borne and air-borne noise transmission in general aviation aircraft. In this paper, numerical and experimental results are used to evaluate damped plexiglas windows for the reduction of structure-borne and air-borne noise transmitted into the interior of an aircraft. In contrast to conventional homogeneous windows, the damped plexiglas windows were fabricated using two or three layers of plexiglas with transparent viscoelastic damping material sandwiched between the layers. Transmission loss and radiated sound power measurements were used to compare different layups of the damped plexiglas windows with uniform windows of the same nominal thickness. This vibro-acoustic test data was also used for the verification and validation of finite element and boundary element models of the damped plexiglas windows. Numerical models are presented for the prediction of radiated sound power for a point force excitation and transmission loss for diffuse acoustic excitation. Radiated sound power and transmission loss predictions are in good agreement with experimental data. Once validated, the numerical models were used to perform a parametric study to determine the optimum configuration of the damped plexiglas windows for reducing the radiated sound power for a point force excitation.

Noise Transmission Characteristics of Damped Plexiglas Windows

Most general aviation aircraft utilize single layer plexiglas material for the windshield and side windows.Adding noise control treatments to transparent panels is a challenging problem. In this paper, damped plexi-glas windows are evaluated for replacement of conventional windows in general aviation aircraft to reduce thestructure-borne and airborne noise transmitted into the interior. In contrast to conventional solid windows,the damped plexiglas window panels are fabricated using two or three layers of plexiglas with transparent vis-coelastic damping material sandwiched between the layers. Results from acoustic tests conducted in the NASALangley Structural Acoustic Loads and Transmission (SALT) facility are used to compare different designs ofthe damped plexiglas panels with solid windows of the same nominal thickness. Comparisons of the solid anddamped plexiglas panels show reductions in the radiated sound power of up to 8 dB at low frequency resonancesand as large as 4.5 dB over a 4000 Hz bandwidth. The weight of the viscoelastic treatment was approximately1 % of the panel mass. Preliminary FEM/BEM modeling shows good agreement with experimental results forradiated sound power.