Shankar Rajaram

Sound transmission loss of honeycomb sandwich panels

Honeycomb sandwich panels used for commercial applications are typically stiff and lightweight. They are optimized for mechanical performance, but have poor acoustical performance. Transmission loss (TL) is one of the metrics used to assess the acoustical performance of honeycomb sandwich panels. Transmission loss for these panels shows inferior mass law performance above coincidence frequency for commercially available panels. For superior transmission loss performance, it is critical to delay (maximize) the coincidence frequency, which is determined by the dispersive panel bending waves. Panel bending waves are characterized by three frequency regimes - total panel bending, core shear, and individual skin bending. These regimes are controlled by panel geometry, panel mass and elastic properties of the core and the skins. The coincidence frequency can be increased by designing panels with core shear wave speeds that are subsonic. In the present study, the influence of different panel design parameters, such as core density, core material, cell size, and cell structure, on the transmission loss of honeycomb sandwich panels is analyzed. Moreover, TL results of panels in three classes of core shear wave speeds - subsonic, transonic, and supersonic - are presented. For panels with supersonic shear wave speed, core density influences TL above the coincidence frequency, but other parameters like cell size and skin type show negligible effects. Panels with subsonic and transonic core shear wave speeds show improved acoustic performance compared to their supersonic counterparts. The mechanical performance of subsonic and transonic panel designs is generally inferior but can be improved when accompanied by weight increase.

Predicting the Sound Transmission Loss of Sandwich Panels by Statistical Energy Analysis Approach

A statistical energy analysis (SEA) approach is used to predict the sound transmission loss (STL) of sandwich panels numerically. Unlike conventional SEA studies of the STL of sandwich panels, which consider only the antisymmetric (bending) motion of the sandwich panel, the present approach accounts for both antisymmetric and symmetric (dilatational) motions. Using the consistent higher-order sandwich plate theory, the wave numbers of the waves propagating in the sandwich panel were calculated. Using these wave numbers, the wave speed of the propagating waves, the modal density, and the radiation efficiency of the sandwich panels were determined. Finally, the sound transmission losses of two sandwich panels were calculated and compared with the experimentally measured values, as well as with conventional SEA predictions. The comparisons with the experimental data showed good agreement, and the superiority of the present approach relative to other approaches is discussed and analyzed.

Small-scale transmission loss facility for flat lightweight panels

The design, construction and qualification of a small-scale sound transmission loss (STL) facility are described. STL measurements were made using the sound intensity technique based on ASTM E 2249-02. The volume of the irregular-shaped reverberant source chamber was 15 m3, and the volume of the regular-shaped anechoic receiver chamber was 20 m3. The facility was tested between 315 Hz and 10 kHz. Good spatial diffusion, and good repeatability for same and repeat installations were demonstrated in the above frequency range. The results from the small-scale facility were compared to tests conducted at a full-scale facility. The STL values of flat, lightweight sandwich panels measured at the small-scale chamber were greater than those measured in the full-scale facility. However, the results from the small-scale facility showed trends that were consistent with the sandwich panel theory above 1 kHz. The results demonstrated that the small-scale STL facility could be successfully used for qualitative comparisons of lightweight, sandwich panels above 1 kHz

Measurement of sound transmission losses of honeycomb partitions with added gas layers

Air, nitrogen, argon and helium barrier layers were mounted on honeycomb sandwich panels and the sound transmission loss (TL) measured. All the gases were maintained at 1 atm pressure during the sound transmission loss measurements. The TL was measured in a small-scale facility with a reverberant source room and an anechoic receiver room. A sound intensity probe was used to measure transmitted sound. Except helium, none of the other gases showed any significant transmission loss improvement due to insufficient impedance mismatch with the air medium. However, the sound transmission loss of honeycomb sandwich panels was substantially improved by the addition of a helium barrier layer on the incident side. The improved TL was attributed to the significant impedance mismatch between air and helium, which resulted in reduction of incident acoustical energy at the interface of the two media. Increases in the sound transmission losses by 6-8 dB were obtained in the range of 2-10 kHz. The helium layer also increased transmission loss at lower and middle frequencies. The addition of a helium barrier layer did not alter the coincidence frequency of the floor panel.

Evaluation of Force Density Levels of Light Rail Vehicles

The vibration forces generated by train wheels rolling on steel rails are characterized as the force density level (FDL). FDLs are a key component in predicting vibration levels from future train projects and generally are assumed to be independent of the measurement position and the soil properties. FDLs are dependent on several factors, which include track type, vehicle suspension systems, train speeds, and condition of wheels and rail surfaces. Presented are results of train vibration measurements at three sites in Seattle, Washington, along Sound Transits Central Link, and a comparison of the results with previous FDL results from Central Links start-up phase. The effects of speed, train length, and track type on FDLs were measured. A test train was employed for the measurements at three sites. The tests at the three sites also included supplemental measurements with revenue service trains. The study showed that the vibration of the current Sound Transit fleet was lower than the previous measurements apparently because the wheels were in better condition than for the earlier tests. Rail roughness was also measured at all the test sites, and their effects on FDL were explored.

Design and Performance of a Permanent Vibration Monitoring System with Exceedance Alarms in Train Tunnels

Sound Transit has installed nine permanent vibration monitors (VMS) to continuously monitor the revenue service train 1/3 octave vibration velocity in the University of Washington (UW) campus area. The monitors allow verification that the vibration levels generated from the newly opened University Link (U-Link) extension light rail operations are within the thresholds established in the Master Implementation Agreement (MIA) signed by UW and Sound Transit. There are also three wheel flat detectors located along the light rail alignment outside of the campus area to serve as an early warning system that allows Sound Transit operations to implement mitigation measures in real time before trains with wheel flats reach the vibration-sensitive zone in the University of Washington. This paper discusses the design of the vibration monitoring program and the functioning of the system during the first three months of U-Link revenue service. Some of the key results are that the system is functioning as designed and consistently records and notifies vibration events that exceed alarm thresholds. The vibration levels from train passbys have been well below the MIA thresholds and preliminary investigation indicates that the alarms are not caused by the trains. A surprising result, however, is that several non-train vibration sources that coincide with train recording sessions are generating alarms. Vehicular traffic on adjacent roadways and mechanical equipment such as sump pumps located in the vicinity of the VMS units are suspected as the potential sources of vibration alarms.

A Comprehensive Review of Force Density Levels from Sound Transit’s Light Rail Transit Fleet

Sound Transit has conducted many wayside train vibration measurement campaigns since 2007 to characterize vibration emissions from the trains and track. The vibration emissions are defined by force density levels (FDL) that are usually based on empirical vibration data. Because there are many competing factors that influence the FDLs, correlation of the empirical data with controlling mechanisms is not straightforward. A more robust dataset developed from multiple test campaigns under different conditions is required to develop an overall understanding of the underlying vibration mechanisms. This paper looks at results from seven major vibration test programs between 2007 and 2015 and attempts to tease out the key features of FDLs from the existing Sound Transit LRV fleet. The existing Sound Transit LRV fleet vibration spectra show three distinct frequency regimes. The low frequency peaks in the FDL spectra are predominantly influenced by the primary resonance frequency of the motored truck and the mid frequency peaks are influenced by the center truck characteristics. The track stiffness, damping characteristics and resilient wheel determine the amplitude and shape of the high frequency peaks. Factors such as speed also influence the FDLs. For a given track and vehicle type combination, the key factors that influence FDLs are wheel-rail profile match, maintenance of the wheel-rail surfaces and frictional forces in the wheel-rail interface.

Light Rail Vehicle Noise: Evaluation of Rail Roughness and Noise from Wheel–Rail Interface

Sound Transit opened its first light rail line, the Central Link, in Seattle, Washington, in 2009. There were many community noise issues immediately after the line went into revenue service that were generally attributed to poor quality rail grinding when the initial mill scale grinding was performed. As a result of that experience, Sound Transit has been cautious when predicting noise and designing noise mitigation for new alignments. Future alignments include the University, Northgate, and Lynnwood Links to the north; the Federal Way and South 200th Links to the south; and the East Link to the east. Over the next 15 years, the system will be expanded from 16 mi to more than 50 mi. The detailed noise data collected as part of the final design of East Link suggest that measures could be taken to reduce the need for substantial noise mitigation on future Sound Transit extensions. This paper summarizes the measurements and key observations from that noise study. One key observation suggests that through implementing acoustic rail grinding and maintaining the current wheel truing program, there is potential for minimizing the amount of sound walls required to achieve community noise goals. The results also suggest that through a detailed, information-based investigation, reference noise levels used to determine the need for noise mitigation could be reduced by approximately 4 dB. However, the results of this study, combined with experience from rail roughness studies in general, suggest that achieving a true acoustic rail grinding that leaves no artifacts is not straightforward, because it requires careful monitoring by the rail grinder operator as well as quality control measurements by the grinding company or the transit system.

Variation in measured force density levels of light rail vehicles.

The procedure recommended by the Federal Transit Administration for detailed predictions of ground‐borne vibration is based on measurements at an existing rail system and measurements at the target site. A force density level (FDL) is derived from train vibration and line source transfer mobility (LSTM) measurements at the existing system. This FDL is then combined with the LSTM measured at the target site to develop predictions. This empirical procedure usually provides more reliable predictions than computer modeling procedures. FDL is assumed to characterize the trains and track support system and LSTM is assumed to characterize the effects of local geology. Implicit assumptions when applying this procedure are that FDL is independent of the local geology and that the vehicle and track support system will be the same at the target site as they are at the vehicle test site. This paper considers the differences in FDLs measured at light rail systems in Los Angeles, Minneapolis, Phoenix, and Portland. Iss...

Prediction of sound transmission loss of honeycomb sandwich panel by higher order approach

People have studied the sound transmission loss (STL) of sandwich panels since the 1970s. However, most of the existing prediction methods have been based on single‐layer dynamical models, neglecting the symmetric (dilatational) movements of the skins. Consequently, the symmetric coincident frequency of the sandwich panel cannot be predicted using those approaches. To account for this dilatational motion of the sandwich structures, different methods were utilized. However, most of them were based on one dimensional sandwich beams theories. The authors have also applied the consistent higher order beam approach to calculate the sound transmission loss of a unidirectional sandwich panel. Although the one dimensional approximation is good in predicting STL, the effects of some factors, such as the anisotropy and orientation of the principle axis of the panel, cannot be estimated. In the current work, the authors extended that one dimensional beam model into two dimensions, which allows us to calculate the ST...