Carl Hopkins

Prediction of high-frequency vibration transmission across coupled, periodic ribbed plates by incorporating tunneling mechanisms

Prediction of structure-borne sound transmission on built-up structures at audio frequencies is well-suited to Statistical Energy Analysis (SEA) although the inclusion of periodic ribbed plates presents challenges. This paper considers an approach using Advanced SEA (ASEA) that can incorporate tunneling mechanisms within a statistical approach. The coupled plates used for the investigation form an L-junction comprising a periodic ribbed plate with symmetric ribs and an isotropic homogeneous plate. Experimental SEA (ESEA) is carried out with input data from Finite Element Methods (FEM). This indicates that indirect coupling is significant at high frequencies where bays on the periodic ribbed plate can be treated as individual subsystems. SEA using coupling loss factors from wave theory leads to significant underestimates in the energy of the bays when the isotropic homogeneous plate is excited. This is due to the absence of tunneling mechanisms in the SEA model. In contrast, ASEA shows close agreement with FEM and laboratory measurements. The errors incurred with SEA rapidly increase as the bays become more distant from the source subsystem. ASEA provides significantly more accurate predictions by accounting for the spatial filtering that leads to non-diffuse vibration fields on these more distant bays.

Measurement of the Vibration Reduction Index, Kij on Free-Standing Masonry Wall Constructions

In the determination of coupling parameters between concrete/masonry walls in the laboratory there are potential advantages in using free-standing test constructions instead of flanking laboratories. To assess this potential, free-standing masonry walls were built in the laboratory to test two common types of rigid T-junction. The measured modal overlap factors were typically less than unity over the frequency range 50Hz – 1kHz. In this range, empirical rules imply that it is not possible to state with statistical confidence that there is any significant difference between the two types of junction connection. Below 1kHz many solid concrete/masonry separating elements are likely to have modal overlap factors less than unity in both the laboratory and the field. This is problematic for laboratory measurements because the vibration reduction index, Kij is unlikely to be a ‘situation invariant’ parameter when determined with elements that have low modal overlap.

Modulation of Visually Evoked Postural Responses by Contextual Visual, Haptic and Auditory Information: A 'Virtual Reality Check'

Externally generated visual motion signals can cause the illusion of self-motion in space (vection) and corresponding visually evoked postural responses (VEPR). These VEPRs are not simple responses to optokinetic stimulation, but are modulated by the configuration of the environment. The aim of this paper is to explore what factors modulate VEPRs in a high quality virtual reality (VR) environment where real and virtual foreground objects served as static visual, auditory and haptic reference points. Data from four experiments on visually evoked postural responses show that: 1) visually evoked postural sway in the lateral direction is modulated by the presence of static anchor points that can be haptic, visual and auditory reference signals; 2) real objects and their matching virtual reality representations as visual anchors have different effects on postural sway; 3) visual motion in the anterior-posterior plane induces robust postural responses that are not modulated by the presence of reference signals or the reality of objects that can serve as visual anchors in the scene. We conclude that automatic postural responses for laterally moving visual stimuli are strongly influenced by the configuration and interpretation of the environment and draw on multisensory representations. Different postural responses were observed for real and virtual visual reference objects. On the basis that automatic visually evoked postural responses in high fidelity virtual environments should mimic those seen in real situations we propose to use the observed effect as a robust objective test for presence and fidelity in VR.

Vibration transmission between coupled plates using finite element methods and statistical energy analysis. Part 1: Comparison of measured and predicted data for masonry walls with and without apertures

Abstract In Part 1 of this paper a study is made to assess the prediction of vibration transmission between masonry walls using Finite Element Methods (FEM) and Statistical Energy Analysis (SEA) through a comparison of measured and predicted data. Masonry walls typically have low modal overlap and low modal density at low frequencies, hence FEM is used to predict the large fluctuations in the vibration level difference. The issue of uncertainty in the physical description of masonry walls is addressed by using Monte Carlo methods with FEM. Two types of masonry wall junction are studied, an L-junction and a T-junction. The importance of in-plane wave transmission was demonstrated for L-junctions with apertures and T-junctions. The agreement between FEM, SEA and measured data at high frequencies indicated that the conversion between bending and in-plane waves in the FEM models was correct.

Sound transmission across a separating and flanking cavity wall construction

Abstract Two parameters that can be difficult to model accurately for cavity wall constructions are the cavity damping and the foundation coupling. In order to investigate dominant sound transmission paths across masonry cavity wall constructions, measured cavity damping and foundation coupling data have been incorporated in a statistical energy analysis (SEA) model for a test construction in the BRE flanking laboratory. This paper presents measured data to validate the SEA predictions for the direct and flanking transmission paths that were dominant. For the masonry cavity wall construction that was tested, structural coupling due to the separating wall foundations was found to be a dominant path for transmission across the separating cavity wall.

On the efficacy of spatial sampling using manual scanning paths to determine the spatial average sound pressure level in rooms

In architectural acoustics, noise control and environmental noise, there are often steady-state signals for which it is necessary to measure the spatial average, sound pressure level inside rooms. This requires using fixed microphone positions, mechanical scanning devices, or manual scanning. In comparison with mechanical scanning devices, the human body allows manual scanning to trace out complex geometrical paths in three-dimensional space. To determine the efficacy of manual scanning paths in terms of an equivalent number of uncorrelated samples, an analytical approach is solved numerically. The benchmark used to assess these paths is a minimum of five uncorrelated fixed microphone positions at frequencies above 200 Hz. For paths involving an operator walking across the room, potential problems exist with walking noise and non-uniform scanning speeds. Hence, paths are considered based on a fixed standing position or rotation of the body about a fixed point. In empty rooms, it is shown that a circle, helix, or cylindrical-type path satisfy the benchmark requirement with the latter two paths being highly efficient at generating large number of uncorrelated samples. In furnished rooms where there is limited space for the operator to move, an efficient path comprises three semicircles with 45°-60° separations.

Impact Sound Insulation Using Timber Platform Floating Floors on a Concrete Floor Base

This paper looks at the performance that can be achieved using timber platform floors on concrete floor bases in order to design a platform floor that could provide at least 29dB ΔLw (both loaded and unloaded) for use in floor constructions described in Approved Document E (2003 Edition). Reconstituted open cell foams were identified as resilient materials that could provide suitable static and dynamic stiffness under lightweight timber floating floors. Using two layers of reconstituted foam a platform floor comprising 22m tongue and grooved chipboard achieved 29dB ΔLw when loaded, and 30dB ΔLw unloaded. Indicative tests on double platform floating floors indicated that they may be beneficial in floor designs where negative values of ΔL can not be tolerated due to critical impact sound insulation and structural stability requirements. A double floating floor could use dynamically stiffer resilient materials than would be needed in a single floating floor and still provide the required impact sound insulation and structural stability.

Sound Measurements and Natural Ventilation in Schools

Abstract Natural ventilation using open windows is an energy efficient and effective method of providing a good indoor air climate in schools. However, open windows not only let fresh air into the building but also external noise. The availability of automatically controlled windows enables precise control over the window opening distance in comparison with manually operable windows. This paper considers the benefits of using small opening distances to provide sufficient ventilation for a good indoor climate as well as attenuation of external noise. Airborne sound insulation tests on a window with different opening distances show that the sound insulation varies significantly depending upon the opening distance; the smaller the opening distance, the higher the sound insulation. In many cases, an opening distance as small as 1 to 2 cm can not only provide sufficient ventilation for a good indoor climate, but also adequate sound insulation. The results of sound attenuation tests, performed at BRE in the United Kingdom, are presented in this paper. In addition, empirical results from 3 different naturally ventilated schools, including two in Denmark and one in the UK are described. These results indicate that often only small opening distances are required to obtain a comfortable thermal and atmospheric indoor climate based on measurements during the year.

Vibration transmission between coupled plates using finite element methods and statistical energy analysis. Part 2: The effect of window apertures in masonry flanking walls

Abstract Part 1 of this paper demonstrated the validity of predictions of vibration transmission across junctions of masonry walls using Finite Element Methods (FEM). Part 2 uses numerical experiments with FEM to calculate the vibration transmission between masonry walls with window apertures at different positions in the flanking wall(s). Results from the numerical experiments are used to assess a simple “rule-of-thumb” estimate for calculating the change in the coupling parameters due to the introduction of an aperture into a flanking wall. Conclusions are drawn concerning use of the “rule-of-thumb” estimate for the coupling loss factor in Statistical Energy Analysis and the vibration reduction index in European standard EN 12354.

Dynamic stiffness as an acoustic specification parameter for wall ties used in masonry cavity walls

Abstract One of the factors affecting the acoustic performance of masonry cavity walls is the type of wall tie used in the construction. The dynamic stiffness parameter allows wall ties of equivalent dynamic stiffness to be identified without involving other variables involved in laboratory airborne sound transmission across masonry cavity wall constructions such as cavity damping and structural coupling at the boundaries. This is useful when one type of wall tie is established in national building regulations and architects or construction companies require the use of non-generic wall ties.