Structure-borne Vibration Research Articles

Structural Floor Vibration and Sound Isolation Design for a Magnetic Resonance Imaging System

This is a case study about noise and vibration control problems, and the design constraints and solutions for a proposed installation of a magnetic resonance imaging system (MRI) in an existing medical research facility. Manufacturer's data indicated that airborne sound level emissions over a broad frequency span could exceed permissible noise criteria for nearby occupied rooms. The building structure also required reinforcement to accommodate the MRI magnet's concentrated load, but invasive disturbance to a transgenic research mouse vivarium on the floor below was prohibited. The structure borne vibration paths needed attenuation or isolation. Design parameters included structural strength, stiffness and the specific platform resonant frequency (non-coincident with known vibration sources or building structure). In addition, acoustical containment was required for anticipated noises from the magnet room, to prevent excessive or annoying and distracting noise in the MRI control room or other adjacent (but unrelated) research, animal holding and office spaces. Structural “de-tuning” and architectural “decoupling” concepts were employed. A resonant frequency criterion was recommended for the new structural floor design. A combination of vibration spectrum analysis, dynamic analyses of alternate structural concepts and existing physical conflict constraints led to the design of an independent platform floor above the existing building floor. Post construction floor vibration measurements were compared to earlier measurement data to show quantitative change in performance. The modifications satisfied acoustical criteria and occupants' subjective evaluations.

Active control of connected plates using single and multiple actuators and error sensors

Active control of the structure-borne vibration transmission in resonant, built-up structures which typically represent a ship hull is analytically and experimentally investigated. Using feedforward active control, the control performance is compared for both dependent and independent control force arrangements. Multiple actuator control forces and multiple error sensors are used to actively control the frequency response. The global response of the coupled plate structure is also presented for active control at discrete resonance frequencies.

Chesapeake Instrument Corporation Project Toad: Principles and Applications of Towflex Hydrophones

The principles of a new method for reduction of self-noise in sonar systems are presented, together with corroborative experimental data taken from both surface ships and submarines. The technique is one of towing or streaming one or more hydrophones, called Towflex Hydrophones. The self-noise reduction observed in such Towflex units is due to several contributing factors. In the first place, the many structure-borne vibration paths feeding energy into the hydrophones are virtually eliminated, except for the cable itself. Because of the flexible construction, the transmission of cable vibration, in turn, is also reduced substantially since the fluid-filled flexible tubing discourages transmission of vibrations to the hydrophone elements. Moreover, the hydrodynamic flow around the hydrophones is at all times nearly axisymmetric and the hydrophones stream stably at speeds up to 30 knots with a minimum of flow noise. Additionally, the sensitive elements in Towflex hydrophones are so designed as to respond only to acoustic pressure and to cancel vibration and acceleration inputs. The net result is a hydrophone array system of remarkably low self-noise. In tests of Towflex units streaming from the sail of the submarine ALBACORE noise reductions of as much as 20 to 40 dB were observed when compared to the hull-mounted unit of conventional design located below the Towflex unit. The levels observed with the Towflex hydrophone on ALBACORE are 2 to 12 dB lower than those measured by USNUSL for the BQR-2 installations on other submarines, depending on operating conditions. This difference represents a detection range improvement of as much as a factor of four. When towed as little as 200 yards astern of the converted seaplane tender J. WILLARD GIBBS, noise levels approximately 10dB above sea-state 1 ambient and some 20 dB below the ship noise level expected for a omni-directional hydrophone were found, because of pattern discrimination against the noise from the towing vessel. There is reason to expect that these low self-noise figures may extend 20 knots or above. Towflex hydrophones are simple, inexpensive and suitable for many ASW applications including helicopter sonars, surface ship sonars, and submarine sonars. Both passive and active continuously scanning systems are feasible. The optimum design parameters for such systems using Towflex hydrophones have yet to be determined.

Noise and Vibration Spectrum of Structure-borne Noise from Railway System

In this study, the spectrum and resonance frequencies of the structure-borne noise and vibration excited by the train traffic on concrete viaduct structure are examined. Comparisons of air- and structure-borne noise spectrum between local and overseas trains are taken. Real-time octave band and FFT (Fast Fourier Transform) mode are used to analyze structure-borne noise spectra in details. Based on the spectrum results of the structure-borne noise, air-borne noise lies on high frequency component and structure-borne noise lies between 20 Hz and 200 Hz, which is a tonal noise. From the FFT analysis, the vibration resonance frequencies are 43 Hz and 54 Hz and acoustics coincidence are 121.5 Hz and 128.5 Hz. The theoretical relationships between sound and vibration are in agreement with the experimental results. The results will be useful for the prediction of structure-borne noise from railways.

Space-time visualization of vibration of a ribbed cylinder

Data from an axial array of sensors along a ribbed cylindrical structure may be analysed to allow the evolving vibrational response to impulsive driving to be plotted as a function of space and time. The process is carried out separately in a series of frequency bands, by first carrying out a time-frequency analysis at each position. Aspects of the behaviour not previously discussed are highlighted by this analysis. These aspects are discussed in the light of a previously published theoretical model [1]. Since reflections from the cylinder's ends also play a role here, it has been necessary to develop further theoretical modelling to interpret the measurements. The new experimental approach, which is described here as the method of ‘space-time cross-sections', has scope for application to other problems involving structure-borne vibration. This illustrates the power of time-frequency analysis and its extensions for engineering vibration analysis.

On the sound radiated by electrical equipment : an active structural acoustic control of harmonic sound

In this paper a technique is presented for the active structural acoustic control of harmonic sound radiated from an electrical machine. In the study, we consider the noise radiated by the heat-exchanger enclosure of a a large size induction motor; the sound is radiated harmonically by the enclosure panels, excited by structure borne vibrations. The control source is a spatially distributed piezoelectric ceramic patch actuator attached to the enclosure panels, and the sensor is a vibration transducer or a microphone. A digital controller and digital processing software are designed based on a feedback algorithm. The paper describes: the functional characteristics of the piezoelectric patch actuators for this application, the finite element model of the enclosure for determining the optimum system configuration, the experimental dynamic analysis to map the panel vibrations, the baseline noise measurements, and the implementation of the control algorithm. Final results are presented showing a 20 dB noise reduction of the harmonic sound radiated from the electrical machine.

Structure-Borne Vibration Transmission in a Tire and Wheel Assembly

Abstract Vibration transmission through tires due to forces generated by the road-tire interaction during rolling over a rough road surface is an important source of noise in the vehicle interior over a wide frequency range. Vibration transmission along the tire tread and across the sidewalls to the rim was measured by striking the tread with an impact hammer in generating transfer functions for comparison with results from an SEA model of the wheel. The effects of tire and rim resonances are noted. Transfer functions to acoustic levels in the tire cavity were also measured.

Noise generated by intermittent structure borne vibrations

Professor Grootenhuis discusses noise levels due to structural vibration and describes how they can be predicted with some confidence under steady state conditions. His case studies relate to flats in a building in the Barbican in London over railway tracks, and also measurements taken in the Royal Festival Hall.

Active control of far-field sound radiated by a rectangular panel—A general analysis

In this paper a general analysis is presented for the active control of the far-field harmonic sound radiated by a rectangular panel that is built into an infinite baffle. In this analysis, the panel vibration may be generated by either airborne sound (incident sound field) or by structure borne vibrations. The far-field radiated sound is controlled either by acoustical sources or vibration sources. Minimization of both the local sound pressure and the total power output is considered. Analytical results for the particular case involving minimization of the sound pressure at a single point are compared with experimental data. The physical mechanisms involved for different control sources (vibration or acoustic) are demonstrated analytically. For the case of vibration control sources, the panel modal velocity components are adjusted to produce far-field sound control. This can be done either by decreasing their amplitudes, and/or by changing the temporal phases of the panel modes. However, for acoustic control sources, the far-field sound is minimized by alteration of the radiation impedance seen by the vibrating panel and the control sources.

Fabrication of low frequency structureborne vibration isolation mount

First Page

Structural power-flow analysis using a mobility approach of an L-shaped plate

In the analysis of the vibration response and structure-borne vibration transmission between elements of a complex structure, statistical-energy analysis (SEA) or finite-element analysis (FEA) are generally used. However, an alternative method is to use vibrational power-flow techniques that can be especially useful in the midfrequencies between the optimum frequency regimes for FEA and SEA. Mobility power-flow analysis has, in general, been used for one-dimensional beamlike coupled structures or for point-coupled structures. In this paper, the power-flow technique is extended to two-dimensional platelike coupled structures joined along a common edge. Because of the inherent advantages in using this power-flow approach, frequency or spatial averaging is not required, and thus the resonant modal response of the coupled structure is determined. The mobility power-flow results are compared to results obtained using FEA and SEA. The agreement with FEA results is good at low frequencies. However, the power-flow technique has an improved computational efficiency as the frequency increases. Compared to the SEA results, the power-flow results show a closer representation of the actual modal response of the coupled structure.

Modelluntersuchungen zum körperschallverhalten von u-bahn-tunneln und deren umgebung

The prediction and the control of structure-borne vibrations in subway railway systems are well known problems. The investigations described in this paper were concerned with possibilities of using simple and practical models to give answers of these problems. The scale factor was 1:10. The results show that model experiments together with theoretical approximations can be acceptable tools for the determination of the mean structure-borne sound behavior of both a subway tunnel itself and for the areas near the tunnel. In cases where a receiver is relatively far away from the tunnel the model leads only to qualitative results, because of the increasing inhomogeneities and frequency dependencies. Also the models are practicable only for simple soil structures.

Development of a rating procedure for impact noise transmission through walls

The development of standardized tests for rating the impact transmission behaviour of different wall constructions and the improvement of deficient constructions are considered. Sound transmission of single impacts on walls was investigated in terms of both sound pressure and panel acceleration signatures, and the noise generation and transmission mechanisms were identified. A literature review of the subjective annoyance response to impulsive noise led to the development of an objective technique based on a peak-hold sound level meter having a 5 ms response time and C weighting. Various wall constructions were compared using a standardized impactor and the peak hold meter. Impact noise transmission was found to be predominantly due to structure-borne vibration; consequently transmission may be reduced by vibration isolation techniques. A procedure to rate impact noise transmission through wall systems is suggested, based on the subjective annoyance response to impulsive noise.

A thin-plate model for the moment mobility at the intersection of two perpendicular plates

The global response of a T-intersection configuration due to a moment excitation has been analyzed. The moment is applied at the intersection and the moment vector is parallel with the intersection line. A thin-plate model has been used to derive the moment mobility for arbitrary combinations of plate elements. The imaginary part of the mobility is found to be dependent on both size and rigidity of the indenter whereas the real part is essentially independent. Numerical results are presented for the practically important case of three identical plate elements, constituting the T-intersection configuration. These results clearly confirm the necessity of taking into account a potential moment excitation for the determination of structure-borne sound and vibration transmission at an intersection point.

Detecting internal abnormalities in turbines

Undesirable internal changes in a running turbine are diagnosed by monitoring the dynamic pressure of the stream of fluid and the structure-borne vibration of the stationary shell. Incipient erosion of blades, material deposit buildup, and other abnormalities are detected by analyzing the dynamic pressure and vibration signal utilizing several signal processing techniques. A better maintenance schedule can be derived, this also serves as a turbine efficiency monitor.

Acoustic Analysis of Quiet Ball Bearing Failure Modes

Qualitative analysis of rotating machinery structureborne noise and vibration can identity ball bearing installation problems. Indications of improper lubrication and impending bearing failures are identified. The problem of nonuniform motion of ball bearing components in determining expected bearing frequencies is noted together with the effects of grease deterioration. The philosophy of structureborne noise versus vibration measurements and an appreciation of several analysis techniques in the context of determining maintenance requirements and evaluating subsequent repair are discussed.

Observations of the Marine Atmosphere Using a Ship-Mounted Acoustic Echo Sounder

A monostatic acoustic echo sounder with a vertically-pointed antenna was installed aboard the NOAA research vessel Oceanographer and was tested during a recent cruise in the Pacific and Caribbean Oceans. Acoustic returns produced by turbulence-induced temperature fluctuations were received from up to 300 m in height. The data revealed the existence of a rich variety in the structure of the marine atmosphere. Thermal convective plumes were usually observed when the sea water temperature exceeded the air temperature by as little as 1°C. Under more stable conditions the echo sounder detected the presence of layered structures associated with temperature inversions that were often perturbed by gravity waves or wind shear. Doppler frequency shift of the returned echo signals was used to estimate the vertical velocity of atmospheric scatterers. The results of the tests indicate that it is possible to probe remotely the lower layers of the marine atmosphere from a moving ship using acoustic echo sounding techniques. Wind-generated noise, ambient acoustic noise, and structure-borne vibrations proved to be the major limitations on the performance of the echo sounder.

The assessment of performance of anti-vibration materials

In noise problems the limitation of structure-borne vibration is often important. Properly designed resilient mountings can be employed to give the required isolation; these may take the form of unit mountings used as point supports or pads of sheet-like material cut to the required size. Anti-vibration mountings employing pads of material in sheet form will often be suitable and are usually cheaper than unit mountings.Information on the attenuation to be expected from such materials is not usually available from the manufacturers. The paper gives details of a simple method which may be employed to give useful and reliable information on the dynamic performance of such materials.

Power Test Code Thermometer Wells

Design of thermometer wells involves strength and thermal considerations. Criteria for thermal evaluation have been well established, those for strength have not. This paper presents thermometer-well designs proposed for Power Test Code work. Forces imposed by static pressure, steady-state fluid impingement, vibration induced by Karman trail vortexes, and structure-borne vibration are discussed. Methods and equations are given to enable the selection of wells for given service conditions. Although these methods are confined to wells of Power Test Code design, the Appendix contains derivations of general equations that may be adapted for any cantilever-type thermometer well.

Lead-Zirconium-Titanite Microphones for Measurement of High Noise Levels

A series of small lead-zirconium-titanite microphones has been developed for the measurement of random air-borne noise fields of sound pressure levels in the range of 45 to 200 db (rc 0.0002 μb) and higher. With a capacitance 1000 μμf and a sensitivity of 93 db (rclv/μl), special preamplifiers are unnecessary for many applications. A substantially spherical beam pattern, even at the higher frequencies (± 3 db @ 35 bc), and a wide-frequency response (1 cps–40 bcps) make these units exceptionally suitable for high noise-level studies. The design is based on the use of ferroelectric cylinder, coaxially mounted on, but electrically and mechanically isolated from, a center aluminum spindle. The whole acoustic head assembly is molded in a Neoprene or silicone rubber jacket resulting in a water-tight, rugged construction. In order to minimize vibration pickup, the acoustic head and stem assembly are mounted in a modified vibration mount with a counter-balancing weight. The acoustic head is also covered with a flexible conductive coating to minimize electrostatic pickup. These and other design features result in a microphone which is truly capable of measuring airborne noise fields, while at the same time discriminating against structure borne vibration, as well as electrostatic and magnetic pickup.