Anders Brandt

Resonance frequencies of the human skull in vivo

Patients with skin penetrating titanium implants in the temporal bone, for attachment of bone-anchored hearing aids, have made it possible to investigate the free-damped natural frequencies (resonance frequencies) of the human skull in vivo. The resonance frequencies of the skull of six subjects were investigated. Teh resonance frequencies were extracted from two frequency response functions (acceleration/force) measured on each subject: One point measurement where the force and acceleration were both measured at the same point, and one transcranial measurement where the acceleration was measured contralaterally. Between 14 and 19 resonance frequencies were identified for each subject in the frequency range 500 Hz to 7.5 kHz. The two lowest resonance frequencies were found to be on the average 972 (range 828-1164) and 1230 (range 981-1417) Hz. The relative damping coefficients of all resonances were found to be between 2.6 and 8.9%. Due to the relatively high damping coefficients, it is assumed that the resonance frequencies do not significantly affect bone conducted sound. In the transcranial measurements, however, a few large antiresonances were found which may affect bone-conducted sound. Intersubject variations were large, probably due to individual variations in skull geometry and in mechanical parameters. The results were shown to be consistent with previous results obtained on dry skulls. No obvious correlation between lowest resonance frequency and skull size was found.

Linearity of sound transmission through the human skull in vivo

The linearity of sound propagation through the human skull was investigated. One male subject, equipped with bilateral skin-penetrating titanium fixtures for attachment of bone-anchored hearing aids, was studied thoroughly. Three different methods were used: comparison of the frequency response functions estimated at different signal levels (using stepped sine as well as random noise), comparison of the coherence function at different signal levels (using random noise), and the Hilbert transform of the estimated frequency response function. Frequencies from 0.1 to 10 kHz and signal levels up to 77 dB HL at discrete frequencies were used. No indication of any significant nonlinear behavior was found with the three methods used.

Identification of dynamic properties of boring bar vibrations in a continuous boring operation

Vibrations in internal turning operations are usually a cumbersome part of the manufacturing process. This article focuses on the boring bar vibrations. Boring bar vibrations in alloyed steel, stai ...

Impact Excitation Processing for Improved Frequency Response Quality

Impact excitation is the most common excitation type for measurements of frequency response functions for modal analysis and other purposes. The method used is almost always based on setting the data acquisition system up with triggering, fixed FFT analysis settings, and then using an accept/reject step where each impact is either accepted if the impact seems good, or rejected if it contained some error such as double impacts or overload. This method has several drawbacks that often lead to non-optimal frequency responses. In this paper, an improved method based on time recording of all signals and subsequent post processing is proposed. The data acquisition part is made easier with the proposed method, while at the same time the importance of a skilled operator is reduced. It is shown on a real test structure that the quality of the resulting frequency responses can be significantly improved (measured by the coherence function) compared to the traditional method, and at the same time the total acquisition time can be shortened. An automatic optimization procedure which allows for fully automated post processing is proposed.

Structural Dynamics Teaching Example: A Linear Test Analysis Case Using Open Software

Teaching the topic of structural dynamics in any engineering field is a true challenge due to the wide span of the underlying subjects like mathematics, mechanics (both rigid body and continuum mechanics), numerical analysis, random data analysis and physical understanding. With the increased availability of computers many engineering problems in practice are evaluated by means of numerical methods. The teaching task within the field of structural dynamics thus has to include analytical models in order to create a theoretical basis but also has to include computational techniques with its approximations, and knowledge about their limitations. Equally important is for students to have knowledge of the experimental verification of the obtained models. This paper describes a teaching example where a simple plate structure is modeled by shell elements, followed by a model calibration using experimental modal analysis data. By using open software, based on MATLAB® as a basis for the example, the applied numerical methods are made transparent to the student. The example is built on a combination of the free CALFEM® and ABRAVIBE toolboxes, and thus all code used in this paper is publically available as open source code.

Calibration and Processing of Geophone Signals for Structural Vibration Measurements

Geophones are highly sensitive motion transducers that have been used by seismologists and geophysicists for decades. The conventional geophones ratio of cost to performance, including noise, linearity and dynamic range is unmatched by advanced modern accelerometers. However, the problem of this sensor is that it measures velocity, and that the linear frequency range is limited to frequencies above the natural frequency, typically at 4-12 Hz. In this paper an instrument is presented based on geophone technology. The sensor is aimed at low vibration level measurements on large civil structures, thus the problem of correcting the bad frequency response becomes essential. The instrument is based on a digitally wired system principle where time synchronization is obtained by GPS, and a good frequency response is secured by calibration and subsequent correction using inverse filtering techniques.

Operational Modal Analysis for Dynamic Characterization of a Ro-Lo Ship

The dynamic characteristics of ship structures are becoming more important as the flexibility of modern ships increases, for example, to predict reliable design life. This requires an accurate dyna ...

Modal test results of a ship under operational conditions

Sea vessels are exposed to a complex vibration environment, influenced by the sea as well as by operational conditions. Particularly, the hydrodynamic load effects are difficult to estimate analytically. Experimental results are therefore important to verify the analytical models. In the present paper preliminary results from a full-scale modal test of a Ro-Lo vessel carried out for three different operating conditions are presented. Since little full-scale modal testing seems to have been conducted on vessels in operation, the experimental setup together with preliminary modal parameters extracted from the measurements are presented. This preliminary study is focusing on investigating the data with respect to operational conditions and shows a significant variation of the modal damping of the vessel in operation, with approximately 400 %, 200 % and 400 % difference in the first three global vertical bending modes, respectively.

Influence of Noise in Correlation Function Estimates for Operational Modal Analysis

The modal parameters in Operational Modal Analysis (OMA) are often estimated based on non-parametric signatures of the structure’s dynamic response. For time domain OMA methods the non-parametric signatures are often correlation functions (CFs) and the pre-processing step for these methods is thus the estimation of CFs. The present paper demonstrates how measurement noise from sensors and measurement equipment affects the estimated CFs. Furthermore, the influence of the measurement noise on the modal parameter estimates is discussed. It is shown how effects of this noise can easily be avoided by ignoring the first part of the CFs when estimating the modal parameters. This is demonstrated by a theoretical review and on simulated and experimental data. The paper also addresses how to add noise to simulated data, so that it resembles a real-life scenario.

Dynamic Characterization of the Little Belt Suspension Bridge by Operational Modal Analysis

The (new) Little Belt Bridge, opened in 1970, is a Danish suspension bridge with largest span being 600 m, and a total length of 1700 m. During the design and construction phase, detailed analysis of the dynamic properties of the bridge were carried out both by hand calculations and by measurements on a scale model. Recently, the bridge was measured using a setup of 45 simultaneous responses, 30 vertical and 15 lateral, distributed over the main span. Operational modal analysis was carried out on the data set, and the first two vertical bending and torsional modes were compared to those of the model. It was found that the first vertical bending mode was 0.156 Hz, which is very near the frequency predicted by the original scale model. In total nine modes are reported, with frequencies from 0.156 to 0.808 Hz, and with damping values between 0.38% and 9.74%. This paper also demonstrates and discusses the use of inexpensive geophones and general measurement equipment for use in OMA applications based on the experience from this bridge measurement.