Yasunao Matsumoto

Non-Linear Characteristics in the Dynamic Responses of Seated Subjects Exposed to Vertical Whole-Body Vibration

The effect of the magnitude of vertical vibration on the dynamic response of the seated human body has been investigated. Eight male subjects were exposed to random vibration in the 0.5 to 20 Hz frequency range at five magnitudes: 0.125, 0.25, 0.5, 1.0 and 2.0 ms(-2) r.m.s. The dynamic responses of the body were measured at eight locations: at the first, fifth, and tenth thoracic vertebrae (T1, T5, T10), at the first, third, and fifth lumbar vertebrae (L1, L3, L5) and at the pelvis (the posterior-superior iliac spine). At each location, the motions on the body surface were measured in the three orthogonal axes within the sagittal plane (i.e., the vertical, fore-and-aft, and pitch axes). The force at the seat surface was also measured. Frequency response functions (i.e., transmissibilities and apparent mass) were used to represent the responses of the body. Non-linear characteristics were observed in the apparent mass and in the transmissibilities to most measurement locations. Resonance frequencies in the frequency response functions decreased with increases in the vibration magnitude (e.g. for the vertical transmissibility to L3, a reduction from 6.25 to 4.75 Hz when the vibration magnitude increased from 0.125 to 2.0 ms(-2) r.m.s.). The transmission of vibration within the spine also showed some evidence of a non-linear characteristic. It can be concluded from this study that the dynamic responses of seated subjects are clearly non-linear with respect to vibration magnitude, whereas previous studies have reported inconsistent conclusions. More understanding of the dependence on vibration magnitude of both the dynamic responses of the soft tissues of the body and the muscle activity (voluntary and involuntary) is required to identify the causes of the non-linear characteristics observed in this study.

Mathematical models for the apparent masses of standing subjects exposed to vertical whole-body vibration

Linear lumped parameter models of the apparent masses of human subjects in standing positions when exposed to vertical whole-body vibration have been developed. Simple models with a single degree-of-freedom (d.o.f.) and with two (d.o.f.) were considered for practical use. Model parameters were optimised using both the mean apparent mass of 12 male subjects and the apparent masses of individual subjects measured in a previous study. The calculated responses of two (d.o.f.) models with a massless support structure showed best agreement with the measured apparent mass and phase, with errors less than 0.1 in the normalised apparent mass (i.e., corresponding to errors less than 10% of the static mass) and errors less than 5° in the phase for a normal standing posture. The model parameters obtained with the mean measured apparent masses of the 12 subjects were similar to the means of the 12 sets of parameters obtained when fitting to the individual apparent masses. It was found that the effects of vibration magnitude and postural changes on the measured apparent mass could be represented by changes to the stiffness and damping in the two (d.o.f.) models.

Modelling the dynamic mechanisms associated with the principal resonance of the seated human body

OBJECTIVES Simple mathematical models have been developed to obtain insights into resonance phenomena observed at about 5 Hz in the dynamic responses of the seated human body exposed to vertical whole-body vibration. DESIGN Alternative lumped parameter models with a few degrees-of-freedom have been investigated. Rotational degrees-of-freedom, with eccentricity of the centre of gravity of the mass elements, represented responses in the fore-and-aft and pitch axes caused by vertical vibration. BACKGROUND The causes of body resonance are not fully understood, but this information is required to develop cause-effect relationships between vibration exposures and effects on human health, comfort and performance.Method. The inertial and geometric parameters for models were based on published anatomical data. Other mechanical parameters were determined by comparing model responses to experimental data. RESULTS Two models, with four and five degrees-of-freedom, gave more reasonable representations than other models. Mechanical parameters obtained with median and individual experimental data were consistent for vertical degrees-of-freedom but varied for rotational degrees-of-freedom. CONCLUSIONS The resonance of the apparent mass at about 5 Hz may be attributed to a vibration mode consisting of vertical motion of the pelvis and legs and a pitch motion of the pelvis, both of which cause vertical motion of the upper-body above the pelvis, a bending motion of the spine, and vertical motion of the viscera. RELEVANCE The mathematical models developed in this study may assist understanding of the dynamic mechanisms responsible for resonances in the seated human body. The information is required to represent mechanical responses of the body and assist the development of models for specific effects of vibration.

Annoyance of low frequency tones and objective evaluation methods

Annoyance of low frequency pure and combined tones was measured in a laboratory experiment. Three low frequency tones at frequencies of 31.5, 50 and 80 Hz at four sound pressure levels, from about 6 dB to 24 dB above average hearing threshold, were selected as pure tones. The combined tones were combinations of two tones: the four levels of 31.5, 50 and 80 Hz tones and a constant level 40 Hz tone. The results showed that the rate of increase in annoyance of pure tones with increase in the sound pressure level was higher at lower frequencies, as reported in previous studies. The results for the combined tones showed that the increase in the annoyance of the combined tone compared to the annoyance of pure tone was dependent on the level difference of the two tones and their frequency separation. These results were compared with the evaluation obtained from different objective methods. The three methods were Moores loudness model, the low frequency A-weighting and the total energy summation used as objective evaluation methods. Among the methods, the low frequency A-weighting gave the best correlation.

Analytical Damping Evaluation Complementary to Experimental Structural Health Monitoring of Bridges

AbstractStructural health monitoring (SHM) of bridges by visual inspection is not necessarily reliable as many damages are not discovered during the periodic inspection of bridges. A technique to assist in visual inspection is vibration-based SHM. It is believed that structural damages lead to changes in stiffness and damping properties, and change the dynamic characteristics of structures, such as natural frequency, mode shape, and modal damping ratio. Although changes in the modal damping ratio can be used as damage indicators in the field of vibration-based SHM, the method’s accuracy remains concerning. This study investigated the analytical modal damping evaluation as a complementary method to the experimental SHM of bridges. An energy-based damping model was introduced to estimate the damping parameters of a steel arch bridge, such as the equivalent loss factors of structural components, and the modal damping ratios of the bridge were then analytically evaluated using the damping parameters. The resu...

Vibro-acoustic analysis of noise generation from a full scale model of modular bridge expansion joint

Noises generated from modular bridge expansion joints during vehicle pass-byshavebeencausinglocalenvironmentalproblemsrecently.Previousexperimentalstudies showed that possible causes of dominant noise components generatedfromthebottomsideofthejointmightbedifferentfromthosefromthetopside.The objective of this study was to obtain theoretical insights into the mechanismof noise generation from the bottom side of the joint for which a main noisesource might be structural vibration of the joint. Vibro-acoustic analysis wasconducted based on the information from a full-scale model of modularexpansion joint obtained in previous experimental studies. The dynamicbehavior of the joint model was investigated by using the finite element method(FEM) and the sound field inside the cavity located beneath the joint model wasanalyzed by using the boundary element method (BEM). Indirect BEM wasused to calculate the sound pressure inside the cavity with the velocity responseobtained by the FE analysis as a boundary condition. The frequency rangeconsidered in the analysis was 20–400 Hz where dominant frequencycomponents were observed in the noise measured in the cavity beneath the jointin the previous experiment. It was intended to interpret numerical resultsobtained by a model developed with available mechanical properties of the jointcomponents to seek a general understanding of the noise generation mechanismof the modular expansion joint. It was observed that the peaks in the spectrumofnoiseinsidethecavitywereduetoresonancesofstructuralvibrationmodesofthe joint and/or resonances of acoustic modes of the cavity.There was evidencethat showed possible interaction between structural modes of the joint and theacoustic modes of the cavity.

An Investigation of the Perception Thresholds of Band-Limited Low Frequency Noises: Influence of Bandwidth:

Perception thresholds of complex low frequency noises have been investigated in a laboratory experiment. Sound pressure levels that were just perceptible by subjects were measured for three complex noises and three pure tones. The complex noises had a flat constant spectrum over the frequency range 2 to 10, 20, or 40 Hz and decreased at 15 dB per octave at higher frequencies. The frequencies of the pure tones used in this study were 10, 20 and 40 Hz. The perception thresholds were obtained using an all-pass filter, one-third octave band filters, and the G frequency weighting defined in ISO 7196. The G-weighted sound pressure levels obtained were compared with 100 dB which is described in ISO 7196 as the G-weighted level corresponding to the threshold of sounds in the frequency range 1 to 20 Hz. The perception thresholds of the pure tones measured in this study were comparable to the results available in various previous studies. The one-third octave sound pressure levels obtained for the thresholds of the complex noises appeared to be lower than the measured thresholds of the pure tones. The G-weighted sound pressure levels obtained for the thresholds of the complex noises appeared to be lower than 100 dB.

Masked Perception Thresholds of Low Frequency Tones Under Background Noises and Their Estimation by Loudness Model

This paper presents experimental measurements of masked thresholds of low frequency tones under background noises and application of loudness model to estimate the thresholds. The measurements of thresholds for tones at frequencies 20, 31.5, 40 and 50 Hz were conducted under three background noise conditions: one ambient noise and 60–100 Hz band-pass noises at two levels. The measurements were carried out in an uncontrolled environment in relatively quiet times. The perception thresholds of the same subjects were also measured for frequencies 31.5, 40 and 50 Hz inside a cabin with ambient noise levels well below the average hearing thresholds specified in ISO 389–7. Moores loudness model has been used to estimate the masked thresholds. The estimated thresholds from the loudness model have been compared with the results obtained in the experiment. The results indicate that the noise above 50 Hz is effective in masking the low frequency tones at 50 Hz and below, and that Moores loudness model can predict reasonably the average of the measured masked thresholds.

INCE/J Vibration Measurement Manual for Buildings—Part 2: On the assessment based on the vibration

This paper discusses the assessment of building vibration in terms of human responses based on vibration measurements specified by the Vibration Measurement Manual for Buildings developed by the Technical Subcommittee on Environmental Vibration Evaluation of the Institute of Noise Control Engineering of Japan. As stated in some of the current international and national standards, building vibration could cause adverse comments from occupants when the vibration magnitude is in excess of human perception only slightly. For the assessment of building vibration based on the measurement manual, therefore, a range of human vibration perception threshold was derived from the previous studies that determined the perception thresholds in experiment involving human subjects. The vibration perception threshold range implies a range of vibration magnitude for different frequencies that may correspond to lowest vibration magnitudes people with different sensitivities to vibration can detect. The vibration magnitude wi...

Recent research activities for the assessment of vibration in living environment with respect to human perception in Japan

The number of complaints against vibration has been increasing gradually in recent years in Japan. This fact may imply problems of the Vibration Regulation Law that was implemented almost 30 years ago so as to regulate vibrations caused by factories and construction works and to mitigate vibration problems caused by road traffic. A group of experts has been investigating an assessment method that may be able to assess recent vibration problems reasonably since 2004. This paper presents a part of group activity that aims at improving the understanding of human vibration perception and applying it in vibration assessment in living environment. Results obtained from experiments involving human subjects to determine perception thresholds of whole‐body vibration are summarised.