SISTEMAS DE COSTE EN LOS CENTROS SANITARIOS

The statistical significance of the exposure-relationships presented in this chapter is not necessarily proof of a causal relationship between vibration exposure and annoyance due to vibration. At present, little is known regarding the physiological and psychological mechanisms which result in annoyance due to vibration and as such no definite claim can be made regarding the causality of the observed relationships. However, the findings presented in this chapter do suggest that, although not yet fully understood, a relationship does exist between vibration exposure and annoyance in residential environments and that this relationship can be described by curves indicating the proportion of the population expected to express annoyance above a given threshold for a given vibration exposure.

5.12

SUMMARY

This chapter has detailed the formulation of exposure-response relationships for the human response to vibration in residential environments. Relationships have been presented for vibration exposure assessed according to a number of different national and international standards. Due to differences in response between the two vibration sources considered in this thesis, separate exposure-response relationships for annoyance have been derived for railway and construction sources of vibration. Narrow confidence intervals, the statistical significance of the relationships, and a sensitivity analysis suggest

that these relationships are statistically robust. However, differences in response to the different vibration sources and the relatively low explained variance in the relationships suggest that improvements can be made to the relationships through investigation into new vibration exposure descriptors. As highlighted in section 5.11.5, little is known regarding the physiological and psychological mechanisms which result in annoyance due to vibration. Therefore, single figure descriptors based upon perceptual models would also improve the scientific plausibility of the exposure-response relationship. The following chapter details a pilot test designed to investigate the feasibility of using the method of paired comparisons and multidimensional scaling to investigate the perception of vibration from railway activities.

CHAPTER 6

CONSIDERING THE PERCEPTION

OF VIBRATION AS A

MULTIDIMENSIONAL

PHENOMENON

6

CONSIDERING THE PERCEPTION OF VIBRATION AS A

MULTIDIMENSIONAL PHENOMENON

6.1

INTRODUCTION

In the previous chapter, exposure-response relationships were derived for the human response to railway and construction induced vibration in residential environments. In the derived relationships, vibration exposure was expressed in terms single figure descriptors advocated in various national and international standards and guidance. Although the derived relationships were statistically significant and exhibited relatively narrow confidence intervals, it was noted that a large proportion of the variance in the response is unaccounted for and that significant differences in response can be observed for vibration from different sources. The differences and scatter in the response could be due to both non-acoustical factors and the inadequacy of the single figure descriptors to characterise the features of the vibration exposure which are salient to human perception.

In acoustics, in particular the fields of psychoacoustics and sound quality, it is widely accepted that the perception of sound is a multidimensional phenomenon. Multidimensionality in this context refers to the overall perception of a sound being made up of a number of perceptual dimensions which relate to separate objective features of the sound. In areas such as the perception of musical timbre and product sound quality, much research has been conducted to determine the perceptual dimensions which underlie the perception of a given set of sounds. In the case of product sound quality, these perceptual dimensions have been used to develop models which can be used to predict the perceived quality of a product based on objective acoustic features of the product sound. If a similar

may be possible to develop models to predict perceived annoyance based upon objective features of a measured vibration signal.

As highlighted in Chapter 2, much of the previous research into the perception of whole body vibration has been in the form of ranking or magnitude estimation tasks conducted in a laboratory setting using artificial signals such as pure sine excitation as stimuli. Although research of this sort provides a valuable insight into psychophysical aspects of vibration perception such as perception thresholds and subjective magnitude, these subjective test methodologies impose limitations on the researcher; namely, the perceptual dimension or dimensions of interest must be determined a priori (Torgerson, 1952). If the underlying perceptual dimensions of a certain stimulus type are unknown, then it is possible that psychologically relevant dimensions will be unaccounted for in models and metrics used to describe the human response to the stimulus.

The multidimensional nature of sound perception is highlighted by the rich vocabulary available for the description of auditory perception. For example, frequency characteristics of a sound can be described as “bright”, “sharp”, or “dull”; amplitude characteristics can be described as “loud” or “quiet”; and temporal characteristics can be described “fluctuating”, “peaky”, or “undulating”. In comparison to the perception of auditory stimuli, the vocabulary at our disposal for describing the perception of vibratory stimuli is rather limited. This suggests that the acuity of human perception of vibration is much less that that of the perception of sound.

The main aim of the work detailed in this chapter is to determine if the perception of whole body vibration is multidimensional in nature and if so, can these perceptual dimensions be represented by a non-metric representation of a group of objects in a low-

dimensional Euclidean space. The second aim of the work presented in this chapter is to determine if these perceptual dimensions can be related to annoyance responses.