2 - Basic Acoustics and Human Sound Perception

Summary

INTRODUCTION

This chapter introduces some important factors in the understanding of sound, sound propagation and transmission, and sound perception without, as far as possible, any maths or complex scientific descriptions. There are plenty of text books available that do include this greater level of detail, but this chapter, and the one that follows it, are intended as a general introduction to those who may need a working knowledge of some of the concepts involved and, in particular, how they may affect legal arguments and the formulation of legal cases.

It outlines the objective concepts that are used to describe sound, together with a brief introduction to the way sound is perceived and how it can lead to annoyance and, possibly, nuisance.

It is important that any legal considerations are properly informed by reference to objective measurement and assessment, in order that any decision-makers can properly take them into consideration and that those presenting the arguments, on both sides, understand the work of the expert witnesses, on both sides, and their reports.

BASIC ACOUSTICS

Sound pressure level

Sound is, essentially, rapidly fluctuating pressure above and below the resting pressure of the atmosphere. The average atmospheric pressure at sea level is just over 100,000 Pascals; Pascals being the international standard (SI) unit of pressure. The lowest pressure fluctuations that can be detected by the ear are around five billion times less at 20 micro-Pascals. As the pressure fluctuations increase, the corresponding sound gets louder, up to the point where it becomes physically uncomfortable to listen to, which occurs at a fluctuation level of around 20 pascals where physical damage to the hearing mechanism can occur.

It can readily be seen that representing the magnitude of the sound using the physical attributes of the pressure signal, as it oscillates above and below atmospheric pressure, is unwieldy. It needs to be quantified in such a way that an unchanging level of sound can be quantified with a single unchanging number.