3 - The Measurement and Assessment of Noise

Summary

INTRODUCTION

The purpose of this chapter is to explain how noise is quantified using a ‘sound level meter’ with its basic function explained and the various options that are available for assessing the noise in various circumstances, in order to provide objective assessments of noise as it affects people. In this chapter the word ‘noise’ will be used rather than sound, bearing in mind the commentary in the previous chapter on the fact that noise is essentially unwanted sound and, for the most part, this is why it is being assessed. It should, however, be borne in mind that unwanted sound is often heard in the context of ‘wanted’ sound. However, for the purposes of brevity, noise will be used, in most cases, to cover the measurements and assessments described in this chapter.

THE SOUND LEVEL METER

It is rather ironic, perhaps, in the context of the above, that noise measurements are made using a device called a ‘sound level meter’, which is used to quantify ‘sound pressure level’. But, of course, such an electronic device is incapable of making such a distinction! Sound pressure level is explained in Chapter 1 and is a way of quantifying a sound that is constant in level as a single number value expressed in decibels (dB).

Basic function

A sound level meter consists of a microphone, which converts the fluctuating sound pressure into a fluctuating electrical signal corresponding precisely, subject to certain tolerances, to the pressure signal it is being used to measure. This is converted by electronic circuitry to a fixed voltage corresponding to the voltage fluctuations over a specified very short time period referred to as the ‘time-weighting’, as discussed very briefly under the definition of sound pressure level in Chapter 1. The specifications for sound level meters are very precise in respect of microphone directivity, frequency response and linearity (changes in noise level being accurately matched by changes in reported sound pressure level) and are regulated by the provisions of BS EN 61672, Electroacoustics – Sound Level Meters. This document provides specifications for two ‘classes’ of meter: Class 1 and Class 2 with Class 1 meters normally being use for environmental noise measurements due to their superior performance.