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AURACLE ACOUSTICS  -  Background

Introduction

The term 'acoustics' may be used to describe the field of study covering the generation, propagation and control of vibrational waves in gases, liquids and solids.

The term 'sound' is generally used to refer to those acoustic waves which can be perceived by the human hearing system, nominally ranging in frequency from 20 Hz to 20 kHz and most commonly propagating in air.  The field of acoustic therefore includes this study of sound, but also includes topics such as vibration (which may occur in the ground or in buildings, vehicles and other structures, and may be at very low frequencies), ultrasonics (vibrations in air or other materials which have frequencies above 20 kHz) and specialist areas such as electro-acoustics, underwater acoustics and the acoustics of musical instruments.

The most common unit if measurement used in acoustics is the decibel (dB) which is used to express sound pressure levels, sound power levels and vibration levels as a ratio compared to standard reference levels. The decibel scale is a logarithmic scale (rather than a linear scale) and has been adopted because it enables the very large ranges of values found in acoustics (many orders of magnitude for audible sounds for instance) to be expressed using relatively small numbers (up to about 120 or so for audible sounds for instance). Additionally, levels of vibration are often expressed in terms such as acceleration, displacement and peak particle velocity.

An increase or decrease of 3 dB is equivalent to a doubling or halving of acoustic energy, while an increase or decrease of 10 dB is equivalent to a ten-fold increase or decrease of acoustic energy.

Under free-field conditions (ie outdoors away from acoustically reflective surfaces) sound pressure levels due to propagation from a point source (eg at a sufficient distance from an item of mechanical plant) reduce by 6 dB per doubling of distance. Under the same conditions sound pressure levels due to propagation from a line source  (eg a road or railway) reduce by 3 dB per doubling of distance.

Environmental acoustics

The field of environmental acoustics may be taken to include noise and vibration arising from sources such as transportation, industrial premises, minerals extraction sites, construction and demolition sites, and waste transfer and waste handling facilities such as landfills, waste incinerators and civic amenity sites.

Potentially affected sensitive receptors include housing (existing or proposed), schools, hospitals and places of worship. It is frequently necessary to control noise and vibration arising from new or existing sources such that impacts at new or existing sensitive receptors are satisfactory and do not exceed specific target levels.

Under planning legislation individual local planning authorities have a responsibility to take account of potential noise and vibration impacts when considering applications for new developments. To this end they may adopt specific target levels to control noise and vibration impacts (which may be set out in the Local Plan or corresponding documents) or they may rely solely on the guidance provided by central government.

Each local planning authority also has a responsibility to patrol its area and to investigate complaints which may arise from any source of noise and/or vibration.  In this context local authorities rely on nuisance legislation.

UK guidance

The central planning guidance document relating to environmental noise and vibration in the UK is Planning Policy Guidance Note 24 'Planning and Noise' (PPG24). This document was published by the Department of the Environment in September 1994, and has since been adopted by most local planning authorities.

It makes reference to a number of other guidance documents which deal with specific areas of the topic such as the measurement and assessment of the sound insulation provided by building elements such as external facades and internal party walls and floors, the rating of noise from industrial noise sources, and the evaluation of human exposure to environmental vibration in buildings.

A key feature of the document is the setting out of Noise Exposure Categories (NECs) for sites which are being considered for residential development and where noise from existing sources may be an issue. The four NECs, A, B, C and D, are defined for transportation noise (road, rail and air traffic) and for noise from mixed sources, which may include include transportation and industrial noise sources, but where no individual source is dominant.

Guidance is provided for each NEC with a view to enabling local planning authorities to ascertain the suitablility of a given site for residential development so that appropriate and consistent planning decisions may be made.  This guidance is includes some flexibility so that it may be interpreted in accordance with local conditions, and most local planning authorities now follow it reasonably closely.

PPG24 also makes reference to a number of other nationally adopted guidance documents covering topics which include the monitoring and prediction of road traffic noise, railway noise and industrial noise (both permanent industrial sites and shorter-term activities such as construction sites and quarries), the measurement and assessment of the acoustic performance of building elements, and the evaluation of human exposure to vibration in buildings.

Some of these documents are referred to on other pages of this website which can be accessed via the Activities page.

 

Copyright © 2003-2006 Owen Clingan - Auracle Acoustics

 

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