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CPD PROGRAMME


Professional development


The CIBSE Journal CPD Programme


Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.


Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).


CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.


Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current.


You can also complete the questionnaire online, and receive your results by return email. Basic acoustic terminology for building services


This module explains the fundamentals of acoustic terminology used in building services as a basis for setting standards and maintaining acceptable noise levels


Acoustic analysis for building services becomes more demanding with novel forms of passive and active environmental systems. However, the basic principles of acoustics do not change – it is simply their application. The impact of ‘sound’ in buildings can be airborne and so heard directly by the ear, or passed through surfaces so that it is felt as vibrations. Whether that sound is treated as ‘noise’ will depend on circumstance – a hum from a fan may be reassuring sound for the facilities manager but could be an intrusive noise to the office worker. This CPD will explain the underlying acoustic terminology that provides a basis for setting the standards and maintaining airborne noise at an acceptable level appropriate to the application. A very simple (pure tone) sound will


travel through the air as shown in Figure 1. The distance between the ripples of


successive high pressure points is known as the wavelength, λ (m), of the sound and the frequency, ƒ (Hz), is the number of times that the pressure peaks per second at a particular point. The pressure ‘wave’ will travel through the air at speed c (m·s-1


), varying the pressure at each


point somewhere between maximum and minimum pressure; this range is twice the amplitude, P (Pa). Wavelength and frequency are related by λ = c / ƒ.


www.cibsejournal.com time (s) f1 frequency (Hz)


Figure 2: Simple pure tone resolved into a single frequency and rms pressure


wavelength wavelength distance air particles condensation rarefation


Figure 1: A representation of a simple pure tone travelling through air


The sound travelling in the air (by


successive collisions of molecules) will have a velocity of about 343 metres in one second; this can be determined from the simplified relationship c = 20√T, where T is the absolute temperature of the air, K. A person would ‘hear’ the sound as the tympanic membrane in their ear moves in and out, as the sound ‘wave’ changes the pressure on the outside of the ear. The sound will also pass through solid materials, albeit much faster, as the molecules are more densely packed (for example, through brick at about 3,000 m/s). Whether passing through the air


or through a solid, there will be some attenuation of the sound’s strength as it travels, since some of the energy will be scattered and absorbed by the material (and some converted to heat). And, of course, for sound to travel there does need to be a liquid, gas or solid, so that there is a transfer of pressure variations – the more densely packed molecules will transfer sound more effectively. Sound is related to a cycling of


pressures, typically illustrated by a sinusoidal wave (as in Figure 2). This form of representation is a convenience that illustrates the pressure variations with time at a particular point; actually, the sound is a longitudinal wave (moving in all directions), with areas of high and low pressure (known as ‘compressions’ and ‘rarefactions’, as in Figure 1) that ‘travel’ like the ripples on a pond, where the wave moves (losing intensity as it proceeds) without any flow of water. A ‘pure tone’ can be represented simply in terms of its frequency, ƒ1


, with its strength


represented by the height of the line (as in Figure 2). More complex sounds can be resolved


into a number of different frequencies (each with their own amplitudes) that can be represented as combining together to make a sound. For example, the relatively


March 2012 CIBSE Journal 65


pressure (Pa)


pressure


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