This page contains a Flash digital edition of a book.

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 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. 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)


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84