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Green Building Physics - Acoustics
Green Building Physics - Acoustics
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Arguably ‘acoustics’ are one of the most overlooked elements in green building design. According to research by Roy, D (2007), users of green buildings indicated that one of their main complaints with the green buildings they use were acoustic issues concerning the acoustics of rooms and noise privacy issues. (The other main complaint was issues with ventilation). It could be interpreted that due to a focus on benefits that are directly economically beneficial, acoustics are sometimes overlooked. In this green building physics article, Gavin Harper explains the physics of acoustics.

This is a four page article. First published in June 2009

Sound can be understood as a wave. Pressure fluctuations result from the vibration of air molecules. When we speak, our vocal chords vibrate, and when we switch on some music, the cone of the speaker vibrates; this in turn causes the surrounding air to vibrate. Consider the following - we have a crank connected to a piston. With each rotation, the piston moves into the cylinder, out of the cylinder and then back to its original position – the cycle repeats. Each time this happens, the air in the cylinder is alternately squeezed and sucked. We can see in the cylinder how the air molecules are alternately compressed and rarified causing them to ‘bunch up’ in the tube. This is the nature of longitudinal sound waves. The faster the crank turns, the quicker the piston moves backwards and forwards and so the higher the frequency – which we perceive as a higher or more ‘treble’ tone. Now the changes in air pressure that we perceive as sound happen very rapidly. The ‘mains hum’ that can sometimes be heard in cheap audio equipment is the same frequency as the mains electricity – 50Hz, which is to say fifty times a second the air vibrates ‘back and forth’. So to recap – the speed at which the air vibrates back and forth is the frequency, but the amount of energy in those vibrations defines the ‘intensity’ of the sound. We use the units of ‘Watts per square metre’ or ‘W/m2’ to express the intensity of sound. Now as we move further away from the source of the sound, the intensity of the sound wave ...

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