Exploring the nature of sound in buildings

Each and every day, we experience a symphony of sounds to make our lives richer writes Stuart Colam, Acoustic Engineer, SAS International. From the dawn chorus to the soothing sounds of a gentle stream, and the less desirable noises such as machinery and a baby crying; the auditory stimulus around us is vast and elicits a specific response in our minds. But what specifically issound? How is it made? How does it travel? And why can we hear it?

If air was visible then it might make it easier to explain, but sound is essentially bits of air vibrating. These air molecules vibrate and bump into each other, which in turn results in a local increase in air pressure. This chain reaction happens quickly, with the speed of sound in air being about 770mph. One way to illustrate this is by a slinky spring sending pulses backwards and forwards along its length.

If nothing makes your eardrum move you will not hear anything. If there is no variation in the air pressure there is no sound.  Air pressure varies with height, decreasing with increasing altitude. In other words, there are fewer air molecules at 8000 metres than at sea level, for instance. This is why at the summit of Mount Everest the air is thinner.  

Chain reaction

Something needs to happen to make the air molecules move, i.e. for sound to be produced. When this occurs, one molecule will bump into another and this chain reaction results in the molecules being closer together than they would have been. An increase in pressure commences, as the molecules are essentially being squashed together and passing on vibrational energy; molecule to molecule.  The movement of air propagates and that's what we call sound. As they bump into each other there is an increase and decrease in air pressure. This push and pull of sound waves reaches your ear and vibrates your eardrum. This ultimately results in electrical signals being sent to your brain and interpreted as sound. 

Sound travels at different speeds depending on the medium. In steel, sound travels 17 times faster than in air, while in water it travels about four times faster.

Think about when you are swimming in the sea and how hard it is to gauge where the noise of a boat engine is coming from. Sound travels more efficiently and faster through water than air. All sound needs is something to vibrate and bump into, which is why in space the absence of molecules makes it impossible to transmit any kind of sound.

Sound in the built environment is sometimes overlooked and should be an important consideration.  When designing modern interiors there is much more than meets the eye – we must consider the ear as well.  It's an issue that has become particularly important due to the proliferation of open and agile working environments. Sweeping interiors are prominent in today's commercial buildings; therefore there is a growing demand for ceiling designs to suit these interiors, whilst still controlling sound travel.  It's why metal ceilings have become the go-to solution that ensures these open-plan designs are not jeopardised by noise levels.

A client might well place greater emphasis on aesthetics, but a good design must deliver effective sound management and an acoustic landscape which positively impacts on the productivity and wellbeing of building occupants.

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