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The Physics of sound in buildings

When designing a room where music will be played or drama productions performed the architect has to consider how sound will reflect from its inside walls. If the walls are hard the sound will reflect well and the room will have a "bright" sound quality with a lot of echoes. However if the walls are covered with cork or cloth the sound will be absorbed and the room will appear "acoustically dead". Another problem is that different surfaces absorb different frequencies by different amounts. If sound engineers are not careful they may produce a surface that will remove all the bass from a sound far more quickly than the treble leaving it with a thin, cold quality.

An important property in building design is known as the reverberation time.

This gives an idea of the echoing nature of the room. It can be defined as the time taken for the sound level to fall to 10-6 of its original value (a drop of 60 dB). For symphonic music the best reverberation time is 2.0s; Symphony Hall in Boston, one of the finest concert halls in the world has a reverberation time of 1.8s with a full audience.

However even the excellence of this hall is probably surpassed by Symphony Hall in Birmingham. This hall has large doors set in the upper tiers of the hall which can be opened or closed so 'tuning' the hall to vary the reverberation time. When the doors are open the hall is more sound absorbent and when they are closed more sound is reflected and so the reverberation time is longer.

The reverberation time is always measured when there is an audience because one person is equivalent to about 0.5m2 of open window! In other words people absorb sound. Medieval cathedrals have long reverberation times of between 5 and 10s which meant that music had to be sung slowly to avoid confusion for the listener.

The reverberation time of a room can also be changed by altering the wall coverings or by adding suitable reflecting surfaces (the hanging "dishes" in the Royal Albert Hall in London). It is also now possible to change the acoustic quality electronically. The sound is recorded by suitably placed microphones and then transmitted at a suitable volume and after the required delay to give the impression of an echo.

Student Investigation
Design and carry out an experiment using a sound level meter to measure the reverberation time of your laboratory and school hall or sports hall.
If possible try to measure this both with the rooms empty and full of people. If it is available a data-logging device should be used here to plot intensity against time.
© Keith Gibbs 2011