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.