acoustics of buildings
Section through a concert hall showing where the sound waves are reflected.
In designing a concert hall or theatre, architects not only have to take into account the number of people to be seated and the arrangement of the seats in relation to one another, the position of the gangways and exit doors, and details of ventilation and illumination. They must also give careful consideration to the acoustics of the building. Will the full range of sound waves reach every seat in the auditorium? Will there be too much reflection, or is too much sound going to be absorbed by the walls and ceiling?
Sound waves travel in straight lines and will go out from their source in all directions. In much the same way as with light, sound waves can be reflected or absorbed by surfaces which they strike. Thus hard polished surfaces reflect sound, while softer matt surfaces absorb it. A concave surface tends to focus sound waves over a small area.
As the sound absorption properties of a particular substance vary for different frequencies, it is desirable that sound should have an unobstructed path from the source on the platform direct to each member of the audience. For this reason pillars and other obstacles in the auditorium are to be avoided if at all possible. Modern methods of construction, especially the use of pre-stressed concrete, are valuable in this respect.
The final tonal quality of the sound depends largely upon a portion of the sound waves reaching the listener by reflection. Each time a sound wave is reflected, some of the sound is lost by absorption. Thus after it has been reflected a few times, the sound will die away completely. There will always be a slight time lag during which the sound gradually dies away even if the source is stopped suddenly. This time lag is known as the reverberation time and a satisfactory value for concert halls of moderate size is about 2 seconds.
The extent of the reflections will depend partly upon the shape and size of the auditorium, but also upon the nature and reflective proportions of the reflecting and absorbing surfaces. Irregularities in the walls and ceilings, in the form of alcoves and recesses, can cause unwanted reflections and are to be avoided. The height of the roof of the concert hall and the curvature of its surface have to be carefully chosen in order that the sound waves which it collects are reflected back over as large an area of the auditorium as possible. Otherwise there is a risk of there being 'dead' spots where reflected sound cancels out direct sound.
At the design stage the wise architect will check the acoustic properties of a proposed concert hall, either by means of models or by calculations based on the rules for reflection of sound. One method of testing models is to make lengthwise sections of the hall and to place them in a shallow tank of water. Vibrations are then set up in the water from the point corresponding to the front of the concert platform. The ripple patterns formed in the tank give a very good indication of the points in the hall where unwanted reflections will occur. If these are serious, the design may have to be modified.
The refinements in the acoustics will depend largely upon the interior decoration and furnishing of the hall. The use of carpets on the floors, of absorbent materials (e.g. plaster) on the walls and the ceiling, and of suitable upholstered seats will help to put the finishing touches to satisfactory acoustic behavior. The acoustic properties of the paint or other wall covering are at least as important as its decorative value.
Various reflectors and baffles may be attached to the walls and ceiling to turn the sound waves in the most desirable direction. It is often necessary to have reflectors above the platform to throw the sound out into the body of the hall. The presence of an audience has considerable effect upon the acoustics of many concert halls as the members of the audience are themselves quite good absorbers of sound waves. In consequence, there are marked differences in the quality of music played in a half empty hall and when every seat is filled. One way to overcome this is to install seats designed so that they have similar absorption properties to those of the audience who may occupy them.
The purpose of a building should be reflected in its acoustic properties. In a theatre a shorter reverberation time is necessary (i.e. the number of reflections must be kept to a minimum), if the audience is to hear clearly what is being said on the stage. In contrast, the slower mellow tones of church music require that the reverberation time of a cathedral may be made longer ( 6 or 8 seconds).
In conclusion, mention may be made of the Whispering Galleries such as that in St. Paul's Cathedral in London. It is, at first, surprising to find that it is possible to stand at one side of the circular gallery and to be able to hear someone whispering at the opposite side. It has been shown that the listener is at the focal point of a giant concave reflector and that all sound waves coming from the opposite side of the gallery are collected by the reflector and at the focal point they reinforce one another.