Room acoustic
In domestic circumstances the internal acoustics of rooms are not usually critical, but in larger spaces such as lecture rooms, conference and music rooms or in assembly halls and auditoria the satisfactory use of space depends on satisfactory acoustic conditions.
This is a problem of considerable complexity in auditoria and expert consultants will normally be employed to advice on the problem.
There are the planning and shape of the room to give good path of sounds between the speakers or performers to the audience and the rate at which a sound dies away.
Direct sound
Ideally all the occupants of a space should be able to hear by means of a direct path of sound between the speakers or performers and each individual occupant.
In conference venues, council chambers or boardrooms, where any occupant may be called upon to speak, this is achieved by layout of seating so that all the occupants can see one another clearly.
A long narrow board table will be much less satisfactory than circular arrangement.
In rooms where there is a large passive audience, it is possible to improve the direct path of sound by raising the speaker and also by raking the seating so that each member of the audience is less obstructed by those in front.
Sound reflected once from a space can, if it is directed in the right direction, useful augment the direct sound.
Shaping an auditorium to improve the path of sound from source to audience.
Slope of ceiling directs
sound to audience
Slope of ceiling directs
sound to audience at back
Rake of seating improves
direct paths to individual members of the audience
It is important that the extra distance which first reflections have to travel over the direct path should not be too great.
If the additional distance is over 20m, the reflected sound will arrive at the listener noticeably later than the direct sound and give rise to an echo.
Care is therefore needed in disposing reflecting surfaces to ensure intelligible reinforcement of sound without echo.
In general reflecting surfaces should be flat, curved surfaces may give rise to sharp concentrations of sound in some places and lack of reinforcement elsewhere.
In the same way that properly disposed reflecting surfaces can assist in distributing sound, inappropriate surfaces will have an undesirable effect.
Cross-beams projecting below a general ceiling level are very undesirable since they prevent part of the ceiling from reflecting sound to the audience.
Reflecting surfaces do not necessarily have to be part of the main fabric of the building.
In very large halls it is not unusual to hang a reflector over the speaker’s position.
The Building Research Establishment recommends as a minimum thickness and weight a surface of 25mm plasterboard, painted on the side used for reflection.
Hall Shapes
Generally, there are 3 types of shapes describe as an important feature for good acoustics of an auditorium:
a. Rectangular
Koerner Hall in Toronto, Canada
A rectangular plan or ‘shoebox’ is the traditional shape of many successful older concert halls.
The sound waves tend to establish themselves along the length of the hall and all listeners receive a strong component of direct sound.
Reflectors can be used to direct sound to the rear of the hall and absorbers used to prevent unwanted reflections.
Traditional ratios of dimensions for height, width and length are about 2 : 3 : 5.
b. Wide fan
Harrogate International Centre, UK
A fan shape or short wide hall allows more people in the audience to be near the source of sound and gives them better views of the stage.
Wide curves were used for the seating of ancient outdoor amphitheatres can give an acceptable acoustics.
c. Horseshoe
Reverso Squadra, Singapore
A horseshoe shape is common in traditional opera houses where the tall concave shape at the back of the hall is broken up by tiers of seats and boxes.
The audience and furnishings (seats) in these tiers also act as an absorber.