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Concert Hall Acoustics: Art and Science
How unwanted sounds are removed from concert halls
One of the major advances in auditorium design in the past century has been the elimination of outside noise from auditoria.
Imagine all the sources of noise around a concert hall in a busy city centre - yet a modern concert hall is one of the quietest places you are likely to visit.
In older halls like the Free Trade Hall, Manchester, traffic noise could sometimes be heard during a concert. At the new Bridgewater Hall in Manchester, the noise of passing traffic and trams cannot be heard in the auditorium. When a bomb exploded in the centre of Manchester in 1996, workmen in the nearby Bridgewater Hall didn’t hear the blast!
There are many different pathways by which sound can enter a building.
Sound can enter a building by travelling through the air, and then through the walls and roof. This is called airborne sound. Sound can also be transferred into a building by travelling through the ground and building structure. This is called structure borne sound.
To prevent unwanted airborne sound from entering the auditorium, the building must have good sound insulation. This used to be achieved by having thick, massive walls. We nowadays use multiple layer lightweight constructions with a wide air gap and, where possible, sound absorbing material between the layers. An example of this is double glazing. It is important that there are no holes in the walls as even a tiny air gap lets through a surprising amount of sound.
The Royal Festival Hall was one of the first concert halls designed to eliminate external noise. The building materials were carefully selected to give the required sound insulation. The building itself is designed like an ‘egg in a box’.
Traffic and trains can also cause unwanted vibration. This can be a particular problem in cities, especially like those in London which have underground railways. To prevent vibration and structure borne sound from entering a building the builiding has to be isolated from the surrounding ground. This is often done by supporting the foundations of the building on spring mountings and/or on pads. The exhibition includes examples of anti-vibration mounts, springs and bads, used in various concert halls.
The above diagram shows the construction of the Bridgewater hall, click image to see more about the anti-vibration mounting used to reduce the effect tram and motor traffic vibration.
An acoustic consultant will usually specify a low level of background noise for an auditorium. One possible sources of background noise is the noise from the ventilation system. To reduce background noise, all the building services elements are mounted on springs and any connections are made flexible by using elements such as rubber hoses.
Symphony Hall Birmingham has a very low level of background noise, so any noise made by the audience, such as coughing or rusting of sweet papers, is very noticeable. It is said that since Symphony Hall opened, audiences in Birmingham have become quieter!
(c) Bridget Shield, Trevor Cox 1999/2000. The material maybe re-used provided a link to the website is provided and a clear acknowledgement to the curators given.