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Curtainwall Acoustics

• An introduction to the acoustics of curtain walling systems
• Sound reduction through the external wall
• Sound reduction between floors and rooms
• Acoustic testing

An introduction to the acoustics of curtain walling systems

• Compared with most other aspects of a building's performance acoustics is rather more 'black art' than exact science, and this is accepted by many acoustics consultants. It is also an area where the users' perceptions may differ from the measured performance.
• These problems are compounded for curtain walling systems, with large expanses of glass affecting the sound reduction through the external wall and simple aluminium extrusions abutting plasterboard partitions providing the only resistance to the passage of sound from room-to-room.

Sound reduction through the external wall

• The most likely reason to specify a value for the sound reduction through the external wall is when the building is close to a significant noise source, such as a main road or railway, and the client has specific requirements for the noise level within the building - the British Council for Offices provides recommended noise levels within offices, for example.
• In these circumstances it is most likely that an acoustic consultant will be appointed, an acoustic survey of the site will be carried out, and the acoustic consultant will provide the architect with the required performance of the curtain walling to be inserted into the specification.
• The acoustic properties of glass are reasonably well understood but different methods for assessing the theoretical acoustic performance of particular combinations of glass will produce different values - significantly different in some cases.
• Most consultants will express the required values as eight "octave band centre frequencies" but some prefer single "weighted" values.

Sound reduction between floors and rooms

• BS 8233 "Code of practice for sound insulation and noise reduction for buildings" provides recommended values for the sound reduction in various types of building - 38dB between two cellular offices in 'normal' conditions, and 48dB where "privacy is important", for example (clause 7.6.3.1).
• Most standard curtain walling systems will achieve values in the range 30-35dB through typical mullions or transoms with the typical (double) glazing specifications used on most UK commercial projects. This means that some acoustic enhancement will be necessary to reach the values specified in BS 8233.
• Achieving values of the order of 45dB floor-to-floor is rarely a problem where the curtain walling includes spandrel panels, and fire stopping is provided between the spandrel panel and the floor slab edge.
• It is much more difficult, however, to achieve values in excess of 40dB through (a) transoms where we have continuous floor-to-floor glazing, or (b) through mullions in the room-to-room situation. The enhancements necessary to reach these values may have a significant cost.

Acoustic testing

• Although there are a number of established national and international standards for acoustic testing, they must be used with great care as it is generally accepted that laboratory tests rarely replicate theoretical values, and site tests rarely replicate the values achieved in the laboratory.

Acoustic testing - Sound reduction through the external wall

• The normal practice, endorsed by most acoustics consultants, is to specify this requirement by reference to BS EN ISO 140-3 "Laboratory measurement of airborne sound insulation of building elements".
• This enables laboratory testing of, say, different glass specifications to determine the one most appropriate for the project.
• There is recent experience that the test results, on samples which include the surrounding curtain wall framing, fall short of the theoretical values for the glass build-up, however calculated.
• The vast majority of curtain walls are not tested before installation, with both specifiers and contractors relying on the theoretical data. That so many are then considered acceptable by the users questions whether the theoretical values may be more relevant to the end user than the tested values, i.e. a form of construction with a calculated value of 48dB where "privacy is important" may represent sensible specification whereas a value of 48dB under test may represent over-specification. This may be worthy of debate with acoustic consultants.

Acoustic testing - Sound reduction between floors and rooms

• It is obviously impractical, on all but the very largest projects, to mock-up entire sections of the complete building in order to carry out floor-to-floor or room-to-room tests under laboratory conditions.
• Therefore the normal practice endorsed by most acoustics consultants, is to specify this requirement by reference to BS EN ISO 140-4 "Field measurements of airborne sound insulation between rooms".
• This requirement should sit within the curtain walling specification to ensure that the curtain walling contractor makes allowance for these requirements in the design. The test will, however, include the surrounding construction with all the obvious possibilities for dispute when the responsibility for failure - which is almost inevitable for the reasons stated above - is being apportioned.
• On-site testing should therefore only be specified if all parties are clearly aware of the limitations of the test methods and are prepared to be pragmatic when interpreting the results.

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