Acoustic performance of Composite Floors (Part 2) – Acoustic Principles
In the second part of our blog on the acoustic performance of composite flooring, Mark Davies, Technical Manager, ComFlor®, discusses the principles of airborne and impact sound, airborne and impact insulation and the transmission of sound in relation to composite flooring.
Steel construction is increasingly used in residential apartment buildings and mixed-use developments where the benefits of speed of construction, quality and off-site prefabrication are important. Steel is a quality assured, accurate, high strength, long life, adaptable, recycled and recyclable material, manufactured to tight specifications. It does not suffer from distortion or movement due to changes in moisture content. This results in easier fixing of linings and higher quality finishes, avoiding problems such as cracking around door architraves and skirtings.
Composite floors consist of profiled (galvanised) steel decking and an in-situ reinforced concrete topping. The decking not only acts as permanent formwork to the concrete, but also provides sufficient shear bond with the concrete so that, when the concrete has gained strength, the two materials act together compositely.
The main benefits of composite floors include the following:
- Speed of construction
- Safe method of construction
- Saving in storage and transport
- Structural stability
- Easy installation
Composite floors can use deep or shallow deck profiles. There are two generic types of shallow decking are re-entrant (dovetail) and trapezoidal profiles.
Acoustic performance has increased in importance in residential buildings as developers and occupants demand higher standards. Amendments to Part E of the Building Regulations came into effect in July 2003, which introduces a new measurement index, Ctr. This takes into account of the low frequency sounds that often cause problems in residential buildings, e.g. traffic and bass music.
The new regulations set more demanding requirements for the performance of separating floors and walls between dwellings, where composite floors are a very effective way of constructing separating floors with good acoustic performance. They rely on the use of some structural mass, a suspended plasterboard ceiling and a resilient floor system on the top surface to achieve excellent acoustic performance.
The purpose of this supplementary blog is to provide the relevant acoustic background principles, namely, principles of airborne and impact sound, airborne and impact insulation, single figure rating values and direct and flanking transmission, to supplement my main acoustic blog entitled – ‘Acoustic Performance of Composite Floors – Meeting the requirements of Part E of the Building Regulations’, which discusses Part E of the building regulations and its requirements, demonstrating compliance with Part E, and the use of the Tata Steel/SCI web based acoustic prediction tool.
This article is broken down into 4 main sections:
- Principles of airborne and impact sound
- Airborne and impact insulation
- Single figure rating values
- Direct and flanking transmission
1. Principles of Sound – Airborne and Impact Sound
Sound is caused when objects vibrate in air. The movement in turn causes air particles to vibrate giving rise to rapid pressure fluctuations, which are detected by the ear. The manner in which humans perceive sound governs the way it is measured and described. Two important characteristics of sound, which humans can detect, are the level or loudness and the pitch or frequency. Sound levels and sound insulation (i.e. attenuation) values are expressed in decibels (dB), whilst pitch or frequency is expressed in Hertz (Hz). In the case of sound levels, the decibel rating is a representation of the volume of the sound whilst in the case of sound insulation values it is a measure of the amount by which sound transmitted from one room to another is reduced by the separating construction.
The sound insulation properties of walls or floors vary with frequency and, as most sounds are a mixture of several different frequencies, certain frequencies within a sound are likely to be attenuated more effectively than others by a given construction. Low-pitched sounds (i.e. low frequencies) are normally attenuated less than high-pitched sounds (i.e. high frequencies). Therefore, the sound reduction characteristics of walls and floors are measured at a number of different frequencies across the hearing range. There are two types of sound that should be considered in the acoustic design of buildings, airborne sound and Impact sound.
2. Airborne and Impact Insulation
Airborne sound insulation is important for both walls and floors. Airborne sound insulation between rooms can be measured by generating a steady sound of a particular frequency in one room (the source room), and comparing it with sound in a second adjacent room (the receiving room). These measurements are made at a number of different frequencies. The difference between the two levels is referred to as the level difference D.
This level difference is also influenced by the amount of acoustic absorption within the receiving room itself. When a sound wave reaches a surface it will be partly reflected off the surface back into the room and continue traveling in a new direction, and it will be partly absorbed by the surface. The sound absorption of a room can be estimated by measuring the reverberation time T. The reverberation time is the time taken for the reverberant noise to decay by 60 dB. A sound created in a room with a long reverberation time will sound louder than the same sound created in a room with a short reverberation time. In order that airborne sound insulation measurements in different buildings may be compared, the level differences can be adjusted to a standard reverberation time of 0.5 seconds. This gives the standardised level difference DnT. Individual building elements such as partitions, doors or windows can be tested in acoustic laboratories. These laboratories comprise two massively constructed adjacent rooms that are isolated against flanking transmission and connected by an aperture containing a test panel of the building element. The level difference is measured between the two rooms and the result adjusted to be independent of both the area of the panel and the acoustic absorption of the room. The resulting value is the sound reduction index R.
Impact insulation is generally only relevant to floors. A standard impact sound source (a tapping machine consisting of automated hammers) is used to strike the floor repeatedly at a standard rate. The resulting sound in the receiving (downstairs) room is measured and this value is termed the impact sound pressure level L. Measurements in buildings can be standardised to a reverberation time of 0.5 seconds. This gives the standardized impact sound pressure level LnT which is a field measurement. Tests in laboratories, normalized for area and absorption give the normalized impact sound pressure level Ln. This test method means that the better the impact sound insulation, the lower the value of LnT or Ln.
3. Single Figure Rating Values
Sound insulation is measured at a number of different frequencies, usually at 16 one-third-octave bands from 100 Hz to 3150 Hz. However, for many purposes, including the requirements for dwellings given in building regulations, a single figure rating is required.
There are several methods that could be used to reduce the sound insulation values at the sixteen individual frequencies to a single figure value. An obvious method is to take the arithmetic mean, but very high levels of sound insulation at some frequencies can offset poor performance at others. The most common method of overcoming this is to compare the measured results with a set of sixteen reference results i.e. a reference curve. The rating is made, by considering only those sound insulation values which fall short of the reference curve. In this way, one or two very good results have much less effect on the single figure value. The method used for calculating a single figure airborne sound insulation value is shown graphically in Figure 1.
A similar method is used for impact sound.
The single figure values are called:
- Standardised weighted level difference DnT,w when generated from DnT
- Weighted sound reduction Rw when generated from R
- Standardised weighted impact sound pressure level L’nT,w when generated from L’nTw
- Normalised weighted impact sound pressure level L n,w when generated from Ln.
Until 2003, the Standardised Weighted Level Difference DnT,w was used as the single figure index for airborne sound insulation in the Building Regulations. From July 2003, a new measurement index was introduced for airborne sound, DnT,w + Ctr. The Ctr term is a spectrum adaption term, which is generally negative and adjusts the index by taking additional account of the low frequency sounds that often cause problems in residential buildings. Thus a DnT,w + Ctr rating is generally lower than the DnT,w rating for the same construction. The DnT,w + Ctr index places more weighting on low frequency sound. Impact sound transmission is measured by L’nTw, the Standerised Weighted Impact Sound Pressure Level.
4. Principles of acoustic detailing – Direct and flanking transmission
Where a room is separated from another room, sound can travel by two routes: directly through the separating structure called direct transmission, and around the separating structure through adjacent building elements called flanking transmission. These routes are indicated in Figure 2. Sound insulation for both routes is controlled by the following three characteristics, namely, mass, isolation and sealing.
Direct transmission depends upon the properties of the separating wall or floor and can be estimated from laboratory measurements. Flanking transmission is more difficult to predict because it is influenced by the details of the junctions between the building elements and the quality of construction on site. It is notable that, in certain circumstances, such as where separating walls have a high standard of acoustic insulation but side walls are constructed to lower standards and are continuous between rooms, flanking transmission can account for the passage of more sound than direct transmission. It is therefore important that the junctions between separating elements are detailed and built correctly to minimise flanking sound transmission.
Figure 2. Transmission of sound
Sound transmission across a solid wall or a single skin partition will obey what is known as the mass law. This law may be expressed in a variety of ways. In principle the law suggests that the sound insulation of a solid element will increase by approximately 5 dB per doubling of mass. The mass law is applicable between 10 kg/m2 and 1000 kg/m2.
Lightweight framed construction achieves far better standards of sound insulation than the mass law would suggest because of the presence of a cavity and therefore a degree of isolation between the various layers of the construction. It has been demonstrated that the sound insulations of individual elements within a double skin partition tend to combine together in a simple cumulative linear relationship. The overall performance of a double skin partition can therefore generally be determined by simply adding together the sound insulation ratings of its constituent elements. In this way, two comparatively lightweight partitions of 25 to 30 dB sound reduction can be combined to give an acoustically enhanced partition with a 50 to 60 dB sound reduction, whereas the mass law alone would have suggested only a 5 dB improvement. This is the basis of many lightweight partition systems and composite floor suspended systems.
It is important to provide adequate sealing around floors and partitions because even a small gap can lead to a marked deterioration in acoustic performance. Joints between walls and between walls and ceilings should be sealed with tape or caulked with sealant. Where walls abut profiled metal decks, or similar elements, mineral wool packing and acoustic sealants may be required. Where there are movement joints at the edges of walls, special details are likely to be necessary; advice should be sought from manufacturers.
Sources of Information
The Steel Construction Institute
The latest Steel Construction Institute (SCI) guidance on this topic, SCI publication P372, published 2008, entitled ‘Acoustic Detailing For Steel Construction’, discusses the principles of acoustic, and construction detailing required in achieving levels of acoustic performance demanded by current regulations. This SCI publication updates and extends the guidance given in previous SCI publications, namely P128, P320, P321, P322 and P336. The scope of the present SCI publication covers all forms of steel construction appropriate for residential construction, including both shallow and deep composite floor systems. Information on the SCI and the relevant documents are available from the following links:
Tata Construction Centre
Construction Advisory Service
Tel: 01724 405060
Tata Panels and Profiles
Tel 01244 892199
Metal Cladding and Roofing Manufacturers Association