CPD 21 2017: Acoustic performance of composite floors

INTRODUCTION

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 better 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. Composite floors offer structural stability, fast, safe and easy installation, and can lead to savings in storage and transport of materials.

Acoustic performance has become more important in residential buildings as developers and occupants demand higher standards. Amendments to Part E of the Building Regulations came into effect in July 2003, introducing a new measurement index, C_tr. This takes account of the low-frequency sounds that often cause problems in residential buildings, such as traffic and bass music. The new regulations set more demanding requirements for the performance of separating floors and walls between dwellings. Composite floors are an 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.

This CPD discusses the principals of sound transmission and the key considerations in the design of composite floors to ensure compliance with Part E. This information is sourced from the Steel Construction Institute (SCI) publication P372, Acoustic Detailing for Steel Construction, published in 2008.

PRINCIPLES OF SOUND

Sound is caused when objects vibrate in air. This movement causes air particles to vibrate in turn, 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. Both sound levels and sound insulation (attenuation) values are expressed in decibels (dB), while 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. 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 (low frequencies) are normally attenuated less than high-pitched sounds (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.

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 to continue travelling 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 60dB. 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 D_nT. 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 standardised impact sound pressure level L_nT which is a field measurement. Tests in laboratories, normalised for area and absorption give the normalised impact sound pressure level L_n. This test method means that the better the impact sound insulation, the lower the value of L_nT or L_n.

MEASURING SOUND INSULATION

Sound insulation is measured at a number of different frequencies, usually at 16 one-third-octave bands from 100Hz to 3150Hz. 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 (ie, by using 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. A similar method is used for impact sound.

The single-figure values are called: 

• Standardised weighted level difference D_nT,w when generated from D_nT
• Weighted sound reduction R_w 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 L_n

Until 2003, the standardised weighted level difference D_nT,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, D_nT,w + C_tr. The C_tr term is a spectrum adaption term, which is generally negative and adjusts the index by placing more weight on the low-frequency sounds that often cause problems in residential buildings. Thus a D_nT,w + C_tr rating is generally lower than the D_nT,w rating for the same construction. Impact sound transmission is measured by L’_nTw, the standardised weighted impact sound pressure level.

ACOUSTIC DETAILING

Where a room is separated from another room, sound can travel by two routes: directly through the separating structure (direct transmission), and around the separating structure through adjacent building elements (flanking transmission). Sound insulation for both routes is controlled by the three characteristics: 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 building elements and the quality of construction. In certain circumstances flanking transmission can account for the passage of more sound than direct transmission – for example, where separating walls have a high standard of acoustic insulation but side walls are constructed to lower standards and are continuous between rooms. It is therefore important that the junctions between separating elements are detailed and built correctly to minimise flanking sound transmission.

Mass

Sound transmission across a solid wall or a single-skin partition will obey the “mass law”, which may be expressed in a number of ways. In principle the law suggests that the sound insulation of a solid element will increase by approximately 5dB per doubling of mass. The mass law is applicable between 10kg/m² and 1000kg/m².

Isolation

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 25-30dB sound reduction can be combined to give an acoustically enhanced partition with a 50-60dB sound reduction, whereas the mass law alone would have suggested only a 5dB improvement. This is the basis of many lightweight partition systems and composite floor suspended systems.

Sealing

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.

ACOUSTIC REGULATIONS – PART E

The acoustic requirements of residential buildings are normally given in national building regulations and associated guidance documents. For England and Wales, acoustic performance requirements are contained in Building Regulations Part E and in Approved Document E. Equivalent documents exist for use in Scotland and Northern Ireland.

Part E covers:

• Acoustic insulation of separating walls and floors between newly built dwellings, and dwellings formed by a material change of use
• Acoustic insulation between hotel rooms, boarding house rooms, and other rooms used for residential purposes such as student halls of residence and key worker accommodation, formed by new-build or by a material change of use
• Acoustic insulation between rooms within a dwelling formed by new-build or by a material change of use
• Acoustic characteristics of common parts of apartment buildings
• Acoustic characteristics of schools

Only requirement E1 of Part E of the Building Regulations has an influence on the construction and detailing of the structural frame and floor. Requirement E1 relates to separating walls and floors, and their junction details. The other regulations in Part E refer to surface finishes, partition walls and other building types.

Requirement E1 states that “dwelling-houses, flats and rooms for residential purposes shall be designed and constructed in such a way that they provide reasonable resistance to sound from other parts of the same building and from adjoining buildings”.

Rooms for residential purposes, include rooms in hotels, hostels, boarding houses, halls of residence and residential homes but do not including rooms in hospitals, or similar establishments, used for patient accommodation.

Approved Document E provides guidance on how the regulations may be satisfied and sets acoustic performance standards.

DEMONSTRATING COMPLIANCE WITH PART E

Approved Document E describes two methods of demonstrating compliance with Part E: pre-completion testing (PCT) and use of Robust Details (RD).

PCT is carried out on site and the onus is on the builder to demonstrate compliance. It is recommended that 1 in 10 of each type of construction detail is tested. PCT only applies to separating walls and floors and is not necessary for internal walls and floors. PCT should be carried out when the rooms either side of the separating element are essentially complete, except for decoration. Tests are generally required to be carried out without non-permanent decorative floor coverings such as carpet, laminate flooring or vinyl. In some cases, integral soft floor coverings are permitted, provided the floor covering is glued to the concrete slab below.

Robust Details (RD) were developed as an alternative to PCT. A range of details has been developed which have been proved through testing to consistently satisfy (and exceed) the acoustic performance requirements specified in Approved Document E. The available RD and their specification requirements are published in a handbook by Robust Details Limited. To use a Part E Robust Detail in the construction process, builders must first obtain permission from Robust Details Limited and pay the requisite fee for each dwelling. Provided that the Robust Details are built correctly, this will be accepted by Building Control bodies in England and Wales as evidence that the homes are exempt from PCT. Further information can be found at www.robustdetails.com.

The Steel Construction Institute (SCI) produces guidance on this topic, and the latest is contained in SCI publication P372, Acoustic Detailing for Steel Construction, published in 2008.

Composite floors may have a deep or shallow deck profile. There are two generic types of shallow deck profiles: re-entrant (dovetail) and trapezoidal. A typical shallow composite floor system RD is presented in Section 3.1.1 of SCI Publication 372. Where this is the case, the RD reference is given. Some of the recommended junction details in Appendix B, B.2 and B.3 are also RD, provided that the wall, floor and junction are specified to comply with the requirements given in the Robust Details Handbook.

SCI publications P321 Acoustic Performance of Slimdek and P322 Acoustic Performance of Composite Floors provide alternative composite floor constructions, such as screed, platform, raft and cradled floors, again to RD specification.

Acoustic insulation is often associated with high-mass constructions. However, this is economically inefficient and is inappropriate for dry assembled construction. Furthermore, there is a need for resilient layers to be introduced to deal with the effect of impact sound, even in concrete floors. Composite floor constructions provide a mixture of mass and resilient layers. Multiple layers are used to provide very good levels of acoustic insulation. The mass is provided by the composite slab, and acoustic testing in buildings has shown that generally the effective mass of the slab per square metre of floor area can be used to predict performance.

The resilience is provided by mounting the plasterboard on a proprietary metal-frame ceiling. This decouples the ceiling from the slab and reduces sound transfer. In addition, a variety of acoustic floors can be used on top of the slab to decouple the floor finish from the slab.

SCI P322 sets out four principal options for shallow-deck constructions. In increasing level of likely performance, these are screed, platform, raft and cradle floor, as set out in the table below. Similar construction details for Tata Steel deep-deck systems are provided in SCI P321.

Floor type	Construction	Single-figure rating (dB)
		DnT,w + Ctr	L’nT,w
Screed floor	Sand and cement or proprietary lightweight screed. Resilient layer of dense mineral wool, plastic insulant, or a polyethylene layer carefully installed to ensure continuity. The resilient layer should be turned up at the edges of the floor to isolate the walls from the screed. Re-entrant or trapezoidal composite floor slab. Gypsum plasterboard ceiling.	50-57	40-50
Platform floor	18mm chipboard or similar finish layer. Optional 19mm gypsum board. A resilient layer of dense mineral wool or plastic insulant. The resilient layer should be turned up at the edges of the floor to isolate the walls from the chipboard. Re-entrant or trapezoidal composite floor slab. Gypsum plasterboard ceiling.	52-57	40-45
Raft floor	18mm chipboard or similar finish layer. Optional 19mm gypsum board. Proprietary timber batten bonded to foam strip, optional thin layer of insulation between the battens. Re-entrant or trapezoidal composite floor slab. Gypsum plasterboard ceiling.	54-58	35-45
Cradle floor	18mm chipboard or similar finish layer. Optional 19mm gypsum board. Proprietary cradle floor supporting timber battens, optional thin layer of insulation between the battens. Re-entrant or trapezoidal composite floor slab. Gypsum plasterboard ceiling.	54-58	35-45

The composite slab may use either a re-entrant or trapezoidal galvanized steel deck, typically 50-100mm deep, with a concrete floor covering to give a typical overall slab depth of 130-200mm, typically 250-425kg/m². The plasterboard ceiling can be fixed using a proprietary metal frame system fixed to the underside of the deck. One layer of wall board is generally sufficient, but an improvement can be achieved by using acoustic boards or two layers of wall board. Indicative performance figures are based on 45 test results from nine buildings using composite floors.

The ceiling usually consists of one or two layers of gypsum plasterboard fixed to a metal frame system that is fixed to the underside of the steel deck. This reduces the rigidity of the connection between the plasterboard and the structure above, reducing the acoustic vibration that is transmitted.

Impact sound transmission is reduced by:

• Specifying an appropriate resilient layer below the finish floor board with correct dynamic stiffness under imposed loadings
• Ensuring that the resilient layer is durable
• Isolating the floating floor surface from the surrounding structure at the floor edges. This can be achieved by ensuring a resilient strip is included around the edges of the walking surface.

Guidance constructions, that when built correctly should provide the required acoustic performances, are provided in Approved Document E. However, these are generally conservative solutions and still require PCT to be carried out. 

Both methods of compliance (PCT and RD) can be used for steel construction, although pre-completion testing is probably the most appropriate because it allows greater flexibility in the design and detailing. The scope of the current range of RD for steel construction is not sufficient to include all the junction details that are typically present on a residential development. Therefore, even if RD have been used there will usually be some junctions where Building Control could request site testing. The range of RD is continually growing, so over time more there will be fewer instances where RD cannot be used. The approval process for new RD requires significant amounts of site test data for similar details, for the tests to show that the details exceed the Building Regulation requirements by 5dB on average and for the details to become public. Hence there is often little incentive for system suppliers to share information if they are satisfied with the results obtained from PCT.

The Steel Construction Institute, with funding from Tata Steel, has developed an online tool to provide structural engineers and architects with a quick, easy-to-use system for working out the likely level of acoustic performance for different walls and floor systems used in steel construction, in the residential, health and schools sectors. This allows users to carry out a preliminary “what if” analysis before embarking on a detailed design. Airborne sound insulation performance is predicted for both walls and floors, and impact sound performance is also predicted for floors. To access the tool go to: bcsatools.steel-sci.org/Acoustics