Sound system

The clock has started ticking for specifiers. On 5 July, the government announced tough new standards for sound insulation between dwellings under proposed changes to Part E of the Building Regulations. The proposal calls for housebuilders to produce a series of standard construction methods for walls and floors to comply with the regulations.

Specifiers now have 18 months to develop a series of solutions and to prove to the government that these designs work. If they succeed, then housebuilders will be freed from the expensive burden of having to test a sample of new homes on each site. However, if they fail, the government will introduce the requirement for acoustic testing on 1 January 2004 – and the industry will have to foot the bill.

It is a tough task, but fortunately for specifiers they do not have to start the development process from scratch. Housebuilders in Scotland have been building and testing dwellings for the past 15 years and as a result they've had to develop floor and partition designs with consistently high standards of sound insulation.

The House Builders Federation is responsible for developing the design solutions and has already been in contact with an acoustic consultant in Scotland. The Robin Mackenzie Partnership is the consulting division of Napier University, and it has recently carried out a study on the acoustic performance of dwellings in Scotland.

The study, based on 1400 post-construction field tests in Scotland, revealed what construction details were most likely to fail acoustic tests. The findings give a clue as to the type of standard details the HBF will be developing.

The Napier report found that the best performing party wall was the timber-frame wall with a typical twin frame of 90 × 45 mm studs with 75 mm cavity and 90 mm quilt insulation and plank and plasterboard on both sides (see diagram of timber wall, page 08). The worst performing wall in the study was the 215 mm block wall with plasterboard on dabs, which had a fail rate of 49%.

Overall, timber floors were found to give a lower pass rate than concrete floors, but this was due principally to the use of the platform floor, which was used by industry in the early 1990s. Today this floor type is rarely used in Scotland, and raft floors are currently the predominant construction. Furthermore, recent advancements in timber floor design have permitted performance levels to consistently exceed concrete floors.

Other findings from the report likely to influence the development of the standard details in England are listed below.

Walls

• The use of 13 mm render (3:1 ratio of sand and cement) to wall faces dramatically improves sound-insulation performance.
  
• Increasing cavities in block walls to 75 mm markedly improves sound insulation.
  
• Incorrect bonding of solid party walls with external cavity walls is still continuing, despite the Scottish Building Regulations stating that this feature should be avoided.
  
• Best performing wall was the standard timber frame partition wall with twin 90 mm studs and 75 mm cavity, which had an average performance of 62 dB (see graphic).

Floors

• The most common core concrete floor construction is 200 mm precast concrete slab, which consistently performs well. However, 150 mm normal density precast slabs are marginal at best.
  
• Beam and block flooring is almost non-existent in Scotland, due to its poor pass rate.
  
• Raft floor constructions, consisting of either double-layer closed and open-cell foam backed battens or single foam battens with 13 mm quilt, are the most commonly tested and easily meet the impact sound transmission requirements.
  
• Platform timber floors using a thick layer of mineral-fibre batt insulation had a 33% fail rate, and even with independent ceiling joists some still failed to meet the impact requirements.
  
• Increased use of laminate flooring with insufficient resilient underlays has resulted in an exponential increase in sound insulation complaints.

Ceilings

• The use of resilient bars (which run perpendicular to the joist span) are increasingly being used to improve both impact and airborne sound insulation. Improvements of around 4-6 dB are typically found.
  
• Drop-down ceiling hanging bars, which are only connected to one joist, sometimes reduce the dynamic stiffness of the floor and can lead to "poor insulation".

Other findings from the Napier report

• Contractors and architects are increasingly requesting expert acoustic advice at design stage to produce cost-effective sound insulation details and avoid possible future testing complications.
  
• Further care is required by specifiers when choosing products based on sound test performance from laboratory test data. Field performance can be typically 5 dB below laboratory data due to flanking transmission.

  In addition, measuring the same wall in different laboratories can result in a 7 dB performance variation.
  

• The proposed testing requirements for England and Wales will specifically target the performance of walls and floors at low frequencies, as these are the frequencies that can cause most disturbance and are the most difficult to design out.

One of the key issues to emerge from acoustic testing in Scotland, which will be of concern to the government in London, was the failure of many constructions that were "deemed to satisfy" (that is, based on drawings in Part H of the Scottish Building Regulations). The problem, says Napier University's Sean Smith, is that even if a construction is "deemed to satisfy", it doesn't tell you that the construction satisfies the requirements because performance depends on the construction design, materials and workmanship. According to Smith, some "deemed to satisfy" constructions have been identified as consistently having a high failure rate.

As a result, says Smith, the Scottish construction industry has adopted more advanced "non-deemed to satisfy" constructions, which are known to give a higher performance level. Contractors would prefer to specify construction details with a larger margin for error than use deemed-to-satisfy constructions laid out in the Scottish regulations.

Smith, who carried out the Napier University study, believes that two decades of acoustic testing has been instrumental in forcing up the standard of sound insulation in Scotland. He says many Scottish housebuilders appreciate the benefits of testing and no longer regard it as a problem. Graham Aggett, chairman of Edinburgh developer Applecross Developments, is one of those.

He believes the benefits of testing far outweigh the downside and says that his counterparts in the South have nothing to fear if acoustic testing does come about for new dwellings.

For the time being, though, many English housebuilders remain wary of acoustic testing. According to Smith, his recent Part E seminars for housebuilders at the House Builders Federation have caused a bit of a stir. "When I first explain the implications of failure I put the fear of God up them," he says. However, after explaining how they can avoid the pitfalls, Smith says the audiences appear to be more reassured. "They relax a bit, anyway," he laughs.

Many housebuilders will have to come to terms with acoustic testing even if the government makes new dwellings exempt. Developers involved in flat conversions will have to carry out tests from 1 July 2003, as will builders of new hotels and hostels. Those worried about the affects of acoustic testing on their designs would do well to learn from the experience of their counterparts north of the border. For people building new dwellings, the pressure is now on for specifiers to come up with robust standard details and prove to the government that the industry is capable of self-regulation.