Sound advice

What’s the one issue virtually guaranteed to lead to complaints in the office? Noise. But now, designers, specifiers and occupiers have a place to turn for help. David Frise explains

It’s a widely held belief – and one backed up by the government’s chief construction adviser Paul Morrell – that 83% of the cost of a building over its lifetime is the people in it. And increasingly businesses need to get more out of their people, which means providing them with the best environment for them to carry out their work.

In response, the Association of Interior Specialists, the trade association for the fit-out and refurbishment industries, saw the need for a guide to office acoustics that looked at the subject from a range of perspectives, not just the ceiling or partitioning manufacturer’s but everyone’s from the delivery team to the end user.

Work on the guide began in early 2009, when we formed an AIS Acoustics Forum, comprising a crosssection of AIS members and other industry experts. The forum commissioned Bridget Shield, Professor of Acoustics in the Faculty of Engineering, Science and the Built Environment at London South Bank University, to carry out a review of available acoustic research. Two years on, the guide was officially launched in May 2011.

Acoustics is often described as ‘a fuzzy science’. Although detailed guidance has been laid down for acoustics in schools in BB93, nothing exists for offices beyond outline design guidance such as that in the BCO fit-out guide, BREEAM for Offices and BS 8233.

Typically, this has meant the acoustic performance of office areas is not usually considered in depth at the design stages, often leading to last-minute fine-tuning and additional post-occupancy spending on acoustic adjustments. AIS is keen for acoustics issues to be considered from the start of a project, not just at the end.

A prime example is a recent project (one of five case studies in the guide) to create a new office for Ordnance Survey in Southampton. An acoustic wall of more than 95,000 bricks was built around the building – an ambitious four-wing construction arranged around an atrium – to cut out noise from a dual carriageway. This wall then cut through the atrium to form a feature. But rather than leaving the wall in brick – which would have served the office’s acoustic needs well – value engineers recommended a block and render finish. This increased the reverberation times in the office, which would have disturbed its occupiers with a layering of sound and increased noise levels. Had a specialist acoustician been consulted at the time of value engineering, some of the additional cost and time involved in recalculating the sound absorption and making fit-out amendments could have been saved.

One highlight of the guide is a table that consolidates long-held practices to help designers and acousticians calculate the level and type of partitioning needed to maximise privacy. This can be calculated from adding the sound insulation value to the background noise level value. Anything over 75 is good – this method is known as the speech privacy potential and is explained in detail in the guide.

The key is understanding what the numbers mean. The guide goes beyond BB93 in that it acts as a guide for a wide range of users, even those with limited knowledge. It brings together best practice from a variety of fields and perspectives.

For example, acousticians will insist that the dividing partitions between spaces should be installed from the structural floor to structural slab. Yet we know that in 95% of cases partitioning sits between the raised floor and the suspended ceiling because it’s cheaper and more flexible. The guide recognises both these perspectives.

It also tells you why you have to do things, not just what to do. This is a practical guide, not just a theoretical one. Ten years ago every office had a suspended ceiling,cellular spaces and air conditioning. But now you find that offices are open plan; they have no suspended ceiling, high soffits, exposed concrete and natural ventilation.

These kinds of design decisions – increasingly made for environmental reasons – also bring acoustic problems. For example, as air conditioning systems are phased out, natural ventilation using thermal mass has two effects: by removing suspended ceilings, reverberation times increase; and there is less background noise from air handling units to mask conversations.

Of particular use to decision makers is the chart which looks at the acoustic issues to consider when designing and installing an office fit-out. For example, it starts by asking what the space is going to be used for, as different uses require a different approach for privacy or communication.

 

WHAT’S IN THE GUIDE

• Basic acoustics – how noise works and legal requirements

• Comprehensive design guidance – everything from ceilings, partitioning and flooring, to items of furniture

• Terminology – a complete jargon buster

• Case studies – five recent high-profile projects: Ordnance Survey, Southampton Ropemaker Place, London Greenfields House, Essex KPMG’s headquarters in Amstelveen, Netherlands Seven Trent Water, Coventry

• Standards – full rundown of available standards

• Research – work done to date

 

 

KEY LEARNING POINTS

Basic Acoustics

• Good acoustics are essential to productivity and creativity in the workplace. Indeed they can be key to the success of a building.

• Acoustic problems and disturbance in a room are often derived from either long reverberation times, which give a room an echo-y feel or from noises outside the room and poor sound insulation.

• Sound is a series of waves or pressure fluctuations, which start with an object vibrating.

• As it travels, sound dissipates. If it hits a hard surface it can reflect. The reflection can lead to a build-up of sound energy. If it hits a soft surface some of the energy can be absorbed. As the sound encounters objects such as walls, the energy passing through them is reduced.

• When sound travels through air it is described as airborne; when it moves through a solid it is termed structure-borne.

• Sound is measured in terms of the frequency of the wave, expressed in hertz (Hz), with the wavelength and pressure level, expressed in decibels (dB).

• Decibels are a logarithmic scale and are best described using typical noises: eg shouting (80dBA), a pneumatic drill (100dBA). Sound levels above 120dBA would be the threshold for pain in most humans. We perceive an increase of 10dBA as a doubling of sound.

• Reduction figures of 10dB are describing a perceived reduction of 50%. Humans can detect a difference of about 3dB.

• To communicate effectively, normal speech needs to be between 10dB and 15dB above the background noise level.

 Positive feedback

Reaction to the guide has been extremely positive. Stephen Grundy, principal consultant with Cambridgeshire acoustics consultancy the Noise Advisory Service, says: “For those not in the know, acoustics is a bit of a black art – sometimes it works, sometimes it doesn’t, and they don’t really know why. This guide has brought all the information together in one place.”

Alex Ahern, section manager (acoustics and structures) at The Building Test Centre, welcomes it as “a comprehensive overview of acoustic design considerations – helpful in avoiding the need for potentially costly remedial work later in the development cycle”.

And Rockfon project support manager for Rockwool Tim Spencer simply comments: “Follow this guide and you won’t get acoustics wrong.”

Acoustics has long been practiced. The Greeks devised amphitheatres to maximise acoustic performance; the fountains in the Palace of Versailles gardens were built to give princes the privacy of background noise; the voids under floors taken up at London’s Hampton Court were filled with sea shells to provide sound insulation.

But until now the full details of this science have not been fully appreciated by a wider lay audience. We are confident that our new guide will finally give architects, contractors, manufacturers and occupiers a way of making sense of it all.

Acoustic control

• Sound can be controlled in three distinct ways:

• Absorption, which deals with reverberation within the space.

• Insulation (attenuation), which deals with the control of sound from one space to another.

• Diffusion, which scatters the sound.

 • Sound reflects in a similar way to light.

• The acoustic quality of a room can be expressed by measuring the reverberation time (RT). If a room has a long reverberation time, one spoken word does not have time to die out before the next reaches the listener.

• In order to reduce reverberation times, sound absorbing products such as ceilings, rafts, wall panels, carpets and free-standing structures can be introduced.

• The sound absorption coefficients of a particular material are expressed as αw – 0.0 being no absorbency and 1.0 being 100%.

• A product’s sound insulation performance is expressed as a weighted sound reduction index described as Rw.

 • If office background noise is too high, productivity is likely to suffer. If background noise is too low, privacy can suffer.

• Research has shown that ceilings have the biggest impact on the acoustic quality of open plan offices.

• The sound insulation performance of the ceilingmay be compromised when it is penetrated by lighting fixtures and ventilation ducting grilles.

• Sound can also be transmitted through building elements; this is known as flanking which can be defined as sound travelling around a sound resisting element.

• When installing acoustic performance partitioning it should be made as airtight as possible.

• Wall sockets should not be installed back to back.

Design Considerations

When selecting the performance rating of a partition, background noise levels need to be taken into account.

The cumulative effect of different building elements will affect the overall room to room performance. The speech privacy potential (SPP) combines the partition sound insulation performance with the background noise level in the receiving room. The higher the resulting SPP, the higher the level of privacy between the rooms.

Speech intelligibility defines the degree of privacy in a space. The higher the intelligibility, the better for promoting communication in a space; the lower it is, the better for privacy.

A key learning points wallchart can be download from www.acousticguide.org 

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