Acoustics of schools: a design guide

Education Business discusses the importance of acoustic considerations in the design of classrooms, as well as noise control and sound insulation. Highlighting the key factors in the planning of such rooms, we detail why schools benefit from proper, well designed acoustic classrooms and how attention to acoustic detail has the potential to enhance learning.

For children who have impaired hearing or have more generic difficulties with hearing, school halls and classrooms can be a difficult space to navigate. However, the problem is not only an issue for the hard of hearing. Classrooms with poor acoustics can also leave children with normal hearing unable to properly determine what is being said by a teacher, which can be detrimental to their education.

With schools becoming continuously busier, with larger school grounds, more students and an array of extra curricular activities, controlling external noise and the resulting acoustics is becoming more important. This problem is exemplified by the current trend of designing open spaces in which to learn, which in turn contributes to a louder environment.

In November 2015, the Institute of Acoustics and the Association of Noise Consultants produced Acoustics of Schools: a design guide, providing revised guidance and recommendations on the acoustic design of new and refurbished schools. The guide replaced the guidance previously published in the 2003 edition of Building Bulletin 93: Acoustic Design of Schools, and aims to instruct and encourage the belief that the design and construction of school buildings provide acoustic conditions that enable effective teaching and learning.

Noise control
Noise surveys are typically carried out quite early in the process of designing a new school or assessing a site’s suitability to have a school erected upon it.

But there is usually a conflict of interests. Schools need to be near to accessible transport links, which increase the noise levels. Likewise, space for open fields is often deemed beneficial for recreation and sport, but they can provide their own issues.

Playgrounds, outdoor recreation areas and playing fields are generally considered to be of relatively low sensitivity to noise. Indeed, playing fields may be used as buffer zones to separate school buildings from busy roads where necessary. However, where used for teaching, for example sports lessons, outdoor ambient noise levels have a significant impact on communication in an environment which is already acoustically less favourable than most classrooms.

The external noise level can be established by carrying out a noise measurement survey. The measurements should be taken during school hours over a suitable time period to be able to quantify the representative sound pressure level, likely to occur during teaching hours and should include noisy events – such as peak time road traffic, but should exclude rare, intermittent events, such as the mowing of school lawns.

If the noise measurement survey shows that the ambient external noise levels on the site are below 45 dB and prediction work shows that they will remain below 45 dB in the future, no special measures are likely to be necessary to protect the buildings or playing fields from external noise. However, consideration should be given to any potential increases in noise levels due to future developments – such as increases in traffic flow or new transport schemes. Where road traffic noise levels are likely to increase, it is reasonable to base the sound insulation requirements on the best estimate of noise levels in 15 years’ time. The situation intensifies if a school is located near to a railway or airport, where noise control will be far more of an issue.

While the immediate thought on how to combat such problems is to increase the distance between the source and the school, basic noise barriers are often more beneficial. In its simplest form a noise barrier can be a continuous close-boarded wooden fence, with a mass of not less than 16 kg/m2. There is relatively little point in increasing the weight of the barrier beyond this because a significant proportion of the noise passes over the top, or round the ends, of the barrier. The attenuation of a barrier is a function of the path difference, that is, the extra distance that the sound has to travel to pass over the top of the barrier, relative to the direct sound path from the source to the receiver.

The issue of noise control is double ended and schools should also be aware of local and residential developments and how noise from the school can disturb those areas. Playgrounds, playing fields, music rooms and halls used for events outside normal school hours, such as concerts and discos, can all create problematic noise for those places nearby.

The sound of speech
The basic dynamics of a school environment require the clear facilitation of clear communication between teacher and student. Ignoring for a moment the use of audio-visual aids and other classroom technologies, without good design for speech, teaching, at its most basic form, becomes redundant. The teacher’s voice needs to be clearly heard above all background noise. According to the Acoustics of Schools: a design guide, the standard sound power output created by speech is 10 microwatts, which amounts to a sound pressure level of 60 dBA at one metre in front of the speaker. This then increases to 100 microwatts (70 dBA) when spoken in a raised voice, and 1,000 microwatts (80 dBA) when shouting.

A well designed classroom should remove the necessity for such measures, partly for the teacher’s health and partly for the delivery of the lesson. Take the study of drama for example – the importance of conveying nuances and emotion are integral to the delivery of speech, and could be lost by changing the level of voice. The report claims that ‘the intelligibility of speech depends upon its audibility as well as its clarity. Audibility is affected by the loudness of the speech (signal) relative to the background noise level and is often expressed as a ‘signal to noise ratio’ (SNR)’.

Background noise can have a major effect on speech, and can be a combination of many factors, including ingress noise from external activities (e.g. road traffic), building services, classroom equipment (e.g. projectors, whiteboards etc.), noise from adjacent spaces and general occupancy noise. Another factor that interrupts the intelligibility of speech is the reverberation time caused, as this can cause prolonged syllables.

This means that large amounts of fixed acoustic absorption are often required. Reverberation (the amount of echo in a room) affects how well speech is understood. Basic classroom furniture – desks, chairs, benches – can be beneficial in scattering sound and deflecting it onto other absorbent surfaces. Such fixtures should be evenly distributed around the room so as to ensure a reliable reverberation time equation.

In classrooms, the two main approaches to locating the acoustic absorption is to make the soffit predominantly absorbent, or to leave the ceiling acoustically reflective and add acoustic absorption to the walls.

This is perhaps more difficult to manage in school halls that are frequently used for assemblies and meetings, and, when crowded concede high noise levels and poor speech intelligibility due to excessive reverberation.

With an acoustically well designed room it is possible for speakers to achieve good speech intelligibility, even with large audiences. Quieter and untrained speakers, however, may not be able to do this and a speech reinforcement system is likely to be required for some functions. The key aim of such a system is to increase the loudness of the direct sound, particularly for more distant listeners, whilst keeping the sound as natural as possible. The distribution of loudspeakers and their directional characteristics is a key issue in achieving high speech intelligibility. In school halls, column loudspeakers can be located on side walls, or in a central cluster.

The halls are alive with the sound of music
Music rooms require particular acoustic attention, as they can strongly vary in size and purpose – being used for anything from small practice rooms for one musician to large performance rooms for rehearsals and performances. As the loudness and sound of the instruments changes, so will the sound insulation requirement, making flexibility desirable. Flexibility, however, can prove expensive, and a more desirable option, designating particular rooms for particular instruments, involves earlier planning.

More so than drama studios, music rooms will have a more recognisable impact on the whole school building. Rooms for percussion and brass instruments will generate higher noise levels and should be located on the ground floor to limit the transmission of impact vibration into the building structure. Higher ceilings are preferential to allow for higher volume levels without damage to those inside the room. Smaller rooms with high volumes can cause noise-induced hearing loss.

Special hearing requirements
Due to the inclusive nature of modern day schools, children with special hearing requirements are quite likely to be taught within a mainstream school setting (85 per cent according to a recent survey). This means that schools must be accommodating, especially when it comes to acoustics and school design – meaning that every learning space must be accessible and for those with special hearing requirements.

A survey by the British Association of Teachers of the Deaf (BATOD) indicated that 67 per cent of pupils with hearing impairments were using an auditory-oral approach and a further 26 per cent used an approach which combined sign with auditory-oral components. For these groups in particular a poor acoustic environment can be a significant barrier to inclusion.

Open plan teaching
Schools are expanding, not only in size, but also in the ways in which they construct a classroom and deliver lessons. While you will regularly still find thirty seats in a square room facing the same direction, the aesthetics of the classroom have moved beyond their previous dimensions. In that small, square room the capacity to achieve good speech intelligibility is relatively straight forward. However, open plan spaces require additional specification as they are significantly more complex acoustic spaces. Noise arriving from adjacent learning spaces and circulation spaces increases background noise levels, making speech intelligibility lower.

Open plan spaces, which can either be fully open plan layout, semi-open plan layout or flexible open plan layout, are generally designed for flexibility of use in the short term and adaptability in the long term, yet the need for flexibility should be balanced with the need for good acoustics, to ensure effective learning. Research has shown that in many large open plan ‘flexible’ areas certain activities are severely restricted, or have to be dropped, because of noise interference.

Further Information
www.ioa.org.uk