The correct specification and scheduling of cross-talk attenuation is crucial to ensure effective school design, says Mark Balsom
Building Bulletin 93 (BB93) sets out the acoustic criteria required to comply with building regulations when designing schools. BB93 contains criteria that will apply to the mechanical services installation, most notably the criteria for the maximum internal ambient noise level and the performance standards for the minimum sound insulation between spaces.

The issue for many noise control manufacturers who take on the acoustic design and selection of cross-talk attenuation is they can't just consider the traditional mechanical and raised speech aspects without taking in to take account other noise sources via the building façade which impact on the space.

Commonly on commercial projects, noise criteria will be given specifically for the mechanical services installation (usually as an NR value). However, the criteria given for the upper limit for internal ambient noise level in BB93 are not just for mechanical services noise; they are shared with external noise breaking in through the façade from other sources, such as traffic noise.

For example, the upper limit for the internal ambient noise level in a typical classroom is given as LAeq,30min 35dB. This is approximately equivalent to NR29. This is already a reasonably stringent criterion for mechanical services noise, but becomes even more stringent when it has to be shared with the external noise breaking in through the façade.

Not in complete isolation

In an extreme example, if the external noise breaking in through the façade is already giving us LAeq,30min 35dB then the mechanical services installation would have to be designed to LAeq,30min 25dB; this is approximately equivalent to NR19. This shows the importance of not carrying out the acoustic design of the mechanical services installation in complete isolation from the other acoustic aspects of the project.

Traditionally cross-talk attenuators are designed and selected to reduce the noise levels of speech in the source room to 10dB below the mechanical services noise in the receiver room (thereby making the speech inaudible). But, due to the performance standards for the minimum sound insulation between spaces, BB93 requires the cross-talk attenuators to be designed and selected in conjunction with the sound insulation performance of the separating elements between spaces.

For example, the minimum sound insulation performance between neighbouring classrooms is given as 45dB DnT(0.8s),w. This means that the sound insulation given by the partition and the common ductwork adjoining the spaces must perform together at the testing stage to achieve the criteria.

At the testing stage, a loudspeaker is placed in the source room, generating noise levels of approximately 90dB. Sound level measurements are taken in the source room and then in the receiver room. Imagine we measure a sound level of 46 dB in the receive room then, 90dB - 46dB = 44dB DnT(0.8s),w (the 0.8s is the reverberation time in the classroom).

If the cross-talk attenuator has been incorrectly selected, it becomes obvious at this stage and remedial works to correct the problem become expensive.

To comply with BB93, at practical completion of the project, all of these acoustic aspects will be tested to show compliance with the criteria. This is why it is important to have an acoustic consultancy on board at the early design stage to make sure the acoustic considerations of the project are assessed holistically and not in isolation from each other.

• Mark Balsom is consultancy associate director with Lee Cunningham Partnership, a Group business of Caice Acoustic Air Movement