If a tree falls in the woods and nobody is around to hear it, does it make a sound? – Ask an acoustic consultant and he’ll know it might make a sound but definitely doesn’t make any noise.
The trusted BS4142:1997 standard used by many acoustic consultants, environmental health officers and planners has been very well used and sometimes perhaps abused. It offers a method of rating a sound level and assessing what point it becomes a noise level. The document has recently been updated and reissued at the end of October 2014.
The scope of the standard describes methods for rating and assessing sound from industrial and commercial nature and now includes assessing sound at proposed new dwellings used for residential purposes. Previously it would have only been applied for rating sound levels at existing receivers.
The assessment criteria has changed, now the criterion is based on the significance of the impact of the rating level when compared to the existing background levels.
|dB||BS4142 1997||BS4142 2014|
|+10||A difference of around +10 dB or more indicates that complaints are likely.||+10||A difference of around +10 dB or more is likely to be an indication of a significant adverse impact, depending on the context.|
|+5||A difference of around + 5 dB is of marginal significance.||+5||A difference of around +5 dB is likely to be an indication of an adverse impact, depending on the context.|
|-10||If the rating level is more than 10 dB below the measured background noise level then this is a positive indication that complaints are unlikely.||+0||The lower the rating level is relative to the measured background sound level, the less likely it is that the specific sound source will have an adverse impact or a significant adverse impact. Where the rating level does not exceed the background sound level, this is an indication of the specific sound source having a low impact, depending on the context.|
On the face of it it appears good news for developers gaining a 10dB margin by where their equipment would have been rated complaints unlikely to now low impact. This will not be the case on all sites though as ultimately it is the decision of the local authority. There could be a local planning policy which requires -10dB below background to avoid overall background levels creeping up over time.
A common point the document refers to is the ‘context’ of the source and receiver. The document now allows for more consideration of the proposed source and the local environment in which the source will be located. A refrigeration unit is less likely to have an adverse impact when installed on a busy industrial estate compared to next to neighbours back garden. Previously, the local authority may request the same assessment criteria be applied to both these scenarios. With more consideration for the context this may allow for more appropriate criteria to be applied per basis.
Three methods for assessing the application of an acoustic feature correction have been introduced. A sound level of -10dB below background level doesn’t necessarily mean it will be inaudible. The character of the sound has to be taken into account. General humming noise is more likely to blend in where residual sound is dominated by steady road traffic noise then say a cyclic thumping noise from a press works. This is where the context of the new installation has to be considered with its surroundings.
The 1997 document included a single +5dB penalty where the source could be described contains a distinguishable, discrete, continuous note, distinct impulses or irregular enough to attract attention. The 2014 subjective method includes penaltys between 0dB-6dB for tonality and up to 9dB for impulsive noise.