Environmental noise and tonal assessment.

It would appear that some consultants, consultancies and noise measurement equipment hire companies are comfortable with the full reliance on unattended noise measurement equipment and software  to determine if a facilities noise emissions are tonal or not tonal. This in our view is fundamentally incorrect and not in line with best practice, standards or guidance documents produced by the Irish EPA.

Unfortunately many are willing to present evidence (for court or planning cases) based on unattended noise measurement data followed by the use of post processing software that uses the FFT ‘reference method’ to decide if a source is tonal or not. Essentially this type of assessment is based on a ‘computer says its tonal approach’ and combined with unattended measurements, it is not in line with best practice.

Computer says its tonal

It is well recognised that clearly distinguishable tones in a sound can make it more noticeable to a human observer than the sound level alone would indicate. Tonality may be assessed by a number of methods, either subjectively or objectively.

One such assessment can be made using BS 4142:2014. This Standard indicates that the tonality of a sound should be made, either subjectively or objectively and gives one subjective method and 2 objective methods, the Simplified ‘1/3 Octave method’ and the FFT ‘Reference method’. It does NOT suggest that one method is preferable to another.

ISO 1996 Part 2 suggests that tonality of a sound may be assessed objectively and gives two methods for such assessment in Annex C, the Reference Method (FFT), and Annex D, the Simplified (or 1/3 octave) Method. This standard does NOT indicate that the Reference method is preferable to the Simplified method either. It would appear that there is some confusion in some consultants reports where it states that the Reference method is preferred to the Simplified method. The method to be used depends rather on the context in which the tonal assessment is used.

The Irish EPAs-NG4 Guidance Note for Noise is very definitive and suggests that a tonal noise source can normally be identified subjectively by a competent person familiar with noise impact assessments. However, prior to the application of a rating penalty the Agency would recommend that the simplified methodology is used for the objective identification of tones which is advocated in Annex D of ISO 1996-2.

If there is any uncertainty about whether a tone ‘does or does not’ exist subjectively, then the use of the 1/3 Octave Simplified Method is an appropriate method of assessment of tonality if used by a competent person.

So should a noise consultant ever present evidence with regard to an alleged tone, it would be important to ensure that their evidence has been subjectively witnessed and objectively assessed using the 1/3 octave Simplified method as a minimum. Only if there is still further uncertainty about the presence of a tone or if the exact frequency of the tone is required, the the FFT -Reference Method could be considered. Most modern instrumentation offers 1/3 octave and FFT analysis as standard, however there appears to be some confusion about the interpretation of the standards and guidance documents.