The Irish EPA (NG4) are very clear on how a tonal assessment should be conducted when carrying out an industrial noise survey.
There are three basic steps to tonal assessment:
(1) A tonal noise source can normally be subjectively identified by a competent person familiar with noise impact assessments.
(2) However, prior to the application of a rating penalty the Agency would recommend adoption of the simplified methodology for the objective identification of tones that is
advocated in BS 4142: 2014: Annex C (normative): Objective method for assessing the audibility of tones in sound: One-third octave method. This methodology requires that for a prominent, discrete tone to be identified as present, the time-averaged linear sound pressure level in the one-third-octave band of interest is required to exceed the time-averaged linear sound pressure levels of both adjacent one-third octave bands by some constant level difference.
(3) The EPA suggest that the 1/3 octave method will suffice for the majority of situations where tonal noise is suspected to be present. In the event, however, that there is a difficulty in identifying a tonal noise source using this method, or there is disagreement over the presence of tonal noise, then the recommended approach is to adopt the methodology outlined in BS 4142: 2014: Annex D (normative): Objective method for assessing the audibility of tones in sound: Reference method (FFT assessment) .
The following examples illustrate when it may be appropriate to apply the more detailed
reference method as opposed to the one-third octave method: In the event that an interested party believes that a tone is clearly audible even though the one-third method suggests that a tone is not present;
• If measurements conducted under different conditions and/or at different times return
contradictory findings, i.e. application of the one-third method suggests the presence of a
tone for one or more measurements whilst also suggesting the absence of a tone for
another measurement (or measurements), and;
• If the level differences are within +2dB of the values stated below and there is a dispute
over the presence or otherwise of a tone.
• 15dB in low-frequency one-third-octave bands (25Hz to 125Hz);
• 8dB in middle-frequency bands (160Hz to 400Hz), and;
• 5dB in high-frequency bands (500Hz to 10,000Hz).
Application of the reference method is a complex procedure, hence it should only be undertaken by a suitably competent person.