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Laboratory Evaluation of Annoyance of Low Frequency Noise

5. Objective Analysis of the Noise Examples

5.1 One-third octave spectra

Based on the 1/3-octave analysis of the noise at the listening position in the listening room, a range of objective measurements have been made for each noise example. All the levels and spectra are given in Annex B. As an example, the spectrum of the traffic noise is shown in Figure 5 below. The spectrum is shown for the three nominal presentation levels: 35 dB(A), 27,5 dB(A) and 20 dB(A). The spectrum is a typical low frequency spectrum with a maximum around 20 Hz. The spectra of the other noise signals are shown in Annex B.

Figure 5.
One-third-octave spectrum of the traffic noise shown for the three presentation levels: Solid line: 35 dB(A); thick, dashed line: 27,5 dB(A); thin, dashed line: 20 dB(A).

5.2 Comparison to criterion curves

In this section the different objective measures, resulting from the different assessment methods are discussed. The discussion is limited to the results from the presentations at the highest level 35 dB, since there are no systematic differences between these results and the results from the two presentations of the same noises at lower levels (apart from the change in level). The objective results are used as parameters in the comparison with the subjective evaluation, Chapter 6.

In Table 7 it is shown for each noise example how much each of the criterion curves is exceeded, and at which frequency the highest excess occurred. Also the Danish measure L_pA,LF is given, and the German A-weighted level (of the frequency bands exceeding the hearing threshold)

Table 7.
Comparison of the results from the different assessment methods, exceedence of the criterion curves (in dB) at frequency (Hz).

Comments to the individual noise examples:

	Traffic noise. The noise has no tones. All excess occur at 80 Hz
	Drop forge. The noise has a faint tone at 20 dB, below the threshold. Only the equivalent noise level is considered even though the noise has a pronounced impulsive character. All excess occur at 50 Hz apart from German (80 Hz) and Dutch (80 Hz)
	Gas motor. According to the German method, the noise has three tones, at 16, 25, and 50 Hz. The 16 Hz tone is below the threshold; excess occur at 25 Hz or at 50 Hz.
	Fast ferry. According to the German method, there are no tones in this noise. All excess occur at 63 Hz.
	Steel factory. No tones in this noise, all excess at 63 Hz
	Generator. This noise has a tone at 80 Hz, and all excess occur at this frequency
	Cooling compressor. It has tones at 50 Hz and 100 Hz, and the excess are found at these frequencies.
	Discotheque. The noise has no tones but has an impulsive character

It is generally seen that the Swedish and / or the Sloven method gives less excess than the German method, the Polish method, or the Dutch method (for audibility).

With a view to the Polish method, it has to be remembered that the background noise criterion, which normally will relieve the criterion curve at higher frequencies, has not been considered here. Furthermore the German criterion curve has not been used correctly in this comparison: firstly it only applies to tones in the noise, and secondly the German method specifically mentions a relief of the criterion curve in the 80 and 100 Hz bands which was not applied here. Finally, the Dutch method (for audibility) is not a method for assessing annoyance, so it is not surprising that it gives a stronger assessment than some of the other methods.

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