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Laboratory Evaluation of Annoyance of Low Frequency Noise

7. Results obtained with the Special Group

Apart from the reference group of eighteen young normal hearing test persons, also a special group of four persons that have reported annoyance due to low frequency noise in their homes was included in the listening tests. The results obtained with this special group of subjects have not been used in the analysis in the previous Chapter. Instead they are analysed and discussed in the present Chapter.

7.1 Comparison of the two groups of test persons

In the first series of analysis, the evaluations made by the two groups of test persons are compared. This is illustrated in the following four Figures (14-17), showing how the reference group and the special group evaluates the same noise examples with respect to:

	Loudness
	Annoyance, if heard at day or in the evening
	Annoyance, if heard at night
	Annoying? (yes / no)

Figure 14.
Illustration of the relation between the assessments of loudness made by the reference group (young people with normal hearing) and the special subject group.

There is a good correlation between the assessments of loudness between the two groups, and the correlation coefficient is calculated to be 0.82. The special group generally finds the noise examples somewhat louder than the reference group does. The points are rather close to a line that would be offset from but parallel to the line indicated in Figure 14 (showing a fictive 1:1 relationship).

Figure 15.
Illustration of the relation between the assessments of annoyance during day/evening made by the reference group (young people with normal hearing) and the special group.

The relation between the assessments of annoyance (day / evening) of the two groups, Figure 15, is less clear. The correlation coefficient drops to 0.75, and especially the group of points from the highest nominal level (triangles) shows a considerable scatter. In every case the special group finds the noise example more annoying than the reference group does.

On the average the special group rate the annoyance at day/evening about 2 to 3 scale units higher that the reference group. An increase in the rating of 2 to 3 units corresponds roughly to an increase in level of about 10 dB.

Figure 16.
Illustration of the relation between the assessments of annoyance at night made by the reference group (young people with normal hearing) and the special group.

For the assessment of annoyance at night, Figure 16, the picture is shifted. The special group finds the noises much more annoying at night than at day (or evening), and the difference between the assessments of the two groups increases significantly. Figure 16 shows a ‘saturation’ phenomenon, that is, one or more of the test persons in the special group uses the maximum indication of the annoyance scale, and this will break the linear relation between the points. The correlation coefficient drops to 0.73.

On the average the special group rate the annoyance at night about 4 to 5 scale units higher that the reference group. Such an increase in the rating of 4 to 5 units corresponds roughly to an increase in the level of about 17 dB.

Figure 17.
Illustration of the relation between the assessments of annoyance (yes/no) made by the reference group (young people with normal hearing) and the special group.

Finally the assessments ‘annoying yes / no’ of the two groups are shown in Figure 17. The scales show how many percent of the group that have marked the noise as annoying. Here the saturation is obvious, where all (four) persons in the special group have marked several noise examples as annoying. The Yes/No parameter has not been used in the previous analysis of the data from the reference group.

7.2 Comparison of the assessments of loudness and annoyance by the special group

The relation between the assessments by the special group of loudness and annoyance (day / evening) of the same noise examples is illustrated in Figure 18. It is seen that the relation is almost linear, and the correlation coefficient is as high as 0.96. The loudness scalings are less than the annoyance scalings and thus the noises are perceived more annoying than loud.

Figure 18.
Relation between the assessment made by the special group of annoyance (day/evening) and of loudness of the same noise examples.

The relation between the assessments of annoyance at day / evening and at night is illustrated in Figure 19. The non-linear relation due to saturation is clearly seen.

Figure 19.
Relation between the assessment made by the special group of annoyance (night) and of annoyance (day) of the same noise examples.

This indicates the need (in this case) for a longer response line – or rather the need for a stronger assessment than ‘very annoying’. The group of points from the middle level (filled squares) is evaluated 2 – 3 ‘units’ more annoying when they occur at night than at daytime, but the points from the loudest presentation are only indicated 1 – 2 ‘units’ more annoying.

For the special group the annoyance generally increase by two ‘units’ from day to night corresponding roughly to a 10 dB change in the noise level. This can be compared to the results from the reference group, Figure 6 (in the beginning of Chapter 6) where the annoyance at night generally was rated about one ‘unit’ higher than at day – at all presentation levels. Such a one-unit change in the rating corresponds approximately to a 5 dB change in the noise level and supports thus the 5 dB penalty in the noise limits at night.

7.3 Comparison of subjective evaluation of annoyance with objective measures

In the same way as in Chapter 6 the subjective evaluation of annoyance in the night period was compared to a number of objective measures, but in this case only the subjective evaluations from the special group were used. Table 10 below shows the subjective evaluation by the special group, and Table 11 shows the results of the statistical analysis made for each of the different objective assessment methods.

Table 10
Results of the subjective evaluation of annoyance during the night time made by the special group.

The general result from Table 10 is – again – that the special group assesses the noise examples as much more annoying than the reference group does. This is seen more clearly if Table 10 (for the special group) is compared to Table 8 in Section 6.1 (for the reference group). It is also seen that the subjectively assessed annoyance increases with increasing level (apart from the noise from the generator, which apparently is equally annoying at both a nominal level of 20 dB and at 27.5 dB). The annoyance found by the special group at a nominal level of 20 dB corresponds almost to the annoyance reported by the reference group at a level of 35 dB.

A very interesting result that is obtained by comparing Table 8 and 10 is that it is not the same noise examples that are evaluated as most annoying by the two groups. The reference group clearly found the drop forge, the discotheque, and the cooling compressor the most annoying. This rank would hold at any of the three presentation levels. In contrast, the special group found the generator the most annoying (at the lowest presentation level) and the discotheque as one of the lesser annoying sounds.

Table 11.
Summary of results of regression analysis of the relation between the assessments made by the special group and the different assessment methods.

It can be seen from Table 11 that none of the assessment methods give any particularly successful correlation to the subjective assessment made by the special group. Two examples are illustrated below in Figure 20 and 21, the Swedish and the Danish method. The groups of points from the intermediate and the highest presentation level both line up reasonably well with a slightly sloping line in the upper part of the Figures, while the group of points from the low presentation level appears very different in the two Figures. In Figure 20 showing the Swedish method these points have a curved tail-like appearance, while they in Figure 21 showing the Danish method appear as a diffuse cloud.

Figure 20.
Illustration of the relation between the Swedish assessment method and the subjective evaluation made by the special group
  

Figure 21.
Illustration of the relation between the Danish assessment method and the subjective evaluation made by the special group

The other assessment methods show results without any particular trend like it is seen with the Danish assessment method. Obviously there is no strong connection between the subjective assessment made by the special group and the objective results found by the objective measuring methods.

Since only four persons have made these assessments, it was evaluated that no more effort should be paid to optimise an objective assessment method to fit the special group’s evaluations. However, the results obviously give rise to a number of questions about how low frequency noise in the environment is experienced and how it can be assessed.

7.4 Discussion of results from the special group

The various scaling results show clearly that the special group made the annoyance evaluations differently from the reference group. The overall scaling value (averaged over all annoyance evaluations, presentation levels and noises) was 3,5 for the reference group and 6,7 for the special group. In other words, the special group evaluated the noises to be almost double as annoying as the reference group did.

This may also be illustrated by ordering the noises from the most annoying to the least annoying. This is done in Table 12.

Table 12.
Ordering of the noises from most annoying to least annoying for the two groups of test subjects.

The reference group has the Drop forge and the Discotheque on top of the list. These two noises are heard as ‘impulsive’ and thus a 5 dB penalty is added to the calculated level in the Danish evaluation method. The Generator and the Traffic noise are in the middle of the list and the Gas turbine is evaluated as the least annoying.

For the special group the Generator is on top of the list whereas the Discotheque and the Traffic noise are evaluated as the least annoying. It is interesting that Traffic noise gives the lowest overall scaling. The value 5,6 is well below the next one (Discotheque) at 6,2. The order of the noise signals could indicate that the special group put more attention to those noises, which resemble the typical low frequency noises that they are complaining about.

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