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Laboratory Evaluation of Annoyance of Low Frequency Noise

3. Primary Results of the Listening Tests

The primary results of the listening tests are the scalings made by the test subjects. The scalings are given as values between zero and ten as measured from the test subject’s indications on the response lines. All the results are shown in Annex A of this report. In chapter 6 it is shown that the annoyance during day/evening and the annoyance at night are very closely related and thus it suffice at this point to look at only one type of annoyance recordings. Table 2 below shows the average subjective evaluation - made by the reference group of listeners - of the annoyance during night from the various sound examples.

Table 2.
Subjective assessment by the reference group of the annoyance from the noise examples if the noise was heard at night. Annoyance rating is given on a scale from 0 (not annoying) to 10 (very annoying).

It can be seen from the results in this Table 2 that the subjectively assessed annoyance increases when the same type of noise is played louder; this is a general as well as an expected result. It can also be seen that the different types of noise are not assessed equally annoying; apparently the noises from the drop forge, the discotheque and the cooling compressor are evaluated as more annoying than the other types of noise. This gives some promise for interesting results from a closer inspection of the different objective assessment methods – they should be able to give some form of explanation why these types of noise are considered more annoying than the others. On the other hand it can be seen that the traffic noise is just as annoying as the main part of the low frequency noise examples. It was the intention that this type of noise should serve as a reference noise (which was not a particularly low frequency noise), and the listening experiments should then indicate how much more annoying a number of different low frequency noises would appear. But it seems that the filtering applied to the traffic noise has turned this into another low frequency noise example.

3.1 Statistical Analysis of Primary Results

In order to investigate the structure lying behind the average data, the raw data from each listening test were typed into a spreadsheet. For each data the following information was also recorded: the age and gender of the test person, the repetition number (round 1 or 2), the sound example number, the nominal presentation level, the measured A-weighted level (dB(A)), and the A-weighted level of the sound in the frequency range 10 Hz – 160 Hz, L_pA,LF.

All data were subsequently transformed to a statistical analysis program (Statgraphics 4.0). It was found that if one disregarded data near the endpoints of the scales, the responses almost followed a normal distribution curve. However since many of the responses were near one or the other of the endpoints, the primary results were not normal distributed. Despite this lack of normality in the distribution of the data it was decided to perform an analysis of variance. The analysis was made for each parameter separately.

Table 3 shows the significance levels of the influence from a number of different factors upon the evaluations of the reference group. If the number is less than 0.05, this factor has a significant effect on the evaluation on a 95% level or above (this means less than 5 % probability for drawing a wrong conclusion). If the number is above 0.05 it cannot be proved that this factor has a significant effect upon the relevant evaluation.

Table 3.
The significance level of different factors that may influence the evaluation by the reference group.

It is seen from Table 3 that – as expected – the noise example, the nominal level, the dB(A) level and the low-frequency level (L_pA,LF), all have a significant influence upon the evaluations from the test persons.

The repetition number (round 1 or round 2 with the same presentation) has no significant influence, which shows the absence of a training effect. The gender of the test persons has influence on the evaluation of annoyance during the day and during the night but not on the evaluation of loudness and on the yes/no question about whether the noise is annoying or not.

3.2 Statistical Analysis of the Results from the Special Group

A corresponding analysis was made with the data from the special group. Since this group has only four persons the data are very uncertain and highly dependent on random variations. The result of the analysis is shown in Annex A. It is found that the noise level influences the evaluations. The influence from noise example on the annoyance evaluations is just at the limit of being significant.

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