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Laboratory Evaluation of Annoyance of Low Frequency Noise

8. General discussion

8.1 The Experimental Method

The present investigation has been performed as a typical laboratory experiment in contrast to a field investigation. The advantage of a laboratory experiment is that it is possible to control almost all the experimental conditions (noises, levels, duration, presentation sequence, test subjects, etc). The disadvantage is that the presentations may not be realistic enough.

The test subjects made the scaling by a mark on a 10 cm long horizontal line. The results from the special group of subjects showed in some cases a saturation effect that made the results more difficult to interpret. A way to alleviate the saturation effect could be to change the layout of the response line. An example of one of the annoyance scales was

but the saturation effect might have been less pronounced if the scale was made like this:

Another way of reducing the saturation effect could be to exchange the word ‘very’ with a stronger adjective like e.g. ‘extremely’.

The response sheet contained all the questions on one sheet. It has been argued that the different questions should have been on separate sheets in order to avoid a smudging effect from one question to the next. Also the wording of the questions might be revised even though no test subjects reported any difficulties with the questions.

The amount of subjects (18 in the reference group; 4 in the special group) could be increased in order to obtain more certainty in the results. For the reference group it is believed that an increase in the amount of test subjects would not change the general results dramatically. For the special group, an increase in the amount of test subjects would certainly improve the validity of the findings as group results. On the other hand it may be difficult to handle the persons with low frequency problems as a homogeneous group of subjects. The problems they report are very different and thus it might be more relevant to handle the results from these test persons individually. Such an individual analysis has not been done in the present report.

The noises constitute a reasonable broad selection of low frequency sounds. The noises were selected to represent typical low frequency noise known to produce community claims. In retrospect it would have been an improvement to include more noises with an impulsive character in order to better ‘test’ the impulse penalty in the Danish method. All noises had clearly a low frequency character partly because of an outdoor-to-indoor filtering of the noises recorded out of doors. Traffic noise was included in order to serve as a reference noise, but due to the outdoor-to-indoor filtering the traffic noise was converted into another low frequency noise. In possible future research a real reference noise should be included.

8.2 Low Frequency Hearing Thresholds

The low frequency pure-tone hearing threshold was measured for both subject groups. The result showed that the special group was less sensitive to low frequency sounds than the reference group. Although this is an interesting result, it might be more informative to measure the loudness growth curve for the test subjects. It is believed that the loudness growth curve would be a much better predictor for annoyance than just the hearing threshold. Measurement of loudness growth is very time consuming and is certainly not straightforward at low frequencies.

8.3 Criteria and Measurement Methods

The results from the two groups seem to be different. All the subjects in the reference group evaluate the noises in almost the same way whereas more variance is seen in the special group. This could be caused by a bias in the special group who tend to put more emphasis on the type of noises that they complain about.

There is a good agreement between the annoyance evaluations from the reference group and the Danish measurement/calculation method (including the impulse noise penalty). The same good agreement is not found for the special group. This raises a question about the aim or objectives of a measuring method. Shall such an evaluation method be made for the average person (the general population) or shall a method be made with special emphasis on the persons who react more pronounced to low frequency noise?

The criteria and evaluation methods used in this investigation are all based on some kind of measurement of the noise level. For the reference group there is a clear connection between the noise level and the experienced annoyance and thus it makes sense to use such criteria and evaluation methods. For the special group this connection between level and annoyance is less clear and thus an evaluation method based on noise level measurements may be of little value for this subject group.

8.4 Statistical analysis

In the statistical analysis it was seen that the data deviated somewhat from a normal distribution partly caused by the saturation effect from the fixed endpoints of the scale. Despite this deviation in the distribution of the data the statistical analysis showed the expected effects and thus no attempt was made to correct for the saturation effect in the data.

For the special group of subjects the saturation effect was pronounced for the annoyance ratings at night. In this case a simple linear regression may not be suited for a description of the data. A logarithmic or polynomial model would probably give a better fit to the data. Due to the relatively small amount of data in this subject group, no attempt has been made to do this.

8.5 Noise limits, Criteria curves and equal loudness level contours

In chapter 5 and in chapter 6 a comparison with the noise limits and the criteria curves was made and as input data for the analysis the maximum excess of a criterion curve or the excess of a noise limit was used. It was decided to do it in this way because this is the way the criteria curves and the noise limits are used in practise. For the use of noise limits there is no problem because the level of the noise is calculated according to some rule and compared to the limit. The calculation of the level is based on the low frequency spectrum of the noise within a certain frequency range.

For the criteria curves, on the other hand, the procedure may constitute a problem as only a single frequency band of the noise is used in the comparison and not the whole spectrum. Only the band where the maximum excess occur is taken into account and the excess at other frequency bands are neglected. From Figure 4 (in chapter 4) it is seen that the course of the different criteria curves differ somewhat above 40 Hz and this means that the various criteria curves will give very different results if the excess occur in this frequency range. It also means that the excess decision will be very dependent on the inherent measurement uncertainty in the measurement of the spectrum. The calculation of a level – based on a spectrum – is much less sensitive to the measurement uncertainty as the uncertainties are ‘averaged’ in the calculation process.

The measurement uncertainty is inversely proportional to the bandwidth of the analysing filter and also inversely proportional to the duration of the measurement (i.e. the integration time). This means that a one-third-octave analysis of a low frequency noise must be extended over a long period of time in order to keep the uncertainty below a certain limit. It is common practise to require that the standard deviation of repeated measurements shall be less then 0,2 dB. This corresponds to an integration time (in seconds) greater than 471/B where B is the bandwidth in Hertz of the analysing filter. For the one-third-octave filter at 10 Hz this means an integration time of almost five minutes. At 40 Hz a one-minute integration time is necessary and at 1000 Hz two seconds are needed. The noise signal shall be stable over this period of time but this is not always the case in practise.

Uncritical use of criteria curves may be misleading. Some of the curves (e.g. the German and the Dutch) are hearing threshold curves and can therefore only be used to predict whether a nose is audible or not. If the curve is exceeded, the amount of excess cannot predict neither the loudness of the noise or the annoyance. This will depend on the shape of the spectrum of the noise.

The use of weighting functions (such as G- and A-weighting) will not automatically give a loudness or annoyance measure. In the conventional audible frequency range it is well known that neither the loudness level contours nor the A-weighting can predict the loudness of complex sounds. Loudness can only be predicted by the loudness level contours when a pure tone (no harmonics) is heard in isolation. The A-weighting resemble the hearing sensitivity at low levels but should not be used for loudness ratings.

It is believed that loudness is a major component of annoyance. Loudness is related to the level and the spectrum of the noise. Annoyance is therefore also dependent on level and spectrum but annoyance is also influenced by (or dependent on) many other factors and these factors cannot be described by physical measurements of the noise.

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