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Laboratory Evaluation of Annoyance of Low Frequency Noise

4. Assessment Methods for Low Frequency Noise

A number of different methods have been suggested for the assessment of low frequency noise. In this investigation the Danish method [1] is compared to the standardised German method [15], to the Swedish method [16], to a recent Polish method [18], and to two different methods from the Netherlands [20] and [19]. These methods are used in different situations to assess the annoyance due to low frequency noise, based on the indoor noise level. They give different guidelines or criteria for the allowed noise level. Furthermore, the administrative procedures used in the individual countries to enforce the criteria for low frequency noise are very different. This question, however, has not been regarded in this project but is the object of another investigation initiated by the Danish Environmental Protection Agency [21].

4.1 Danish Method

The Danish method [1] is described in "Information No. 9/1997 from the Danish Environmental Protection Agency". It also gives the recommended limit values for low frequency noise and infrasound. The recommended measurement method is specified. The noise is measured in several positions indoor, and is analysed in 1/3-octave bands. The nominal A-weighting corrections are added to the spectra, and the weighted spectrum is summed to form the A-weighted level of the noise in the frequency range 10 Hz – 160 Hz. The resulting level is called L_pA,LF.

It would not be possible make a direct measurement of the A-weighted level, L_pA,LF, since the minimum limit of the tolerance for the A-weighting filter is undefined (i.e. minus infinity) below 20 Hz. The nominal A-weighting is shown in Table 4.

Table 4.
The nominal A-weighting corrections

In the Danish method a table of recommended limit values is used for assessment of the noise. In dwellings the A-weighted equivalent level (averaged over 10 minutes) shall not exceed 20 dB L_pA,LF in the evening and the night (18 – 07) or 25 dB L_pA,LF in the day period (07 – 18). In offices, teaching rooms etc the A-weighted level shall not exceed 30 dB, and in other rooms in enterprises the limit is 35 dB. If the noise has an impulsive character, the limits are reduced by 5 dB.

4.2 German Method

In the German method [15, DIN 45 680 / 1997] low frequency noise is defined as noise where the C weighted noise level is at least 20 dB higher than the A-weighted level, based on either equivalent levels or maximum levels.

If the noise is evaluated to be ‘low frequency’, a 1/3-octave frequency analysis is made. The method considers the frequency range 10 Hz – 80 Hz, but in special situations the 8 Hz and / or the 100 Hz band can be included. The method applies to rooms in dwellings where people stay or rests. In an Annex to the method a range of limit or criteria values are given for the day period (06 – 22) and for the night period (22 – 06).

In the German method, distinction is made between tonal noise and noise without tones. If the level in a particular 1/3-octave band is 5 dB or more above the level in the two neighbouring bands, the noise is said to be tonal.

For tonal noise, the level of the frequency band with the tone is compared to the hearing threshold (LHS) in the same band. It is then found how much the tone is above the threshold. The levels in the other frequency bands are not taken into account. The limit value for the equivalent level of the tone in the day period is: 5 dB in the 8 Hz – 63 Hz bands, 10 dB in the 80 Hz band, and 15 dB in the 100 Hz band. The same assessment method applies to the maximum level of the noise; here the limit values in the same three frequency ranges are 10, 15, and 20 dB. In the night period all the limits are reduced by 5 dB, and thus the limits for the equivalent level of the tones are 0 dB, 5 dB, and 10 dB.

If the noise is not tonal, the limit for the A-weighted equivalent level (10 Hz – 80 Hz) is 35 dB during daytime and 25 dB during the night. The A-weighted level is calculated by adding the A-weighting corrections to only those levels that are above the hearing threshold. As opposed to the Danish method, the contributions from levels below the threshold are disregarded. The corresponding levels for the maximum levels are 45 dB and 35 dB.

The hearing threshold, L_HS, used in the German assessment method is given in Table 5:

Table 5.
One-third octave values for the hearing threshold given in the German standard DIN 45 680

4.3 Swedish method

The recommendations from the Swedish National Board of Health and Welfare (SOSFS 1996:7) [16] give guidance to an assessment of whether noise under different conditions may have health effects. The recommendation comprise a criteria curve of recommended maximum levels of low frequency noise in rooms used for living (see Table 6).

The curve covers the frequency range 31.5 Hz – 200 Hz and applies to the equivalent level of the noise. A measurement method is specified and is described in a report from the Swedish Testing Institute [17]. If the noise level exceeds the criteria curve in any 1/3-octave band, the health and environmental authorities may characterise the noise as a sanitary nuisance.

Table 6.
Values of the criteria curve for low frequency noise, applied to the equivalent noise level per 1/3.octave band in the Swedish assessment method.

4.4 Polish Method

Also the Polish method applies a threshold curve. This is defined in the frequency range 10 Hz – 250 Hz, and corresponds to 1/3-octave levels each giving an A-weighted level of 10 dB (i.e. 10 dB above the inverse A-weighting correction). The criterion curve is called LA10 [18].

The noise is considered annoying if both of these conditions are met:

	The spectrum of the noise exceeds the criterion curve LA10 in one or more 1/3-octave bands, and
	The spectrum of the noise exceeds the spectrum of the background noise.

It is mentioned in [18] that usually the background noise is somewhat higher than the criterion curve at the highest frequencies, above 100 Hz.

4.5 Dutch Proposed Method, Sloven

This proposed method [19] is intended for use along with granting environmental permission to industries and enterprises. Also this method uses a criterion curve. It is defined in the frequency range 10 Hz – 200 Hz. In the upper part of the frequency range the criterion curve agree well with the Swedish criterion curve. At the lowest frequencies, where the Swedish curve is not defined, it corresponds with the hearing threshold as specified in the German method.

It is expected that annoying low frequency noise will occur if the criterion curve is exceeded in one or more 1/3-octave bands.

4.6 Dutch Criterion for Audibility

This method is described in [20]. It is intended for use in cases where people complain about low frequency noise to decide if audible low frequency noise occurs. The aim of the method is not to verify whether the noise is annoying or not.

The method employs a hearing threshold based on the best 10 % of a non-selected population aged 50 – 60 years. The threshold curve is used in the frequency range 20 Hz – 100 Hz.

4.7 C-weighted Sound Pressure Level

In the German method the difference between the C-weighted and the A-weighted sound pressure level is used to determine if low frequency noise is present. Similar rules of thumb have regularly been mentioned in the literature.

4.8 Comparison of Criterion Curves

In Figure 4 the criterion curves from the different assessment methods are compared. The order is arranged after the criterion value at 100 Hz.

The curves Sloven, Swedish, and Polish are criterion curves directly aimed at assessing if the noise is annoying. The curves Sloven and Swedish differ only in the frequency range 50 Hz – 80 Hz, where the Swedish curve is clearly lower than Sloven. In the entire frequency range the Polish curve is lower than the two other curves. Here it must be remembered that the background noise is also part of the Polish criterion, which will often have a relieving influence on the criterion curve at high frequencies, but this part of the method is not considered here.

Figure 4.
Comparison of criteria curves from the different assessment methods

The curve ‘German’ is a hearing threshold curve and is used as a criterion for tones in the noise. It is allowed for tones to exceed the curve by 5 dB during daytime, and higher a higher exceeding is allowed at higher frequencies. The curve ‘Dutch’ is used in cases with complaints in order to decide whether there is audible noise in the relevant frequency range. The curve is not used to determine if the noise is annoying. It can be seen that the German and the Dutch threshold curves are almost identical, and that they almost coincide with the curve ‘Sloven’ below 40 Hz.

The Polish criterion curve is clearly lower than the threshold curves in the frequency range below ca. 30 Hz.

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