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Laboratory Evaluation of Annoyance of Low Frequency Noise 1. Background
1.1 InfrasoundSound in the frequency range below 20 Hz is defined as infrasound. Previously it was believed that infrasound was inaudible, and the three decades 20 Hz – 20 kHz was often called ‘the audible frequency range’. We now know that infrasound can be heard provided it is strong enough. The threshold of hearing is determined at least down to 4 Hz, and there is a reasonable agreement between the recently measured thresholds. The G-weighting function has been standardised in ISO 7196 [3] and has a close relation to the shape of the hearing threshold in the infrasound region but also includes a sharp cut off at frequencies above 20 Hz. A determination of equal loudness level contours in the infrasound region at 20, 40, and 60 Phones was reported as early as in 1984 [4]. The slope of the lowest loudness level contour was comparable to the slope of the threshold curve, while the higher loudness level contours had slightly less slope. As a first approximation it may thus be expected that the G-weighted infrasound level would predict the strength or loudness of the infrasound, at least when the level is not too high above the threshold. Infrasound is usually not perceived as a tonal sound but rather as a pulsating sensation, pressure on the ears or chest, or other less specific phenomena. The loudness and annoyance due to infrasound increase very quickly with increasing level. The hearing threshold for single tones is usually about 95 dB(G), and tones with a 20 dB higher level are expected to be sensed as very loud. It can be assumed that infrasound below the hearing threshold is not annoying. The recommended limit value for infrasound in dwellings in Denmark is 85 dB(G). The Danish Environmental Protection Agency explains that the purpose is to secure that even persons with a very low hearing threshold are not annoyed by infrasound. After the ‘Information No 9’ was published in 1997 a number of measurements of environmental infrasound has been made in dwellings. Only at rare occasions levels have been seen near the recommended limit value. In most situations the level is at least 20 dB lower than the 85 dB limit. 1.2 Low Frequency NoiseWhereas infrasound is a well-defined concept, low frequency noise is not. In the Information No. 9/1997 [1] low frequency noise is defined to comprise the frequency range 10 Hz – 160 Hz, but other references – including other assessment methods – use other frequency ranges usually within the interval 8 – 250 Hz. Low frequency noise is noise with a strong content at low frequencies, giving the noise a rumbling character different to e.g. road traffic noise. The concept of low frequency noise was introduced as this type of noise was expected to be more annoying than noise without the strong low frequency content. This expectation is mentioned in ‘Guideline on environmental noise from industry’ [5]. Here it is also mentioned that there ‘is no universally accepted assessment method for low frequency noise at present’. Since then, several countries have published measurement and assessment methods for low frequency noise; reference is made to Chapter 4 in the present Report. Many assessment methods use the spectrum of the noise (1/3-octave spectrum measured indoors), and compare this spectrum to a criterion curve, which must not be exceeded in any 1/3-octave band. In the Danish assessment method the noise is evaluated by the A-weighted level of the noise in the frequency range consisting of all the 1/3-octave bands from 10 Hz to 160 Hz. Depending on the actual conditions, many types of noise can be regarded as low frequency noise. The firing rate of many diesel engines is usually below 100 Hz, so road traffic noise can be regarded as low frequency noise as well as (diesel) train noise or noise from ferries. Similar considerations can be made for engines or compressors in industries or co-production plants. Burners can emit broadband low frequency flame roar. Low frequency noise can be noise or vibration from traffic or from industries, totally or partly transmitted through the ground as vibration and re-radiated from the floor or the walls in the dwelling. By this transmission, frequencies above approximately 20 Hz are attenuated. It is a general observation that indoor noise is perceived as more ‘low-frequency-like’ than the same noise heard out of doors. 1.3 Motivation for the InvestigationThe Danish Environmental Protection Agency has the opinion that hearing and assessment of infrasound is reasonably well described. In addition it is extremely rare that infrasound in the environment is anywhere near the recommended limit value. Contrary to this there is an obvious need for investigations where the subjective annoyance due to typical examples of low frequency noise is compared to different objective measures of the strength of the same noises. There are several different features of the different assessment methods presently in use, and the corresponding limits or criteria values differ. The fundamental assumptions for the assessment methods are, however, largely the same. (I.e. the same investigations and the theories
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