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|Year : 2004 | Volume
| Issue : 25 | Page : 23--28
Effects of low frequency noise on man-a case study
J Feldmann1, FA Pitten2,
1 Institut für Technische Akustik, Technische Universität Berlin, Einsteinufer 25, D-10587 Berlin, Germany
2 Institute of Hygiene and Microbiology, University of Würzburg, Josef-Schneider Str.2, 97080 Würzburg, Germany
Technische Universität Berlin, FG Technische Akustik, Einsteinufer 25, D-10587 Berlin
Based on a real case effects of long-term exposure of infrasound on man are outlined. Beside a description of the background of the case together with remarks on the occurred health problems, the main view lies on the proceeding in identifying the special kind of exposure just as possible technical causes. As a source of annoyance a small heating plant was identified, which imitated into the house of the exposed people very low frequency airborne sound far below the common hearing thresholds. The results show clearly the general deficit of research on the effects of low level infrasound on man.
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Feldmann J, Pitten F A. Effects of low frequency noise on man-a case study.Noise Health 2004;7:23-28
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Feldmann J, Pitten F A. Effects of low frequency noise on man-a case study. Noise Health [serial online] 2004 [cited 2020 Aug 14 ];7:23-28
Available from: http://www.noiseandhealth.org/text.asp?2004/7/25/23/31650
In the course of 1995 Mrs. and Mr. B. moved in their new built one-family house within a less noisy sub-urban colony near a small provincial city. With the beginning of the heating cycle in winter 1995/ 1996 the family first noticed low frequency noise and vibration in their house. During 1996 both physiological and psychological effects occurred with increasing intensity like indisposition, decrease in performance, sleep disturbance, headache, ear pressure, crawl paresthesia or shortness of breath. The effects were so strong that the family decided to sleep in another location in the neighborhood. In the meantime they spent few weeks in a recreation clinic. The speculation at this time was, that the possible emission source is the central heating plant of the colony with a power of 2 x 750 kW located roughly 60 m from the home of the family B. but both, the kind of source mechanism and transmission paths were not clear. The annoyance depended on both season and weather in such a manner that it was stronger in winter and in cases of wind. Complaints of other people in the colony were not known. Also the authors did not perceive the signals during their activities. Because a contact with the management of the heating plant was not very successful a thorny road for the afflicted persons began. In the following five years several activities can be noted, in principal without a real success until now. Beside of inefficient building acoustic measures, examinations of experts inclusive the local administration in roughly six expertise of different quality came to the end that low frequency noise and vibrations were present in the house of the family caused by the heating plant, but both immission were far below the normal perception thresholds and therefore no actions necessary with respect to the German legislation regulations like TA-Larm, DIN 45680 or DIN 4150. The next instance was the court of law. At this point the authors of the present paper were involved into the dispute. They joint together the knowledge of technical acoustic with that of environmental medicine. By the court a new expertise was placed in order to clarify the open problems and contradictory questions, respectively, especially the following point: Is the heating plant really the source, why is the sleeping place in the neighborhood of the impaired house free of annoyance, and why are there no reactions from other persons living in the colony. Family B. had the hypothesis that a vein of clay soil between the heating plant and their house causes a transmission from the foundation of the plant directly via the ground, and not via the air.
To come to an end with such simple seeming questions the authors arranged different investigations, like special acoustic measurements, investigation of the geological situation and specific medical examinations, which shall be described in one of the following sections.
To give answers to the open questions from the technical side of view, special acoustic measurements of both airborne sound and vibrations at different locations were carried out. In addition to the existing results, and to get a better insight into the character of the immission the focus lay on narrow-band analysis. The frequency range was 2 Hz to 100 Hz, resolution 0.25 Hz. The used equipment consisted of a 1" condenser microphone B&K 4144 with amplifiers of type 2639 and type 2610, accelerometers B&K 4381 with amplifiers type 2635, capacitive accelerometers PCB3701G3FA3G including current supplies, Sony multi-channel DAT Recorder PC208 with a frequency range between 0 Hz and 5kHz and a two-channel FFT- frequency analyser. The lowest limit of sensitivity of the equipment for airborne sound was 2 . 10 - 6 Pa (-20 dB), the corresponding limit for the vibrations (velocity) was 50 . 10 - 3 m/s for 0.25 Hz, and at 0.05 . 10 - 3 m/s for 100 Hz, therefore the measurements are much more sensitive than the human perception thresholds. Sound and vibration were recorded simultaneously.
The investigated locations were:
i) The former sleeping room of the family in the impaired house as a place of largest annoyance. Measurement points were located with respect to sound inside the room off-centered, and with regard to vibrations at the outer building-wall inside the room. In this case Mr. B. could perform a part of the measurements himself over several nights by starting and stopping the recorder. The aim was, to detect the signals under conditions so realistic as possible in dependency of the subjective degree of annoyance. This was completed by a protocol, which contained both a subjective description of the events and the corresponding boundary conditions, like weather. For example, statements were: strong, very low frequency, hum, drone, intermittent, pulsating, diffuse pressure fluctuations, pain in the legs or in the area of stomach, additionally the annoyance was stronger at windy days. On this way for the first time it was possible to get a high correlation between the subjective statements and the physical signals. It turned out, that essentially frequencies below 10 Hz were most responsible for the annoyance, [Figure 1]. Additionally in some cases components around 48 Hz respectively 100 Hz where measured which could be identified as a contribution of the circulating pump of the heating system. But these components had the expected importance rather in vibration perception than in airborne sound.
ii) The sleeping apartment in the neighborhood in roughly 50 meters linear distance to the house. Here the results are very similar to those measured in the impacted living house. Therefore, the cause why at this place the annoyance was less could not explained by only physical factors.
iii) Another house near by (family O.), but nearer located to the heating plant. Here the measured spectra in principal were similar, but not so significant regarding the characteristic frequency ranges as in the home of family B. In this house family B. perceives the annoying signals less strong.
The results with respect to the characteristic behavior are summarized in [Figure 2]. The described differences are presented very clearly. With regard to the same place the spectrum in the case of less annoyance (ID20) lies 20 dB to 30 dB below the worst case (ID15). With a value of 65 dB the highest level are lying at 1 Hz. The alternative sleeping place of the family (ID02) is not free of the critical frequencies under discussion.
Based on these measurement results the authors formulated some conclusions together with hypotheses about the technical causes of the low frequency immission. Generally the maximal measured level in the most significant very low frequency range did not exceed 80 dB in any case. That means the immission are far below the common perception threshold here normally placed between 100 dB and 120 dB. At this point the question arose why such low levels in principal can lead to such bad health effects. Causes could be (Sueki et al., 1990; Berglund et al., 1996; Leventhall, 2003) i) the time of exposure was long enough to come into the awareness of family B. in spite of the low levels, about such long-term exposition no investigations with clear results exist; ii) individual thresholds can scatter over a larger range than for higher frequencies, this shall be discussed later;
iii) sound in connection with vibrations can amplify the reactions;
iv) infrasound with its very long wavelengths (10 Hz = 34 m) causes in relatively small closed rooms strong pressure fluctuations, which are more detected with the whole body and its inner organs than with the ears; v) annoyance depends as known not only on the height of the exposure level, other facts are also important, like sensitization, anger, stress, state of health or social facts, in parts these aspects are discussed later.
Possible technical sources of extreme low frequency sound (infrasound) were discussed in connection with the heating plant, and in coordination with the subjective statements about wind and weather. Beside the several sources like compressors, pumps, heating boiler, combustion chamber etc., the both chimneys of the heating plant came into the focus of consideration. Two mechanisms were thinkable. Firstly, a free-standing separate grounded chimney with a height of 13 m can be a source in form of wind excited tilting vibrations, here with frequencies in the order of 2.5 Hz. In this case the transmission would take place via both the soil and the base into the impaired house, where the vibrations are radiated as so-called secondary sound. The existence of a special clay vein is not necessary in any case. This transmission could be better in winter where the soil is frozen, but the mechanism requires wind. To suppress such an effect the chimneys have to anchored by wires. Against this hypothesis stood the fact, that the results of the vibration measurements were not comparable with those of the airborne sound, they were still much less significant. In the lowest frequency range the highest acceleration level lay around 0.001 m/s 2 . Therefore the theory about a transmission via the soil could not be maintained, this statement should be validated in a subsequent measurement campaign. Secondly, a chimney can be a direct airborne sound source. Two cases are possible. Either the combustion process generates low frequencies amplified by the exhaust duct including the chimney similar to an organ pipe, or the wind creates vortex shedding at the outer contour of the chimney. The generated directivity pattern is then dipole like. Based on the predominant wind direction the house of family B. together with the alternating sleeping apartment would be located in the main sound field. The dominant frequency is wind dependent. For a moderate wind speed of 5 m/s in connection with the dimensions of the chimney one can calculate a maximum around 1.2 Hz. The argument of the afflicted persons, the immission depends upon the season is not a contradiction with regard of both excitation possibilities, because free-field sound transmission can strongly depend on weather. A suppression of the first mentioned effect is possible by changing the dimension of the exhaust system, by adding a tuned resonant absorber or by influencing the combustion process directly. The second mentioned effect can be suppressed by a so-called spoiler consist of a helical wire applied over the height of a chimney. Other thinkable mechanisms are i) beat or combination frequencies generated by the two chimneys if the combustion process into the two fire chambers is not identical, call into mind that the heating plant consists of two separate parts of 750 kW; ii) resonances of special soil-building interactions.
A second measurement campaign could bring more light in the remaining open questions regarding the role of the plant. A measurement of corresponding sound and vibration could be performed simultaneously at the place of the heating plant and in the house of family B. under the both conditions heating plant in full action and heating plant out of operation. Measurement points again were inside the impaired house in the most annoyed sleeping room, and on the separated both foundations, plant respectively chimney. Representative results in form of level differences are shown in [Figure 3] and [Figure 4].
The results are showing clearly two facts. Firstly, the induced plant ground vibrations in the affected house are restricted to a frequency range only around 50 Hz, which can be identified as the portion of the circulation pump system. Compared to the signals at the heating plant, generally the effect in the house is not very marked, the statement the cause of annoyance lies primarily in ground transmitted vibrations cannot be hold. Another result of these measurements shows, that there were no significant vibrations at low frequencies below 10 Hz on the both foundations of the plant, so the hypothesis of tilting excitation of the chimney could not be maintained.
If one looks on the results of the sound measurements it is ascertainable that in principal the whole investigated frequency range is affected by the heating plant operation. The differences mainly are lying between 8 and 10 dB, also in the identified most annoying frequency range below 10 Hz. These results confirm clearly the hypothesis about the direct airborne sound character of the present annoyance.
To make sure that Mr. and Mrs. B. did not suffer from a neurologic or psychiatric disorder, they were examined by a psychiatrist. He did not know about the specific background of the family when he interviewed them. As a result he clearly excluded any severe psychiatric problem being responsible for the health complaints of Mr. and Mrs. B.
Another theoretical explanation for the complaints of the affected couple was that they were extraordinarily sensitive to low frequency noise and vibration. To get a clear impression of their ability to perceive infrasound they were presented to the psycho-acoustic laboratory at the University of Oldenburg. The following threshold limit values were determined for Mrs. B. (8 Hz / 98 dB, 48 Hz / 50 dB) and for Mr. B. (2 Hz / 115 dB, 5 Hz / 112 dB, 8 Hz / 108 dB, 12,5 Hz / 102 dB, 48 Hz / 55,5 dB). Therefore the infrasound immission detected in their house were by far lower than the immission which they could perceive.
The expertise of the present authors was basis of a second action at court. The arguments were accepted that the source of annoyance definitely can be seen in the heating plant nearby the impaired house of family B. Further is was undisputed that the main source is more directly radiated airborne sound rather than ground transmitted vibrations. But because in any case the levels of the immission are far below the known perception thresholds the physical signals alone are not sufficient to explain the negative health effects. But also the other examinations gave no clear indications. Family B. got no right. As a conclusion of the case one has to state, that in present not enough knowledge exists on the field of long-term exposition of low-level low frequency noise. Research is necessary.
|1||Berglund, B., Hassmen, P. (1996) Sources and effects of low-frequency noise. Journal Acoustic Society America 99 (5): 2985-3002.|
|2||Sueki, M., Noba, M., Nakagomi, M., Kubota, S., Okamura, A., Kosaka, T., Watanabe, T., Yamada, S. (1990) Study on mutual effects of low frequency noise and vibrations. Journal of Low Frequency Noise and Vibration an Active Noise Control 9 (2): 66 -75.|
|3||Leventhall, G. (2003) A review of published research on low frequency noise and vibration. Contract ref. EPG 1/2/50, Department for Environment, Food and Rural Affairs, London, UK.|