Low Frequency Noise Annoyance. Assessment by Low Frequency. Noise Rating (LFNR) Curves *. N. Broner (1) and H.G. Leventhall (2). (1). Vipac & Partners ...
Low Frequency Noise Annoyance Assessment by Low Frequency Noise Rating (LFNR) Curves *.
N. Broner (1) and H.G. Leventhall (2). (1).
Vipac & Partners Pty Ltd., 30-32 Clarement Street, South Yarra, Victoria, Australia 3141. (2). A tkins Research and Development, Woodcote Grove, Ashley Road, Epsom KT18 5BW, U.K. (Received 6 Januarv 1983) Abstract
Over recent years, it has become apparent that low frequency noise annoyance is more widespread than originally believed. Annoyance has occurred where the emitted noise is unbalanced towards the low frequencies even though the dB(A) level has been low. Following laboratory experiments carried out as part of an investigation into low frequency annoyance, combined with field annoyance data, the Low Frequency Noise Rating (LFNR) curves are proposed for the assessment of low frequency noise annoyance complaints. 1.0 INTRODUCTION The number of complaints regarding low frequency noise emitted by industrial plants and service equipment is increasing. Many sources of high level low frequency noise (up to about 100 Hz) and also of infrasound (up to 20 Hz) have been identified and documented. Such sources include ventilation and air-conditioning systems, oil and gas burners, boilers, compressors and diesel engines (eg. Hood and Leventhall, 1971: Leventhall , 1973: Tempest, 1973: Ikegami and Nakano, 1975: Leventhall and Kyriakides, 1976: Bryan, 1976: Broner, 1978a: Nishiwaki and Mori, 1978: Tokita, 1980: Somek, 1980: Yamashita and Koyasu , 1980). Many of these sources are unbalanced towards the low frequencies in that they exhibit a spectrum which shows a general decrease of sound pressure level with increase in frequency and it is now apparent that, in some situations, this type of noise is clearly annoying (eg. Bryan, 1971, Tempest, 1973: Bryan, 1976: Vasudevan and Gordon, 1977: Leventhall, 1980). Subjective reports of effects due to such exposure indicate that disorientation and general unpleasantness as well as a variety of other symptoms may occur. Bryan (1971), for example, found that two residents living near a factory boiler were annoyed even though the level outside their houses was only 55 dB(A). A similar case investigated by Broner (l978b) found one resident disturbed by the noise from a central heating unit even though the level was only 32 dB(A). A number of other low-frequency noise nuisance problems have been reported (eg. Mullin, 1972: Birney, 1972: Anon., 1973a, 1973b, 1973c: Bruel and Oleson, 1973: Leiber, 1974: Ashley, 1975: Nishiwaki and Mori, 1976: Kono et ai, 1976: Ugai and Suzuki, 1979: Chatterton, 1979: Mabey, 1979). There is one area of West London in which complaints of low-frequency noise annoyance occur regularly (Scott, 1978), although the problem appears to be a national one. A report in the London newspaper the "Sunday Mirror" brought a response of over 700 letters, many of which also described annoyance due to a low-frequency rumble and in some cases, due to a feeling of pressure on the ears (Anon. 1977a, 1977b, 1977c). It is, therefore, apparent that annoyance due to low frequency noise is experienced by members of the general population. Now that the problem has been recognised, more complaints are coming forward. A review of case histories suggests that an unbalanced spectrum occurs if the difference between linear SPL and A-weighted SPL (SPL(A» is greater than approximately 20 dB when the SPL(A) is low. Often, noise control of the more common broadband sources is carried out in such a way as to result in a falling spectrum characteristic with increasing freq uency, the rationale being that the higher frequencies are more annoying and contribute more to A-weighted SPL and so should be reduced. Likewise, the transmission loss through a wall is such that it shapes the noise immission spectrum, the normal wall transmission loss emphasising spectrum unhalance. The result is that the annoyance experienced due to the unbalanced noise imrnission may be significant even though the SPL(A) is such "The work reported in this paper was carried out whilst the authors were at Chelsea College, University of London. t .. urnal
of Low Frequency Noise and
Vit-rutio n Vol.2, No.1, 1983
20
LOW FREQUENCY NOISE RATING
that, based on normal criteria, no annoyance would be expected. It is becoming increasingly apparent that the SPL(A) value is not a valid basis for validating a complaint where the intruding noise is unbalanced, so that it contains most energy in the lower frequencies. The common assumption that the assessment of loudness and annoyance are equivalent also breaks down in these cases (Tempest, 1973: Bryan, 1976) and this may be due, in part, to the unsteady nature of much low frequency noise. . One complicating factor is the possibility of low-frequency tinnitus. Common forms of tinnitus are of higher frequency, for example, whistling and ringing sounds, but our investigations have shown us that, although some low-frequency noise complainants do suffer from tinnitus, others do not. As part of an investigation into low-frequency noise annoyance at Chelsea College, annoyance responses to, and unacceptability ratings of, low frequency noise were obtained. The major results are reported elsewhere (Broner, 1979: Broner and Leventhall, 1978a, 1978b, 1980). This paper describes the basis for the derivation of the Low Frequency Noise Rating curves first proposed by Broner (1980) and indicate how they dan be used to assess low-frequency noise annoyance. 2.0 METHOD Test Chamber The test chamber used was a steel enclosure with 100 mm thick double skinned walls. The large size (3.65 x 3.05 x 2.44 m) provided spaciousness and comfort and gave validity to judgements made as if relaxing at home. Noise Stimuli The noise stimuli were produced by ten low-frequency loudspeakers which were mounted in a 2 x 5 matrix on one of the walls of the chamber and were driven by a 300 watt DC power amplifier. The test stimuli were confined to frequencies below 100 Hz. Thus, the noise stimuli consisted of the seven nominally 10Hz bandwidths between 20 and 90 Hz (produced by a Random Noise Generator in conjunction with a variable low frequency filter) and each was presented at an overall SPL of 55,65 and 75 dB. The sequence of 21 stimuli is shown in Table I. Some of the stimuli are near the normal threshold of audibility. The A-weighted SPL range for the stimuli was 19-52 dB (see Table 2). Table 1 The Sequence of 21 Low Frequency Noise Stimuli Stimulus Number
Frequency Range (Hz)
Linear SPL (dB)
1 2 3
50-60 80-90
65 75
30-40
55
4
20-30
75
5
40-50
65
6 7
70-80
55
8
60-70
75 55
9
20-30 70-80
10
40-50
65 75
II
80-90
55
12
75
13
50-60 60-70
65
14
40-50
55
15
80-90
65
16
20-30
65
17 18
50-60 30-40
55 75
19
70-80
75
20
60-70
55
21
30-40
65
21
LOW FREQUENCY NOISE RATING
Table 2
dB(A) Levels for the Low Frequency Noise Stimuli
LSPL
20-30
55 dB 65
19.8 24.3
75
33.2
Frequency Hz 40-50 50-60 60-70
30-40 22.4
24.3
27.5
31.3
31.6 40.5
34.0 44.3
37.0
38.7 49.4
47.9
70-80
80-90
31.8 40.2
31.4 42.0
50.8
52.2
Subjects The total number of subjects used in the study was 75. This paper is, however, concerned only with those 21 subjects who had previously co-operated with the Acoustics Research Group in its investigation of low-frequency noise annoyance phenomena. Some of the subjects had travelled many miles to take part in the study and were very willing to assist in any investigations which could have helped solve the mystery of "their problem". Of the subjects, 10 had had their hearing threshold tested previously (Walford 1978) and their hearing was found to be generally poor. In fact, it was found to be significantly poorer than a control group for frequencies above 50 Hz, though, below 50 Hz there was no significant difference. The mean age of the group of subjects was 54.3 years with a standard deviation of 12.6 years. Unacceptability Rating The subjects taking part in a study of annoyance were to imagine that, after a hard day's work, they had just been comfortably seated at home and had intended to read their newspapers. The main instructions dealt with the rating of annoyance by means of the magnitude estimation technique (eg. see Broner and Leventhall, 1978b). Following the annoyance response the subjects were instructed as shown in Figure 1. After recording your annoyance
respo~se,
I want
yc~
"0
ir~icate
whether
or not you believe the sound you have just heard uoul.d be acceptable to you.
By this I mean whether or not you :eel tiut you could learn to live
with it if you heard it regularly i" your own home. After recording your acceptability response, you may comment if any, that you may have perceived due
to each sound.
the 2ffec"s,
On
Such effects as
chest vibrations or a pressure sensation are of inverest. Finally, would you please indicate in "he space Frcvided whether
yo~
regard
yourself as being sensitive to noise. Thank yo",,).. AGE:
;).
::>
SEX:
M
Do you regard yourself as being sensitive to noise?
TIME:
is'30
YES
REACfION SHEET NOISE NUMBER
RATING
1
70
2
23
3
30
ACCEPTABLE YES NO
.>
COMMENTS
Throbbing
/
.:
Etc Figure I
Unacceptability Rating Instruction and Reaction Sheet
22
---
LOW FREQUENCY NOISE RATING
Test Procedure The first 9 stimuli were duplicated and added to the sequence shown in Table I, thus resulting in a total of 30 stimuli. In this way, a subject could commence at anyone of the first ten stimuli, thus providing a degree of randomness. Each stimulus was presented for 20 seconds with a 10 second interval between stimuli, resulting in a total test session length of 10.5 minutes for the 21 stimuli. The subject carried out the rating task whilst seated in the test chamber. To ensure that no stimulus was missed, a green light came on 0.25 seconds prior to each noise stimulus. 3.0 RESULTS Figure 2 shows the percentage of subjects expressing unacceptability for three linear SPL's (LSPL). It can be seen that for an LSPL of 5-5 dB, an average 20% of the sensitive group of respondents expressed unacceptability whilst for 65 and 75 dB LSPL, the average percentages rose to 50% and 80% respectively. There is also some evidence that the 30-50 Hz range is more objectionable than adjacent frequency bands. It should also be noted that, as the frequency is increased from 70 Hz, the unacceptability again increases and tends towards the rating expected at higher frequencies at the given SPL(A) levels.
ANNOYANCE .55dB
SPECI ALS ~ 65dB
o
N =21 75dB
100
>~
-J
CD
~
0W U U
50
z
::> ~ 0
20-30 Hz
Figure 2
3D-40Hz
loO-50Hz
50-60Hz
60-70 Hz
70-80 Hz
80-90 Hz
Unacceptability Ratings for the Group of "Specials" to the Noise Stimuli
4.0 CASE HISTORIES AND FIELD STUDIES The need for a modification to established noise criteria to account for annoyance caused by low-frequency/low-level noise comes from various case histories and field studies documented by consultants and researchers since 1971. Typically, the background noise within a residence may have been very low so that the lowfrequency noise is not masked. In many cases, the accompanying low-frequency fluctuations and modulations cause a "throbbing" which increases the annoyance. In others, the secondary effect of rattling of windows and doors leads to fear and annoyance. The inability to locate the source of disturbance is also seen to contribute to the annoyance, whereas in the cases where sources are known, vigorous complaints arise. In one such case the source of the disturbing low-frequency "rumble and throbbing" was a set of two vibrating screens within a sand mixing plant approximately 400 m away from the complainant. With the plant running, high SPL's occurred at the screen frequencies of 45 Hz and 63 Hz, giving a noise level of 56 dB linear or 29 dB(A) in the complainant's bedroom. Figure 3 shows the spectrum during plant operation. For comparison, the spectrum after closure is also shown. The linear noise level was found to drop to 52 dB. A level of 22 dB(A) following closure was deemed acceptable by the complainants.
23
LOW FREQUENCY NOISE RATING
ONE THIRD OCTAVE lAND SOUND PRESSURE LEVEL (dB)
50
....- Prtor to plallt Closure
'0
30
20
10
16
31.5
63
125
250
500
1000
2000
'000
FREQUENCY (Hz)
Figure 3
One-third octave spectra showing the noise measured in the bedroom prior to and after plant closure
Another two examples of annoying low-frequency/low-level noise are shown in Figure 4. In both cases, persistent complaints arose within living accommodation in Central London due to machinery in adjacent premises and there were threats of
legal action. The levels are low at 31 dB(A), 53 dB linear due to machinery in the dry cleaners and 32 dB(A), 54 dB linear due to the adjacent air-conditioning plant. The one third octave analysis, which averages the signal, does not reveal (he true nature of the noises. They both had an unpleasant throbbing characteristic of about one per second and may have been tolerable had they not been fluctuating. Low-frequency pure tones just at or above the threshold of hearing have also been found to cause extreme annoyance (eg. Broner, 1978b: Bryan and Tempest 1979: Chatterton, 1979: Leventhall, 1980). Typically, sources were a central heating unit, a cupola furnace in resonance, a residential boiler and an industrial forced draft fan boiler. In Japan, the rattling of windows and floors (especially in wooden houses) due to low-frequency noise has been found to cause anxiety and complaints by inhabitants (Yamada et aI., 1980: Nishiwaki, Mori and Fujio, 1980). Typical sources were again forced draft fans and boilers, reciprocating machines and furnaces as well as the Shinkasen high-speed railway entering and leaving tunnels. Complaints have also been registered with linear levels as low as 65 dB in the range 2-90 Hz (Tokita, 1980). ~:~~~~~~AVEj-·--+---t-------1r---+---+---+---t---t---t--., PRESSURE LEVEL (dB)
DRY
60
CLEANERS, 31 cJB(A)
AIR CONDITIONING PLANT, 32dB(A) 50 I
'0
\
•..
I ....
·U
30
10
16
31,5
150 fREQUl::NCY
Figure 4
500
1000
2000
4000
(H7)
One-third octave spectra showing the noises measured in living accommodation in Central London which caused persistent complaints
24
LOW FREQUENCY NOISE RATING
5.0 CHOICE OF A CRITERION LIMIT It has been shown above that low-frequency noise annoyance is more common than
initially believed and it is our experience that the investigating authorities, basing their assessment on the normal dB(A) criteria, tend to reject the legitimacy of many of the complaints. A decision must, therefore, be made to "protect" complainants by showing that the complaint may be validated. The choice of a criterion limit is then dependent on the degree of complaint validation that one wishes to afford the "sensitive" people. It would certainly appear that such a limit should be proposed, at least for the purpose of assisting Environmental Health Officers to an awareness that the normal criteria do not apply to "sensitive" people in situations where low-frequency noise predominates. 6.0 THE LOW FREQUENCY NOISE RATING (LFNR) The Noise Rating (NR) curves formerly proposed by ISO for the general evaluation of acceptable noise levels in buildings are not appropriate for the evaluation of lowfrequency noise problems such as those described above. It is, therefore, suggested that a new set of curves is required to be used for judging low-frequency noise complaints. The need for such a family of ratings has already been recognized and Challis and Challis (1978) made an initial attempt in proposing the NRM (modified NR) criteria for assessment of low-frequency noise problems due to gas turbine power stations. However, the NRM criteria do not adequately account for the increased sensitivity experienced below 100 Hz when un balanced noise emissions occur. Blazier (1981) proposed new criteria taking spectrum balance into account but these were only derived for heating and ventilation systems. There is evidence for an increased sensitivity to the 30-50 Hz range, based on the unacceptability results reported in section 3.0 above and from Kraemer (1973), Vasudevan and Gordon (1977) and unpublished data obtained at Chelsea College. The Low Frequency Noise Rating (LFNR) curves, shown in Figure 5, are therefore proposed. The complaint assessment is carried out by superimposing a one third octave analysis of the noise, measured inside the house or office concerned and not externally as often recommended in Guides or Standards, on the LFNR curves. Considering all one third-octaves from 125 Hz upwards, the lowest curve which is not exceeded by the noise spectrum is the LFNR rating of the noise. If anyone third octave below 125 Hz exceeds that LFNR rating, then a complaint about the low-frequency noise emission may be anticipated. As can be seen from Figure 5, it is likely that no annoyance will be experienced if a one third octave band spectrum of the noise lies below the LFNR 25 curve. This limit can be written in terms of SPL(A) and linear sound pressure level (LSPL) as: For SPL(A)
~
30 dB,
LSPL
~
55 dB.
As the A-weighted SPL increases, as for example may occur. at work (eg. office or industry), the evidence suggests that the limit on the LSPL need not be as strict, possibly due to different expectations of background noise compared to the relaxed home setting. This means that increasingly more low-frequency noise is acceptable as the SPL(A) increases and this is reflected by the shape of the LNFR curves below 100 Hz as well as in higher A-weighted ambient sound level criteria for places of work. Thus, for LFNR 25, the break point is at 100 Hz and 44 dB while for LFNR 45, the break point is at 63 Hz and 70 dB. The break line slope is -13 dB/(I/3-octave). Similarly, below 25 Hz, where there is a reduced sensitivity as shown in the unacceptability results and in the hearing threshold curve, the LFNR curves allow increased levels. At higher levels the limit may be written approximately as: For SPL(A)
> 30,
LSPL
~
SPL(A) + 30 dB.
Where a noise is throbbing or fluctuating, the evidence suggests that it is more annoying and thus a penalty should be applied to the one third-octave spectrum below 100 Hz before an assessment is made. A penalty of 3 dB is suggested. An example of the use of LFNR curves is shown in Fig. 6. The complaint occurred in an office. Prior to treatment, levels at 125Hz and above gave LFNR 38 as the rating and this curve is exceeded in the frequency region below 125Hz. The complaint therefore was assessed as justified. Following treatment, the rating determined by frequencies above 125Hz was LFNR 41, whilst the lower frequencies are at levels well below this curve. Conditions were judged to be acceptable.
2S
LOW FREQUENCY NOISE RATING lFNR
110
100
90
G
~
iil
80
> ~
Q
10
Z
< lIQ (OJ
~
tJ
o
60
SO
Q
e:l: !-o
"l
40
Z
0
30
10
FREQUENCY
Figure 5
(Hz)
The proposed Low Frequency Noise Rating (LFNR) Curves
ONE nilRDOCTAVE BAND SOUND PRSSURE LEVEL (dOl
70
,,0 50
'" .10
16
Figure 6
31.S
63
I
I
I
12S
ISO
SOO
1000
2000
'000
Noise measured in a small office (After Leventhall and Kyriakides, 1976)
7.0 CONCLUSION Low-frequency noise annoyance problems are more common than originally believed and are becoming increasingly recognised as awareness of the problem develops. A means of validating low-frequency noise complaints has been proposed in the form of the LFNR curves. When using these curves, the increased annoyance due to lowfrequency "throbbing" or fluctuations must also be considered. It is necessary to measure the annoying noise in the environment concerned, not externally as recommended by most guides and standards.
26
LOW FREQUENCY NOISE RATING
REFERENCES Anon (1973a) "Supermarket", Noise and Vib. Bull., Sept, 239. Anon (1973b) "The Infrasonic Bus", Noise and Vib. Bull., March, 84. Anon (1973c) "Oil-fired Boiler", Noise and Vib. Bull., Nov, 286. Anon (1977a) "Have you heard the hum?", The Sunday Mirror, June 19. Anon (1977b) "Hounded by this nagging noise", The Sunday Mirror, June 26. Anon (19 77c) "Hum - That Mystery Noise Drives Sobbing Wife Out of the House ", The Sunday Mirror, July 3. Ashley, C. (1975) "Chairman's Report on the Workshop on Infrasound". J. Sound & Vib., 43(2), 465-466. . Birney, J.G.
(1972) "Infrasound". New Scientist, 54 (791),101 (Letter).
Blazier. (1981) "Revised Noise Criteria for Application in the A coustical Design and Rating of HVAC Systems. Noise Control Engineering 16,64-73. Broner, N. (19 78a) "Low Frequency and Infrasonic Noise in Transportation ", Applied Acoustics, II, 129-146. Broner, N. (1978b) "The Effects of Low Frequency Noise on People - A Review". J. Sound and Vibration, 58(4), 483-500. Broner, N. (1979) "Criteria for the Assessment of Low Frequency Noise". Proc. of Low Frequency Noise Conf., Inst. of Acoustics, Jan. Broner, N. (1980) "A Criterion for Low Frequency Noise Annoyance ", Presented at the 1 Oth ICA Sydney, July, Paper C1.-4.4. Broner, N., Leventhall, H.G. (1978a) "Individual Loudness Functions". Acoustics Letters, 2, 22-25. Broner, N., LeventhaU, H.G. (1978b) "Low Frequency Noise Annoyance and the dB(A)". Acoustics Letters, 2,16-21. Broner, N., LeventhaU, H.G. (1980) "A Modified PNdB for Assessment of Low Frequency Noise". J. Sound and Vib., 73(2),271-277. Bruel, P.V. (1979) "Limits for Infrasound and Ultrasound in Factories". Proc. Inter-Noise 79, Paper L3-1, 849-854. Bruel, P.V. (1980) "Standardization for Low Frequency Noise Measurements". Proc. Conf. on Low Frequency Noise and Hearing, 7-9 May, Aalborg, Denmark, M~ller and Rubak (eds.), Aalborg University Press, 235-240. Bruel, P.V., Oleson, H.P. (1973) "Infrasonic Measurements". Bruel and Kjaer Technical Review No.3, 14-25. Bryan, M.E. (1971) "Annoyance Effects Due to Low Frequency Sound". Proc. of the Fall Mtg. of the Brit. Acoust. Soc., 71.109, Nov. Bryan, M.E. (1976) "Low Frequency Annoyance" in 'Infrasound and Low Frequency Vibration', W. Tempest (ed.), Academic Press, London 65-96. Bryan, M.E., Tempest, W. (1979) "Low Frequency Noise Annoyance". Proc. of Low Frequency Noise Conf., Inst. of Acoustics, Jan. Challis, L.A., Challis, A.M. (1978) "Low Frequency Noise Problems from Gas Turbine Power Stations". Proc. Inter-Noise 78, 475-480. Chatterton. (1979) "A Case History of a Low Frequency Noise Problem ". Noise Control Vibration Isolation, AugfSept., 295-298. Hood, R.A., Leventhall, H.G. "Field Measurement of Infrasonic Noise". Acustica 25,10-13, (1971).
27
LOW FREQUENCY NOISE RATING Ikegami, Y., Nakano, A. (1975) "Infrasound from Diesel Engine". Proc. InterNoise 75, 303-306. Kono, S. et al. (1976) "Vibration of Houses Caused by Infrasound and CounterMeasures Against It". Proc. Inter-Noise 76, 237-242. Kraemer, S.D. (1973) "Annoyance of Low Frequency Sounds". M Sc Project Report, Chelsea College. Leiber, C.O. (1974) "Physiologische Auswirkung des Infrascahlls (Eine Befragung}". Institut fur Chemisch-technische Untersuchungen (CTI), Bonn, June. Leventhall, H.G. (1973) "Man-made Infrasound, Its Occurrences and Some Subjective Effects". Proc. of the Colloquium on Infrasound, Centre National de la Recherche Scientifique, Paris, Sept., 129-152. Leventhall, H.G. (1980) "Annoyance Caused by Low Frequency Low Level Noise". Proc. Con! on Low Frequency Noise and Hearing, 7-9 May, Aalborg, Denmark, M,pller and Rubak (eds.), Aalborg University Press, 113-120. Leventhall, H.G., Kyriakides, K. (1976) "Environmental Infrasound: Its Occurrence and Measurement" in 'Infrasound and Low Frequency Vibration '. W. Tempest (ed.), Academic Press, 1-18. Mabey, D.G. (19/9) "Reduction of Infrasound from the Auxiliary Compressors of a Large Wind Tunnel". J. Sound and Vib., 63(1), 158-160. Mullin, S. (1972) "Infrasound's Effects " New Scientist, 53(789), 709 (Letter). Nishiwaki, N., Mori, T. (1976) "Noise at Infrasound Frequencies Generated by Machines and Methods for Decreasing SPL ". Proc. Inter-Noise 76, No.31. Nishiwaki, N., Mori, T. (1978) "Low Frequency Noise from a Diesel Engine Ship ", Proc. Inter-Noise 78, 785-788. Nishiwaki, N., Mori, T., Fujio, N. (1980) "Vibration Problems due to Low Frequency Noises from an Electric Power Station ". Proc. Con! on Low Frequency Noise and Hearing, 7-9 May, Aalborg, Denmark, Mller and Rubak (eds.), Aalborg University Press, 173-180. Scott, R. (1978) "Annoyance Caused by Low Frequency Sound". Noise and Vibration Bulletin, Sept., 266-268. Somek, B. (1980) "Infrasound in Industry". Proceedings of 1 Oth ICA, Syndey, July, Paper C2-1.1. Tempest, W. (1973) "Loudness and Annoyance Due to Low Frequency Sound", Acustica, 29, 205-209. Tokita, Y. (1980) "Low Frequency Noise Pollution Problems in Japan ", Proc. Con! on Low Frequency Noise and Hearing, 7-9 May, Aalborg, Denmark, M,pller and Rubak (eds.), Aalborg University Press, 189-196. Tokita, Y. et at. (1980) "Proposal on the low Frequency Noise Meter". Proc. Con! on Low Frequency Noise and Hearing, 7-9 May, Aalborg, M,pller and Rubak (eds.), Aalborg University Press, 227-23~. Ugai, Y., Suzuki, S. (1979) "Infrasound Occurred to Centrifugal Fans Used in Subway Ventilating Equipment and Infrasound Control". Proc. Inter-Noise 79, Paper H20-D, 569-572. Vasudevan, R.N., Gordon, C.G. (1977) "Experimental Study of A nnoyance Due to Low Frequency Environmental Noise". Applied Acoustics, 10,57-69. Walford, R.E. (1978) "Acoustical Aspects of Some Hum Complainants". Diploma of Chelsea College. Yamada, et at. (1980) "Hearing of Low Frequency Sound and Influence on Human Body". Proc. Con! on Low Frequency Noise and Hearing, 7-9 May, Aalborg, Denmark, M,pller and Rubak (eds.), Aalborg University Press, 95-102. Yamashati, M., Koyasu, M. (1980) "Infrasound in Daily Lives". Proceedings of 1Oth ICA, Sydney, Paper C2-7.1.
28
