ECOACCESS GUIDELINE FOR THE ASSESSMENT OF LOW FREQUENCY NOISE Cedric Roberts Environmental Operations, Integrated Assessment, Queensland Environmental Protection Agency, 160 Ann Street, Brisbane, Queensland, Australia 4002 Abstract As a result of the escalation in low frequency noise sources appearing in new industrial development and the incidence of low frequency noise complaints experienced by residents in Queensland, a low frequency noise guideline has been developed by the Environmental Protection Agency. The Guideline is applicable to low frequency noise emitted from industrial premises, commercial premises and mining operations (not blasting), and is intended for planning purposes as well as for the evaluation of existing problems. The intent of the established criteria is to accurately assess annoyance and discomfort to persons at noise sensitive premises caused by low frequency noise with a frequency range from 10 Hz to 200 Hz. The technical paper will describe the main elements of this guideline including: use of the G-weighting function to determine annoyance due to infrasound in the frequency range from 1 Hz to 20Hz and low frequency noise criterion adopted for initial screening inside home environments in terms of Linear, A-weighted and one-third octave band sound pressure levels in the range 20 to 200 Hz. Initially auditory perception is established by comparing one-third octave band frequency sound with the ISO median hearing threshold levels (HTLs) for the best 10% of the aged population (55-60 years old) followed by separate noise criteria applied to establish annoyance from tonal and non-tonal noise. Two examples of the application of the guideline to infrasonic noise and non-tonal noise from industrial equipment are given in the paper. Introduction The G-weighting function is used to determine annoyance due to infrasound. The G-weighting for the There has for many years been a high incidence of determination of weighted sound pressure levels of sound low frequency noise complaints reported to the state or noise, whose spectrum lies partly or wholly within the government environmental protection agency from frequency range from 1 Hz to 20 Hz, has been residents living in Queensland. There also appears to be standardized in ISO 7196, (1995) [2]. G-weighted sound an escalation in low frequency noise sources appearing in pressure levels are denoted LpG and are measured or new industrial development in Queensland. As a estimated in dB(G). consequence, a low frequency noise guideline has been The average hearing threshold for single tones is developed by the Environmental Protection Agency. The usually about 95 to 100 dB(G), and tones with a 20 dB Guideline is applicable to low frequency noise emitted higher level are expected to be sensed as very loud. G- from industrial premises, commercial premises and weighted levels below 85-90 dB(G) are not normally mining operations (not blasting), and is intended for significant for human perception and are not annoying. planning purposes as well as for the evaluation of The recommended limit value for infrasound inside existing problems. dwellings during the day, evening and night and inside The intent of the established criteria is to accurately classrooms and offices is 85 dB(G). For occupied rooms assess annoyance and discomfort to persons at noise in commercial enterprises the limit is 90 dB(G). sensitive premises caused by low frequency noise with a (Jakobsen 2001) [3]. frequency range from 10 Hz to 200 Hz. The noise is measured over a 10-minute period and a 5 dB penalty is added for impulsive noise e.g. single Infrasound blows from a press or drop forge hammer. An Low frequency noise spans part of the infrasonic and approximate determination of the G-weighted sound audible ranges and may be considered to range from pressure level may be made by analysis of the signal about 10 Hz to 200 Hz [1]. Sound in the frequency range using one-third octave bands and application of the below 20 Hz is defined as infrasound. Infrasound can be weighting values given in Table A1, Appendix A. An heard (or felt) provided it is loud enough. Infrasound is example of the determination of the annoyance due to usually not perceived as a tonal sound but rather as a infrasonic noise is given in Table B1 Appendix B. pulsating sensation, pressure on the ears or chest, or other less specific phenomena. The loudness and annoyance due to infrasound increases extremely rapidly with increasing level above the threshold of hearing. 2. Determine if L – L > 15 dB. If it does, Low frequency noise above 20 Hz LINeq Aeq measure the noise in 1/3rd octave band levels, in terms Items such as boilers, pumps, transformers, cooling of L for frequencies from 5 Hz to 200 Hz. fans, compressors, oil and gas burners, foundries, LINeq washing machines, electrical installations, diesel engines, 3. Compare these levels with the LHS values of the asynchronous motors, ventilation and air-conditioning median hearing threshold level for the best 10% of equipment, wind turbulence and large chimney the older population (55-60 years old) given in resonances are sources of high level, low frequency noise Table 1 to determine the degree of low frequency having frequency content less than 200 Hz. Generally, noise audibility. low level/low frequency noises become annoying when 4. Check for the existence of an amplitude-modulating the masking effect of higher frequencies is absent. This component, where the noise level changes cyclically loss of high frequency components may occur as a result at a particular 1/3rd octave band frequency. The added of transmission through the fabric of a building, or in perception of loudness caused by this attribute can be propagation over long distances. It may even be accounted for by subtracting a 5 dB penalty from the perceived by a resident who suffers from age-induced, LHS value in Table 1. high frequency hearing loss. For example, the LHS level for 50 Hz could be A problem arises in determining the location of the reduced from 39 dB to a corrected 34 dB. This could source and/or the relative importance of the various be the case for poorly designed HVAC systems that transmission paths of low frequency sound sources as the generate large turbulent fluctuations at low radiation pattern is usually omni-directional. The location frequencies and can cause fan surging with of low frequency noise indoors is complicated by a concomitant noise level surging of 10 dB or more. number of factors such as: room dimensions being Table 1 Median hearing threshold levels (L ) exceeded normally within the wavelength range of interest (1.36 to HS by 10% of an aged population (55-60 years old) 17m) which may give rise to standing waves; airborne noise causing windows and other building elements to 1/3 Octave Centre L (dB) rattle; and difficulty in distinguishing between airborne Frequency (Hz) HS and structure borne noise. 8.0 96 The main elements of an assessment include: The low frequency noise criterion adopted for initial screening 10.0 92 inside home environments in terms of Linear, A- 12.5 88 weighted and one-third octave band sound pressure levels 16.0 84 in the range 20 to 200 Hz; the comparison of one-third 20.0 75 octave band low frequency sound levels with the values 25.0 62 for LHS of the ISO median hearing threshold level for the best 10% of the aged population (55-60 years old) to 31.5 55 initially establish auditory perception. The initial 40 46 assessment is intended for use in cases where an 50 39 individual complains about low frequency noise and a 63 33 decision needs to be made as to whether the particular 80 27 noise is audible. This assessment does not verify whether the noise is annoying or not. A sound that is audible is 100 22 not necessarily unacceptable. 125 18 Initial assessment 160 14 Where a noise immission occurs exhibiting an 200 10 unbalanced frequency spectra, the overall sound pressure Source: ISO/CD 1996-1 level inside residences should not exceed 50 dB(Linear) Annoyance due to tonal noise to avoid complaints of low frequency annoyance. If the If the sound pressure level in a particular 1/3rd octave dB(Linear) measurement exceeds the dB(A) band is 5 dB or more above the levels in the two measurement by more than 15 dB, a one-third octave neighbouring bands, the noise is said to be tonal. There band measurement in the frequency range 20 to 200 Hz can be one or more tonal components in the spectrum. should be carried out. For tonal noise, the level in the frequency band/s with Audibility assessment the tone/s is compared to the hearing threshold level The following checks are made to establish whether (LHS) in the corresponding bands (Table 1). It is then noise contains dominant low-frequency components: found how much (the exceedence) the tonal value is above the threshold level. The levels in the other 1. Determine broad band L and L levels inside Aeq LINeq frequency bands are not taken into account. each affected room for a representative measuring time interval, typically 10 minutes. Acceptable criteria for tonal noise Table 4. Recommended limits for non-tonal low The limit values for exceedence of the threshold table frequency noise (LpA,LF) values by the equivalent level of the tone/s are as given in Type of space L dB Table 2. pA,LF (Note 1) (Note 2) (Note 3) Table 2. Limit values for exceedence for annoyance due Dwelling, evening and night 20 to tonal noise Dwelling, day 25 1/3 Octave Frequency Band Classroom, office etc 30 Rooms with commercial >100 35 Period 8 Hz to Hz and enterprises 80 Hz 100 Hz 63 Hz < 200 Source: Jacobsen, 2001[3] Hz Note 1.