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Natural Science Forum / Physics / Acoustics / July 2006Measuring Background Noise
I have just got myself involved in some noise measurement work and have some received basic training. (I plan to do more training) However, in the meantime I need some advice on measuring background noise. The situation: A local factory/ warehouse has a planning condition that the LAeq (15 minutes) between 0800 and 2200 must must not exceed the background noise (defined as L95) by more than 5dB(A) and from 2200 to 0800 must not exceed by more than 3dB(A). To establish the backgrond noise I am assuming that I need to get an L95 reading for at least 15 minutes during the day and another reading during the night, at times when the factory is not operational. My question is - do I just take one 15 minute reading for day time background and one for night time or do I need to take readings at different times - as the backgound may vary e.g. noise at 8.00 in the morning may not be the same at say 20.00 in the evening. If I do take reading at more than one time - what do I do with the results. Average them? Advice would be appreciated. Doug First, I do not like the idea of mixing measurement methods and basing the limit on an leq exceeding an L95 by some amount. The existing leq may already be 10 or 20 dB higher than the L95. However, it is the regulators and not you who have set that method and you have to live with it. I think you have already answered your question somewhat by recognizing that the L95 with vary with different 15 minute periods. You really need to have some idea how the local authorities will interpret the situation. However, they could easily say that you must meet the limit for the quietest 15 minutes of the day on the quietest day of the year. Did you catch that, the sound will vary from day to day and possibly seasonally as well. In our area, the L95 is often higher in the summer and early fall due to insects, and then lower in the winter and early spring. If you do not have nearby sound sources at night, you will find the nighttime levels are often dominated by distant sources, and levels due to these sources can vary strongly with atmospheric conditions. >I have just got myself involved in some noise measurement work and have > some received basic training. (I plan to do more training) However, in [quoted text clipped - 20 lines] > > Doug Noral, I think you may be correct about the background vs the leq. I just did some quick measurements here in the office and found the L95 to be 41dB while the Leq was 52.9dB. This was only over a very short time of 30 sec. I extend that to 15 mins or 30 mins is there likely to be a difference in the readings? In the job I have to do it looks like the Leq will almost certainly be way more than 3dB or 5dB greater than the background. Is this appoach for setting limits commonly used. Is it a valid approach? As it stands it looks like the guy who own the warehouse is on to a no win situation. As, usual any advice would be appreciated. Doug > First, I do not like the idea of mixing measurement methods and basing the > limit on an leq exceeding an L95 by some amount. The existing leq may [quoted text clipped - 36 lines] > > > > Doug If this is a "simple" compliance situation, proving or disproving compliance by noise monitoring may be a fruitless exercise. Measure the environmental LA95 noise level and then do your close-in Leq noise measurements and start calculating out to the monitoring location. If the noises are uniformly constant this will work. If the noises are short and random, then the criteria may be inappropriate. Possibly an LAmax,15min vs LA95,15min, or an LA01,15min vs LA95,15min criteria may be more appropriate. It all depends on the "character" of the sound. In seeking a "simple" LAeq,15min compliance approach, it is easy to lose sight of the community's potential for a noise annoyance reaction to different type of sound. Psycho-acoustic reactions and industrial noise compliance often diverge to the detriment of those hearing the offending sound. > Noral, > [quoted text clipped - 14 lines] > > Doug Doug, As Brian Marston has pointed out, creating a noise environment that will satisfy neighbors is often a very different matter than just complying with regulations. In some places (common in the US) local regulations are written without much thought about all the possibilities. The 3 to 5 dB difference between the L95 and Leq could be an appropriate criteria if the community is very quiet and if the noise to be introduced is steady. However, a community could actually be very noisy but have a low L95. The Leq could be 20 dB or more higher than the L95. In that case, I think limiting a new noise to have an Leq within 3 to 5 dB of the L95 is too stringent, especially if the new sound is comparable in its character to the other existing sounds in the community. Now, on the other hand, communities in the Northeast US often use limits that say the Leq of the new source cannot be more than 10 dB above the existing L90. That could allow very unreasonably loud new noises if the community is actually a quiet one with the existing Leq close to the L90. Basically, my advice on industrial noise affecting the nearby community is to take a two step approach in evaluating your goals. 1. Evaluate appropriate controls based on the nature of the sound and the characteristics and expectations of the community regardless of the regulations. 2. Evaluate what you have to do to comply with the regulations. When you do this, you will often find that the regulations are not sufficient to prevent problems, but sometimes you can find regulations that are unduly restrictive. If the regulations are unduly restrictive in your situation, then you may have a decision to make regarding whether to take a chance on not meeting them. Some regulations require some type of certification of compliance to get permits for construction etc. In those cases, you must comply. Others are enforced only if there are complaints. That is where you could take some chance if you are confident you have taken reasonable steps to control the noise for the circumstances and fairly confident the neighbors will be satisfied. To answer you question about the effect of measurement period on the difference between Leq and L95, most typically the longer you measure the greater the difference will be until you have measured long enough to establish the long term difference. However, you cannot state that as a general rule. For instance if the first few seconds or minutes of your measurement include both a period of the quietest time that occurs greater than 5% of the period, and a period of the loudest sound that occurs, then the difference could decrease for increased measurement time. > Noral, > [quoted text clipped - 66 lines] >> > >> > Doug Doug, Where are you located? Your state/country should have an environmental body that sets the industrial noise policy, and this document will guide you when carrying out noise assessments. BACKGROUND NOISE To get a genuine L95 reading it will generally be necessary to measure noise levels (for example at 15 min intervals) over a period of a week with an unattended noise logger. If you were being thorough with the assessment, you would then match this data against meteorological data to exclude periods of high winds and/or heavy rain. How you then determine the background noise level from these measurements will vary according the noise policy, however it will involve something like sorting the L95(15min) values in ascending order and selecting the 10th percentile value. FACTORY NOISE To determine the LAeq from the factory, you should make attended 15 minute measurements, so that you can exclude extraneous nosie sources such as aeroplanes, lawn mowers and so on. The number of measurements you make should be based on your assessment of the factory opertations - i.e does the factory equipment operate consistently, or does it vary throughout the day? SEASONS It is true that the background level will vary with the season due to insects, and if possible you should avoid measuring noise during the periods (the insects would be classed as extraneous noise). I don't want to complicate things too much for you mate, but you should also be aware that in rural areas, meteorological phenomena such as temperature inversions (which occur in some areas during winter) will also affect noise levels. Kev Hello in Ireland <www.pateltonra.com> Define operating hours. It looks like the bare minimum will be 24 hours of measurement, if you're in a fully urban area. From what you say, and without knowing the area, I'd recommend nothing less than 24 hours. It can take years of measurement before you get the feel for the daily / weekly fluctuations. Averaging is a debate that can go on for years - logarithmic average, arithmetic average, absolute lowest LA95,15min or average of lowest LA95,15min values over a number of days, statistical trimming of each periods LA95,15min values or a combination. Talk to the authority who set the conditions, and get a copy of the policy those conditions are based on. Talk through what measurements they expect and how and where they expect them to be measured. If you get it wrong, you are playing with the the livelihood of that company so it can't be taken lightly. > I have just got myself involved in some noise measurement work and have > some received basic training. (I plan to do more training) However, in [quoted text clipped - 20 lines] > > Doug |
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