| NOISE TESTING |
| Why measure noise ? This question is important because we are not just trying to reduce noise emissions, we are trying to reduce noise complaints. The two aims are not synonymous. Measuring noise emission on the ground is a somewhat artificial way of assessing the nuisance value of an aircraft, but for most aircraft / engine combinations on a club field, it correlates reasonably well with nuisance value. The important thing to remember is that the noise meter is a tool. Reducing the decibel reading on the meter is not the end goal of noise reduction, but is a useful step along the way. Why not just listen to the noise produced in the air ? I would argue that this should also be done - if a model sounds offensive to other pilots, think how much more offensive it will sound to a potential complainant. However, the noise meter is useful in deciding whether various interventions (e.g. props, silencers) are effective in reducing noise emissions. It also provides a degree of objectivity. Equally compelling reasons for measuring noise are that the Department of the Environment guidelines are given in measured dBA values, and that planning permission and appeals are subject to quantitative noise values. This discussion does not embark on an explanation of decibels or the mechanics of sound transmission; the subject is well covered elsewhere (see bibliography). Such reading is not essential to realise the benefits described herein. How to measure noise. The procedure for measuring noise is outlined in the BMFA handbook (for UK readers). In essence, it entails measuring noise at maximum RPM at a 7 metre distance, with the model held at 1 metre above the ground (away from the body) Measurements should be taken with the propeller facing towards, away, and at both 90 degree points relative to the noise meter. Measurements on a club field are useful and consistent if a few rules are followed: i) The meter should be regularly calibrated against an external standard, preferably before each flying session ii) The temperature of the meter should be kept reasonably constant, i.e. it should not be left out in the baking sun all day. iii) The RPM of the engine should be adjusted and measured at the time of the test to ensure that the maximum RPM is being achieved. iv) Measurements should take place in winds below 5 knots if possible. Measurements in winds of above 10 knots will tend to produce false increases in noise, as wind blowing onto the back of the propeller causes the prop to flex and resonate, increasing the emitted noise. This does not occur in the air, because a head wind will always exist and it is thus not representative of in-flight noise emission. v) One person should be responsible for taking noise measurements. This helps to reduce inter-observer bias vi) All noise measurements should be recorded, not just the maximum decibel level. in addition, some basic data should be recorded (see sample noise log) With this information, a responsible impression is created and comparisons are easily made, e.g. when different props or silencers are used. A further point is that the maximum of the four values recorded is the one that determines whether the aircraft passes the noise test. it is completely incorrect to try and average the four figures; this yields no useful information at all. |
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| DoE noise code BMFA advice on noise testing |
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