Noise Test AnalysisWe have carried out a survey of 5 years worth of HMAC noise test records. 154 tests have been carried out for 42 members and guests. The results discussed below show that small models tend to be quieter than large models, although it must be acknowledged that the smaller engines emit their noise at a much higher pitch and can be perceived to be more irritating than the deeper note of a (louder) large motor. Unfortunately this fact cannot be quantified.
The majority of tests have been for medium size models, with engines in the range 0.25 to 0.61 cu.in. There seems little to choose between two-stroke and four-stroke motors. The engine manufacturers seem to have paid more attention to the two-stroke silencers and their "quiet" silencers, usually sold as an optional extra seem particularly effective. Our main recommendation for reducing the noise of a medium size model is to use a largish propeller to keep the revs down below 10,000 rpm and to choose a scimitar blade profile; propellers by APC and Graupner are popular and very effective choices.
We copied all the noise test sheets into a spreadsheet program for analysis purposes. There's quite a lot of data to be manipulated and it's not obvious how to show trends. Please contact the HMAC Webmaster if you have any suggestions or questions whose answers could be buried in the data.
THE NOISE TEST DATA
Our data was extracted from the noise test records for the five calendar years from 1998 to 2002 inclusive. The annual breakdown of test numbers is 1998:56, 1999:33, 2000:31, 2001:9, 2002:25. It is interesting to note that 16 members required just one test over the five year period and one busy member had 17 tests carried out. The noise test team totalled more than a dozen individuals and the most industrious three of these carried more than 50 tests each! There was a tie for busiest tester; two people carried out 61 tests each.
Turning to the test data itself, we have grouped the test data into three engine sizes, small for engines less than 0.40 cu.in, medium for engines between 0.40 and 0.61 cu.in, and large for engines greater than 0.61 cu.in.
The graph on the right shows the how often a given maximum dB value (usually the head-on value) occurs. Each graph has been normalised by its peak number so that they all peak at 100%. The actual maximum numbers in each group were 7(small), 18(medium), and 10(large). If you look closely at the graph on the right you can see that several small engined models recorded a maximum noise level right down to 70db. More medium sized models had their maximum noise levels in the mid to high seventies than did the large models. Often the maximum noise levels of large models was for a side-on position, probably due to carburetter and/or propeller noise.
The graph on the left shows the maximum recorded noise value plotted against the engine size in cubic centimetres. The data is plotted logarithmically. A "61" size engine is 10cc. The graph shows again that small engines measure quieter than large engines, but it is worth emphasising that a high pitched buzz is more annoying than a deep roar. The data also shows that models can be made quieter by careful attention to good muffler design and to propeller size & shape.
The most popular propeller manufacturer was Graupner (47), followed by APC (31), then Master (8). Smart propellers were often used in large models. Unfortunately many noise test entries failed to record the make of propeller.