To ascertain how this 86dBA is measured you need to follow ISO 362 (practically identical to BS 3425).
There are conditions over 6 pages with all the details but these are the guts of it
-measurements shall be made in a level open area with no reflecting items (buildings, boulders, fences etc) within 50m.
-There shall be a minimum of 20metres square of ashphalt or similar road surface (that has to comply with ISO 10844... I did say it wouldn't be simple!) with a further minimum 10m stretch at either end (at least 3m wide) prior to entering the 20x20 area.
-The microphone is 7.5m away from the centreline in the middle of the 20x20 area
-4 measurements are made per side .. ie vehicle does at least 8 passes
-vehicle must be warmed up first
-As the bike approaches the 20x20m area it must be doing 50km/hr in 2nd and/or 3rd gear (if 5 forward gears or more are fitted). The bike then accelerates hard across the 20m and then the throttle is closed as the rear of the bike passes the 20m line
-if the engine RPM's exceed the maximum net power point, the test is done only in 3rd gear, otherwise both 2nd and 3rd are used
-minimum of 4 consecutive measurements must be done and be within 2 dB
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