The UK has introduced strict noise limits which, by 1996, when the limits were last reduced, had halved perceived noise levels of individual vehicles over the previous 15 years.
At low speeds, similar to the speed used for vehicle noise testing, the noise from the engine, gearbox and exhaust will generally predominate over the noise associated with the tyre/road surface. On dry roads and at a constant speed engine noise generally predominates for speeds up to 50km/hr (30 mph). Above this speed tyre noise becomes the dominant source of noise.
The current noise test for passenger cars, as set out in EU Directive 92/97 as amended, consists of driving the vehicle into the test area at a speed of 50 km/hr and then accelerating at full throttle through it past a microphone. The microphone is placed at a set distance from the line of travel and it measures the maximum level of noise reached which is then compared to the limit value to determine whether the vehicle passes or fails.
The test area is surrounded by an open area to avoid sound reflections and the road surface is carefully constructed to a set standard to ensure consistency of results.
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