How is exterior pass-by noise testing performed?

Pass-by noise testing measures maximum sound level as a vehicle passes microphones at 7.5m distance on an ISO-specified surface. Test procedures (UN R51, SAE J2805) define approach speeds, acceleration conditions, and gear selection. Modern regulations use ASEP (Additional Sound Emission Provisions) addressing sounds not captured in standard test. Testing requires special low-noise surfaces, minimal background noise, and specific weather conditions. Results must meet regulatory limits and influence exterior sound design balancing noise reduction with brand sound character.