37. (A) When you are close to where the pebble fell, the waves are clearly in a circle, centered on where the pebble fell. (B) These are measured to ensure compliance with laws that govern acceptable sound emitted from a vehicle. (C) Engineers can use several analysis techniques to identify noise sources including near-field acoustic holography (NAH) and beamforming. (D) Beamforming presents several challenges including simultaneously measuring a large number of microphone sensors, transmitting and saving this large amount of acquired waveform data, and being able to quickly process large data sets.