Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks. While noise and vibration can be readily measured, harshness is a subjective quality, and is measured either via "jury" evaluations, or with analytical tools that provide results reflecting human subjective impressions. These latter tools belong to the field known as "psychoacoustics". Interior NVH deals with noise and vibration experienced by the occupants of the cabin, while exterior NVH is largely concerned with the noise radiated by the vehicle, and includes drive-by noise testing. NVH is mostly engineering, but often objective measurements fail to predict or correlate well with the subjective impression on human observers. This is partly because the human body has its own frequency response, e. g. the ear's response at moderate noise levels is approximated by A-weighting, but this does not mean that two noises with the same A-weighted level are equally disturbing. The field of psychoacoustics is partly concerned with this correlation. In some cases the NVH engineer is asked to change the sound quality, i. e. adding or subtracting particular harmonics, rather than making the car quieter