the stronger sound completely covers the stimulus of the weaker sound the reversal of pitch modulation direction descending frequencies rather than ascending coupled with the masking effect dramatically reduces psychologically perceived level of urgency for observers in the rear hemisphere of the ev informing them that no response is required of them this at least statistically spares half of the proximate population from the stress of deciding on what means of reaction to employ to effectively preserve personal safety applying the assumption that the motion of the ev is essentially forward along its longitudinal axis the observer can then easily ascertain whether the ev is approaching him or moving away from him this is accomplished by using two sound sources each capable of emitting separate and distinctly different sounds in a spatially directional manner fig3 shows one possible arrangement of sound sources on an ev and a representation of two different sound spatial patterns one directed toward the front of the vehicle and one directed toward the rear the sound patterns depicted in fig3 are not meant to imply that no sound is heard outside the shaded areas but instead represent a locus of constant loudness with less intense sound fields lying outside the shaded areas typically the forward and rearward directed sounds will have equal intensities along a direction perpendicular to the longitudinal axis of the ev the observer learns.