 But these cases are not the norm. Perceptual media are in the norm cognitively transparent: they are imperceptible insofar as they transmit without significant alteration information about some relevant properties of the thing perceived through them. Media become perceivable when this transmitting function is impaired by some event or disturbance occurring in them. Auditorily, this occurs in the case of the Doppler effect. The vibration of the air carries information both about the sounding object and about the effect its speed has on the medium.