A sensation of pitch (psychophysics) often occurs in free nature when the sound of a sound source reaches the ear of an observer directly and also after being reflected against a sound-reflecting surface. This phenomenon is called Repetition Pitch (RP), because the addition of a true repetition of the original sound to itself is the basic prerequisite. The perceived RP corresponds to the reciprocal value of the time delay between the original and the repeated (reflected, delayed) sound. RP is most salient when the original sound is wide band and does not produce pitch itself (like white noise). Probably the first written report of the phenomenon dates from Christian Huygens, who observed such a pitch in the (wide band) sound from a fountain reflected against the steps of a large stone staircase in the garden of the castle of Chantilly in France (1693). In free field, one might be able to observe a gliding RP when a plane flies over. In music, the phenomenon is sometimes deliberately created by electronic means (delay and add) to superimpose a pitch or coloration effect to the original music (see Flanging). In room acoustics and sound recording, the phenomenon often causes an unwanted coloration of the original sound. Blind peoples might use RP to locate obstacles by clicking the street surface with their cane, thus producing a wide-band impulsive sound that is reflected against the obstacle. RP has been subject of various studies. See more for further details, audio demos, and references.