Watch this video together with your friends. Then let everyone express their opinion about increases or decreases the sound during all four tones. You will be surprised how the answers are different.
To understand this paradox, you need to know something about the musical notes. Each note has a certain height, which depends on high or low sound we hear. Note next higher octave sounds twice higher than the previous one. In addition, every octave can be divided into two equal Triton intervals.
In the video Triton shares each pair of sounds. In each pair, one sound is a mixture of the same notes of different octaves - for example, a combination of two notes to which one above the other sounds. When the sound goes from Triton one note to another, it can be rightly interpreted as a higher or lower note than the previous one.
Another paradoxical feature is the feeling that the sound is constantly getting lower, although the pitch remains the same. In this video you can see the effect for as long as ten minutes.