If a speaker's diaphragm is vibrating back and forth at a frequency of 900 Hz, then 900 condensations are generated every second. The amplitude of a sound is represented by the height of the wave. When there is a loud sound, the wave is high and the amplitude is large -- Where sound is twice as loud, the decibel level goes up by six -- humans speak normally at 60 decibels. Frequency and amplitude are independent of each other. Two sine ... the longer the wavelength, the lower the frequency.
A ninety degree shift would require a periodic relationship between two frequencies can be described as X/Y, where X and Y are integers. For example, an octave bears the relationship of 2/1; a perfect fifth is equivalent to 3/2; a major third can be tuned as 5/4. If the frequency of the lowest component sine wave, the fundamental, is designated as 1, the values of the other sine waves, the harmonics, will be {2, 3, 4, . . .}. For example, a note whose fundamental is 110 Hz (concert A), will have harmonics at frequencies of 220 Hz, 330 Hz, 440 Hz, etc.
The 90 degree shifted sound
can be heard here.