The harmonic series is an arithmetic progression (1×, 2×, 3×, 4×, 5×, ...). In terms of frequency (measured in cycles per second, or hertz (Hz) where is the fundamental frequency), the difference between consecutive harmonics is therefore constant and equal to the fundamental. But because human ears respond to sound nonlinearly, higher harmonics are perceived as "closer together" than lower ones. On the other hand, the octave series is a geometric progression (2×, 4×, 8×, 16×, ...), and people perceive these distances as "the same" in the sense of musical interval. In terms of what one hears, each octave in the harmonic series is divided into increasingly "smaller" and more numerous intervals.