A piano and a violin, when playing the same note (i.e., sounding the same pitch), are distinguishable because of the harmonic structure of the created sound. When a specific note is played, resonance occurs, producing a sound at a specific base or fundamental frequency. Additional frequencies that are positive integer multiples of that base frequency also are produced. These frequencies are called the harmonic series. For example, if a note is played with a base frequency of 200Hz, harmonics at 400Hz, 600Hz, etc. are also produced. Some instruments only generate the odd harmonics: from the base of 200Hz, 400Hz is skipped and 600Hz remains, etc. The trend of these harmonics is to decrease in power as distance from the base frequency increases. Other frequencies that are not positive integer multiples are also produced with, in general, much lower power, but with a similar decreasing trend. Hence, these differences in harmonic structure produced aural (heard) differences between instruments.
Pipe organs are unique wind instruments in that each sound is generated by its own pipe. This, by nature, creates variations in the harmonics of each pipe (and thus over the entire range) because of slight fabrication differences or purposeful voicing differences by the builder. This is in contrast with other wind instruments where the effective length of a single pipe is changed with valves (e.g., trumpet), holes that are covered (e.g., clarinet), or a slide (e.g., trombone). Thus, other wind instruments, which are based on a single piece of metal or wood, have a similar harmonic structure over their entire range.