Diphthongs involve gliding pitches.
The pitches shown above are averages, which may vary as much as three quartertones in either direction. In the charts compiled by some experts in this field, bands of frequencies are shown, rather than single frequencies. It is not practicable to show those sounds which British speech has and American speech lacks, or vice versa. Examples would be found in such words as not, murmur, pitying, possibility, power, shore, piper, etc. Incidentally, these and other problems are hampering the development of automatic speech recognition machines and robot stenos.