In order to produce a note, the succession of shocks must not deviate
much from isochronism.
If the isochronism is but little impaired, we obtain a note
corresponding to the mean interval of the shocks.
If the intervals between the shocks are alternately t and t', and if the
difference between t and t' is slight, we obtain the two notes t+t' and
(t+t')/2. If the intervals between the shocks are alternately t, t', and
t'', we obtain the two notes t+t'+t'' and (t+t'+t")/3.