Helmholtz and the Sirens 227
Using the double siren, Helmholtz could produce other varieties of “ intermittent ” or
“ beat tones, ” whose sum or difference lies below the frequencies of audible pitches, and
hence are not hearable as a combination tone but felt viscerally as “ a jar or rattle. ” Such
subsonic phenomena probe the differences between hearing and seeing:A jarring intermittent tone is for the nerves of hearing what a flickering light is to the nerves of
sight, and scratching to the nerves of touch. A much more intense and unpleasant excitement of the
organs is thus produced than would be occasioned by a continuous uniform tone. ...
When the separate luminous irritations follow one another very quickly, the impression produced
by each one lasts unweakened in the nerves till the next supervenes, and thus the pauses can no
longer be distinguished in sensation. In the eye, the number of separate irritations cannot exceed 24
in a single second without being completely fused into a single sensation. In this respect the eye is
far surpassed by the ear, which can distinguish as many as 132 intermissions in a second, and prob-
ably even that is not the extreme limit. ...Figure 14.9
Disc for an Oppelt siren, made by Rudolph Koenig (ca. 1865) (Collection of Historical Scientific Instruments,
Harvard University).