Douglas Bush wrote in MMD 981004 (glad that you are on bytes again!)

> Is anyone able to explain what the difference is between the Unda
> Maris and Vox Celeste registers found on Belgian dance organs?
> I understand that they are both string pipes and are tuned celeste
> fashion.

Both these voices are indeed composed from two ranks of string-toned
pipes; one of the ranks is tuned a few Hertz higher to get an
undulating effect. The desirable amount of detuning is a constant
subject of debate between organ tuners and listeners! The main
difference between these registers is the register on which they are
playing:

    The Vox Celeste ("voice of Heaven")is used in Belgian organs as a
    4' countermelody voice. It is acting as a counterpart of the violin
    ranks on melody. Voix Celeste pipes are very narrow, and the effect
    is a little bit like an accordion.

    The Unda Maris ("sea-wave") is used as a 16' melody voice. It is
    used to build up tone and volume in the melody section and the
    slight detuning (less than in the celeste voice) is necessary to
    add just a tiny bit of liveliness to the otherwise rather static
    violin voices.

Mr. Carl Frei of Breda used these voices in a completely different way:
by switching their position (Voice celeste on melody and Unda Maris on
counter melody) they both became 8' registers. Frei voiced these pipes
in another way to get his special street organ effects, and a greater
volume. Frei's pipes also had wider measurements than their Belgian
counterparts. So, what's in a name?

with greetings from Holland,

Hans van Oost, KDV

 [ For consistency I changed the spelling of "Voix" to Vox in
 [ this message.  I hope I did not alter the meaning by doing this.
 [ Jody