The most significant register in Belgian dance hall organs is without a doubt the jazz-flute. Although the register was not invented till 1925 it was applied frequently after that. Not only did it receive a prominent place in newly built organs, but it was also built in older models as a modernisation. For the Belgian music markers it turned out to be an easy to handle register on which the melody could easily be brought forward.
Before 1925 the music arrangers had a choice from several solo voices, like the harmonic flute, piston, baxophone and the (wooden) xylophone, with or without the tremulant. The strings like violin and Unda Maris were usually used in chords and were, of course, very suitable for the plenum. In old arrangements we can still hear violin solo's - with or without tremulant- but this probably was found too dull in the long term. Besides, the melody was frequently arranged for counter melody voices - for instance combinations of cello, cello grave, bariton or flute 8' (sounding as a flute 4'!) and a bourdon céleste, which was frequently disposed on the countermelody of Belgian organs. The melody was then lavishly decorated with trills and broken chords by the carillon and harmonic flute on the melody section. Old arrangements by Abel Frans, Louis Thijs and Jan Gilissen are a good example of this style.
Before WW I the "Vox Humana" register was very popular. We know this register of Limonaire and Gasparini street organs, but pictures of old Mortier organs show that this register was a must in the first decade of the 20th century. Gavioli used the register too. Carl Frei used to call it "female voice" in his Antwerp time. The reed register, with its constant vibrato, remotely resembled a human voice. A singer using his or her natural vibrato is more clearly heard before an orchestra. In dance hall organs the vox humana was disposed in the counter melody section. Such registers can still be found on the "Blauwe Mortier" in the Arnhem open air museum, the Hooghuys organ in the Utrecht museum, and in the van Steenput street organ of Adrie Vergeer.
When the vox humana register had become obsolete in the twenties of the 20th century, Mr. Guillaume Bax -working with Mortier- sought a new "human" sound for the organs. He devised a construction in which not the tremulating organ wind causes the vibrato, but the effect is obtained by little tremolating valves, opening a hole at the back of the pipi, opposite the upper lip (view picture). By opening this hole the tone grows weaker, but also higher.
Mortier introduced this register in 1925, right in the middle of the jazz era; hence the name "Jazz flute". It soon became the most significant register in Belgian organs. In arranging the melody for a jazz flute the markers added steep glissando's at the start of a note. They also made slidings between notes. Presumably the idea was based on the popularity of the "Swanee whistle", an instrument patented in 1924, in which the pitch was regulated by pushing a movable stopper in and out. The instrument was used in some Popper orchestrions. A similar sound was made by the musical saw, and the Lotos flute in orchestrions of German origin. That register was made of a row of stoppered pipes with a normal tremulant. A lady-crooner like Joséphine Baker, singing Dinah on a recording of 1926, almost seems to imitate a swanee-whistle.
Other Belgian makers used similar registers, sometimes called "bourdon-jazz", as a stopped rank without the characteristic Mortier chimneys. Even street organs built for export to the Netherlands often had a jazz-flute. One of these organs even became known as "de Zaag" (the Saw"). Without use of the tremulant the register gave a fluty sound, which often was used for short decorative passages, mostly in combination with the xylophone.
Bax did a new invention around 1930: the Vibratone, a register with the same tremulating valves as the jazz-flute. Near the top of the stopped pipes a hole was made, covered with a membrane that was originally made of mica. The register grew popular in all dance hall organs of the thirties. At first the register was disposed on the melody section, but it soon appeared in the counter melody as well. The vibratone sounded an octave lower than the jazz-flute and made a fine combination. The register seems to have been designed to imitate the mellow sound of saxophone sections in dance orchestra's of the thirties.
Because of their success both the jazz-flute and vibratone were built into almost every dance hall organ; even in the older ones built before 1925, unfortunately at the cost of many original carillon or harmonic flute registers! As fas as it goes the jazz-flute and vibraton did the same with Belgian organs as the bourdon céleste in Dutch street organs.
After WW II the sound of unison jazz flute with vibraton was thought too weak and it became a habit to work in octaves with these registers. The music marker Peersman went so far as to make jumps in the melody in order to be able to work in octaves. The resulting sound was dominated by a howling sound. Later arrangers like Albert Decap and Dick Gillet shortened the lowest notes of the octaves to a short accent.
The glissandi at the start of notes remained in fashion. Eugène Peersman and Urbain van Wichelen made steep "stairs" of up till a full octave. At first all chromatic notes were used, but this sometimes led to serious weakening of the cardboard, so some tricks were devised to keep some connection in the books (view picture).
It is a matter of taste of one likes the sound of a jazz-flute; for dance hall organ enthusiasts it is a must, others cannot listen to them for a minute. Fact is that the sound remains very typical for Belgian organs.