Free-Reed Organ

A cut along the reed, to scale: the brass tongue (riveted at the left) swings down through its slot in the plate and out the other side, one cycle stretched over two seconds. The wind presses from above; the air escapes through the gap round the tongue, whose open area (bar, right) is choked to the clearance while the tongue is inside the slot.

The flow through the gap against the tongue’s tip position (into the slot →). Dashed: what the reservoir’s pressure would push through each opening if the air had no mass. The live loop leans off it because the air feeding the reed must speed up and slow down: that lag is what drives the reed.

The sound of the last key over the last four seconds. With the musette on, its reeds are a few cents apart and the sound swells and fades at their frequency difference; ticks mark that predicted beat period.

The tongue chops the wind into pulses, so a free reed is rich in harmonics, with no gaps in the comb. The 16′ and 4′ ranks add an octave below and above; the musette's reeds sit a few cents either side of the 8′.

Each note grows out of the turbulence in about a tenth of a second, all its harmonics at once. With the musette on, its lines swell and fade at the beat; on a weak wind the treble is slow to speak, or does not speak at all.

The pressure every reed blows against, over the last eight seconds. Chords draw air and the pressure sags; the reservoir smooths the pumping, the expression stop does not. Below the red line the last key’s reed cannot start.