Below standing text comes from a page which has been added afterwards in the crank organ plans of Walter Höffle.
Construction and working.
These pipes are almost the same construction as the principals. Because they are smaller, their sound has more overtones and violin like. This register belongs to the strings group. Because of their tall shape the danger exists that these pipes will sound one octave higher. The pipe will be made over length with a tuning slide at the end. Also because of this over length the pipe will sound with more overtones. These pipes are normally not as loud as, for instance, the stopped pipes.
To prevent the effect of playing one octave higher, Gavioli invented the frein (beard)/ It also influences the pitch of the pipe. The construction of the pipe is the same as an open pipe. The angle of the labium and the core are both 9 degrees. Both surfaces should inline. For the air regulation, a screw must be fitted in the foot (brass pipe).
The so called Gavioli beard will be made of 0,5 - 1 mm thick brass plate. Of course you can use aluminum plate as an alternative. To make beards for every pipe the best way is to make a hardwood former to bend the plates. In the length of the plate, two holes should be drilled. The lowest hole is for fastening the frein on the pipe. By bowing the plate a bit away from the pipe, the second hole is to adjust the frein towards the pipe mouth to obtain the best adjustment.
Make the pipe first without frein and voice it one octave higher. This means that there is too much air inside the pipe. With the frein the air will be pulled out of the pipe. By changing the distance between the brass plate and the body of the pipe, the original base tone will be found and the pipe will play one octave lower. By the shape of the frein the air will be suck out.
For making the Gavioli beard the next addition: I have made a different model. It is easy to make it with two slanting cuts. (see page 61).
The original shape is like the letter T. To make this, it is more labour intensive and you should have special tools for punching out the shape.
The upper slide can be made from brass or aluminum. Depending of the chosen thickness you should router the corresponding slit. I do it with an upper router. Try it out first on a piece of wood if you have the right thickness before you router a readymade pipe.