This subject occurs in an adjacent FS thread, and in bits and pieces throughout the history of this forum. I deal with phase lock loops (PLLs) in my work (although they are electrical, not mechanical), so I thought I’d have a go at trying to explain it without the use of mathematics which are simultaneously tedious, boring (I can put my wife in REM sleep in less than five minutes with this stuff 
), and very difficult to enter using the simple text format of this forum.
The basic physics of phase locking is that, like “the force”, mechanical systems with loose coupling (i.e. not bolted or welded together), influence one another. In this system, air is the major coupling medium, and the body of the hollow mainstock a minor contributor. Pendulum clocks, whether swinging at the same frequency, or in octaves (doubled frequencies) will phase lock when mounted to the same wall. It takes longer than for a set of drone reeds to lock, but the clock pendulums have a frequency of ~0.25Hz-2Hz, and the drones are in hundreds of Hz.
In its most basic form, when drone reed A closes, air pressure in the mainstock chamber increases. This increase in pressure propagates through the shared chamber, and when it reaches reed B, it changes the pressure differential between the chamber side of reed B and the “downstream” drone tubing side of reed B. This increase in pressure differential will add to the force trying to close reed B. If the tongue of reed B is already moving toward the closed position, the tongue’s rate of closure will increase, closing reed B earlier than a reed operating in isolation.
Conversely, if the tongue of reed B is moving toward the fully open position, the increase in pressure will slow the rate of opening. When the two reeds are beating in unison, they will tend to try to hold each other closed since with both reeds closed, the pressure is at maximum (encouraging the reeds to stay closed), and when both reeds are open the pressure is at its minimum, applying the least closure force on the reeds. Travel time for this pressure increase to fill the chamber, assuming a mainstock common chamber length of 4”(100mm) is ~0.3milliseconds (standard temperature and pressure), so we can call it simultaneous at these frequencies.
This sounds all well and good for systems where both contributors are trying to work at the same frequency, but what about where the frequencies vary by factors of 2, i.e. octaves? In mechanical systems, the big dog wins (mostly), and in this system the big dog is the bass drone (although if you shut off the bass drone, the baritone drone is quite capable of filling in). Each time the bass reed closes, it will raise the pressure in the chamber, thus either increasing the rate of closure of the baritone and tenor reeds, or reducing their rate of opening.
Since these reeds oscillating at (or near depending on how well they are tuned) two and four time the rate of the bass drone, the “nudging” (increase of closure rate and decrease of opening rate) of the bass drones changing of the pressure “encourages” the other reeds towards a common closed time. When the bass is closed, the resistance to opening of the other reeds is at a maximum, and when the bass reed is open, the resistance to the opening of the other reeds is at a minimum.
Note - If a reed’s behavior follows that of other mechanical devices, and you could observe the motion of the reeds’ tongues, you would find that the baritone and tenor reeds’ tongues do not open as much (we’re talking microscopic amounts) when the bass reed is closed.
Can reeds in a solid body mainstock phase lock?
In a solid body mainstock, the reeds are separated by a more complex coupling system. You have the air surrounding a reed, coupling to the body of the mainstock (which may be damped by the body of the player depending on their body shape), coupling to the air in the second drones reed chamber. This introduces concepts (such as adiabatic heating) which I’m not comfortable in discussing in any environment which doesn’t involve a pint sitting on the table in front of me (if you’re ever in town, ring me up and we’ll make our best guess over glasses of the dark stuff).
I know there are those out there who contribute valuable information, and can add their observations to this discussion. Jump in if you have something to add.
dave boling
), but rather a physical phenomena where two (or more) systems will lock their phase, rather than coexisting, out of phase, at the same (or certain integer multiples) frequency. It can influence a drone that is slightly (maybe a Hertz or less) to play in tune.
