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PostPosted: Fri Nov 18, 2016 9:06 am 
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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 :thumbsup: ), 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

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PostPosted: Fri Nov 18, 2016 10:12 am 
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Thanks for that Dave. I'm glad now for the three semesters of physics I had to take to get my B.Sc. degree!

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PostPosted: Fri Nov 18, 2016 10:53 am 
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I liked watching all those metrognomes, on the other thread, even if it was a bit spooky! I haven't got a hollow mainstock but if I did, would I be able to take my pipes out of their case and start playing in tune just by tuning-up the bass drone?


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PostPosted: Fri Nov 18, 2016 1:08 pm 
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Something to add...or not.

I asked a reputable maker once which he preferred - hollow or solid stocks. He said 'hollow'. I was a but surprised and asked if this was not heresy.

He explained a bit, mentioned the Taylors and M. Egan (as famous solid stock makers) and said he'd not found one to be much better than the other, playing-wise, but said he thought solid stocks were less difficult to make.

I have to wonder why what he said would not be true, since hollow or solid, they are ultimately open and being supplied by the same bag of air.

So do the tongues all really open and close together - phased? The tenor is 4x as fast as the bass? Where does my A drone tongue hang out?

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PostPosted: Sun Nov 20, 2016 8:38 pm 
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The scientific answer is "it depends". Oscillators can lock if they are on the same frequency ( or multiples of a frequency), or very close. Keep in mind that the "beat" frequencies you hear when two drones are out of tune are in small single digit hertz, while the fundamental tones are hundreds of hertz, i.e. the error between them are a few tenths of one percent. The frequency can "pull in" a little, but it don't expect miracles.

Driftwood wrote:
I liked watching all those metrognomes, on the other thread, even if it was a bit spooky! I haven't got a hollow mainstock but if I did, would I be able to take my pipes out of their case and start playing in tune just by tuning-up the bass drone?

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PostPosted: Mon Nov 21, 2016 7:08 am 
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Hmmm. If reeds phase-lock, how come my baritone reed takes a long time to "find" its tuning? I tune the drones, play, and Whammo!, the baritone drone goes flat. Repeat a few times to get it just so. Question is, why doesn't the baritone reed phase-lock the first time it is tuned? Reeds are elder in a hollow stock.


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PostPosted: Mon Nov 21, 2016 7:44 am 
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I think I'm mischaracterizing what phase locking will do. It will not keep a reed that is going away from failing. It is not a mystical (even with elder reeds :o ), 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.
I've inserted a graph of two sine waves (same frequency in this case), of difference amplitudes (essentially volume). Notice how the waves cross the X (time) axis at the same time, and from the same direction. If this is a result of two systems interacting, then it is likely due to phase locking rather than chance.
Image

dave boling

PCL wrote:
Hmmm. If reeds phase-lock, how come my baritone reed takes a long time to "find" its tuning? I tune the drones, play, and Whammo!, the baritone drone goes flat. Repeat a few times to get it just so. Question is, why doesn't the baritone reed phase-lock the first time it is tuned? Reeds are elder in a hollow stock.

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PostPosted: Mon Nov 21, 2016 10:42 pm 
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PCL wrote:
why doesn't the baritone reed phase-lock the first time it is tuned
?


Because there are simply too many variables involved in this art form which would invalidate any preposterous hypothetical, theoretical, or mathematical pipe dream the OP might suggest. The first of all is the variability of the bag arm. 'Twould preclude any other relevant observation.


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PostPosted: Tue Nov 22, 2016 2:25 am 
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PCL wrote:
Hmmm. If reeds phase-lock, how come my baritone reed takes a long time to "find" its tuning? I tune the drones, play, and Whammo!, the baritone drone goes flat. Repeat a few times to get it just so. Question is, why doesn't the baritone reed phase-lock the first time it is tuned? Reeds are elder in a hollow stock.



If one listens to each drone individually it is often possible to detect that whilst two of the drones will each rise in pitch slightly with increased bag pressure, the third drone may go down ( or vice versa).... Finding a set of drone reeds that all react in the same direction, or all remain perfectly stable, is the aim.


Good luck everyone , with this phase locking discussion... a topic I first raised some 35 years ago... interesting to see what ye all come up with :D


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PostPosted: Tue Nov 22, 2016 7:28 am 
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I always thought that the air space connecting the reeds would have to be in resonance for phase locking to occur. Without resonance how is momentary change in pressure behind any one reed felt by the other reeds?
Has anyone performed an experiment to test the hypothesis?

Sean


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PostPosted: Tue Nov 22, 2016 11:57 am 
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Thanks for the explanation Dave ,the idea of phase locking now seems plausible . I was always a bit skeptical having never experienced anything with drones that could be called phase locking. My feeling is that if phase locking does actually occur the "force" involved is so weak that it has no practical influence for your every day piper. I also think the term "locking" is misleading as this implies that there is a considerable force involved . Would phase synchronization be a better term ?

RORY

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PostPosted: Wed Nov 23, 2016 7:20 am 
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rorybbellows wrote:
I also think the term "locking" is misleading as this implies that there is a considerable force involved . Would phase synchronization be a better term ?

RORY


Rory,
I like the word synchronization more than locking, but synchronization describes the result, while locking is a bit of an overstatement. In a "phase locked" system, the components are phase synchronous, but because of the weak mutual influence (in this case the air is compressible and there is no rigid mechanical bond), a better term (which you'd likely never find in a scientific journal not having to do with pyramids or the Bermuda Triangle :D ) would be "nudged". If the two components are capable of producing the same (or integer multiple) frequency, and somewhat flexible in their tone center, they can become phase synchronous with sufficient coupling. Doesn't sound a bit like a politician "clarifying" their position, does it :P

dave boling

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PostPosted: Sat Nov 26, 2016 6:49 am 
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That leaves one question , what is the purpose of a hollow mainstock? Is it that the influence of phase nudging was over estimated by who ever it was who first made a hollow stock for Uilleann pipes or was it a purely weight saving measure ? or maybe some other reason ?

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PostPosted: Sat Nov 26, 2016 8:54 am 
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rorybbellows wrote:
That leaves one question , what is the purpose of a hollow mainstock? Is it that the influence of phase nudging was over estimated by who ever it was who first made a hollow stock for Uilleann pipes or was it a purely weight saving measure ? or maybe some other reason ?

RORY



More like,; " who was the first person to make a solid stock and why did they do that ?"

Hollow mainstocks as we know them today developed from the stocks used on the Pastoral, Hybrid Unions, Northumbrians and Musette de Cour and other bagpipes. These had an open chamber stock that worked fine when it contained either single or double reeds but not both. Combining the two types of reeds within the same chamber can be problematic, and cause tuning problems when the double reeds are used to play several notes,as in a regulator. The same problems can be seen with the Cabrette which if it has a single reed drone with an air feed from the chanter stock the pitch of the drone note can change when some chanter notes are played... This is due to changes in the standing wave behind the reed when the chanter note is changed which can alter the air pressure at the air feed hole for the drone or in the case of a single chamber 'common stock' changing pressure waves when the regulators are used can destabilise the single reed drones.

To cure these problems on the early Union Pipes the back of the hollow chamber stock was closed off and metal tubes added to seperate the single reeds from the doubles. On the Cabrette the drone pipe is usually either a dummy or played with a double reed. The double reed type of accompaniment pipe is called a 'Chanterelle' and has a more 'duet' voice than a drone.

The fact that single and double reeds don't coexist happily points to the influence that the pressure waves of one reed will have on its neighbours.... Phase locking influences ?

So, it is most likely that the Hollow Stock came first: Of the well known early full set makers only Michael Egan appears to have made solid stocks... though exceptions may exist but are more often atributed to later replacements.

Possible Phase Locking aside ,advantages other than weight can be; better drone stability, harmonic blending ( sounds like one drone with lots of harmonics), a sort of Steriophonic sound ( with the clicking of the drone reed tongues clearly audible through the stock wall) , weighted reed tongues don't hit the side of a long drilled hole but sit in a large chamber.


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PostPosted: Wed Nov 30, 2016 1:09 pm 
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Do free reeds do phase lock?


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