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paulbrennan

Tuning by twanging.

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This is a naive question about tuning!  I'm looking at an old bandoneon that needs only spot tuning. Nobody local can really do it. I've tuned a couple of the outside reeds myself using the demo version of Dirk's software with pretty good results. For the inside reeds, I definitely need to remove the reed plate so it's more complicated. Presumably, I need a bellows, or even a table.  I don't really have a shop or space to make one, other than kitchen table, so I'd have to purchase this. So I'm wondering is it possible to tune reeds without a bellows by twanging them on a resonant box like a tuning fork? Of course this would not accurately reflect the pitch when installed but then apparently neither does the external bellows. So, maybe, the procedure would be the same - 1) Use Dirk's to calculate the tuning adjustment for each octave set. 2) remove the reed plate, twang and adjust by the same amount 3) replace the reed plate and retest.....  I have no idea if this is practical, hence the question. 

 

Cost factor - Dirks, 228 USD.  Bellows from eBay, about 100 USD plus shipping. Of the two, Dirks actually seems most useful to me b/c of the ability to calculate both 8ve reeds at the same time. 

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Thank you for confirming it won't work! 

 

I think I see the tuner you mean. Is it polyphonic though? I've got some pretty good monophonic tuning software. What I liked about the Dirks is the ability to detect pitch on both of the 8ve reeds without removing them or blocking one off.  Also, able to calculate the reference pitches for standards other than 440. I was impressed with the software, just a lot to spend on this particular instrument. 

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I wouldn’t dismiss twanging, you can get good at it and it is used in the profiling process when making a reed by hand. The issue is developing a consistent twang. If you have the reedplate off you could try blowing through them. In order to not rust the reeds afterwards you need to suck rather than blow so make sure they are clean first. 

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It is possible to very quickly mould a small box with a hole in the top to suck or blow through out of balsa wood, or a cut down match box with the sides and ends taped to size. (Use a shim to narrow the box if by chance the distances between the reeds changes with reed size (due to their pitch).

 

The balsa box fits over the removed reed plate with a single reed below, (or with the blow and draw reeds below, but sometimes the valves get in the way if you are not removing them to tune). Blow / suck through the hole and record the pitch, having first recorded it on the instrument. Then adjust the reed. Be sure to clean any moisture from the reeds after breathing on them.

 

Before I meddled with concertinas this is how I used to tune my harmonicas.

 

I have tried twanging reeds when I make them, there is a real art to it, which God did not give me, unfortunately.

 

david

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I'm just pressing the reed shoe onto a wooden surface of the next table or desk available and get satisfying results for the digital tuner. Of course there may occur a deviation, but it's common knowledge how to handle this (feel free to ask of course).

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Thank you all for these interesting opinions! It sounds like consistency might be an issue with twanging, which is interesting in itself, does anybody know technically why this would be the case?  I'm thinking of the violin played pizzicato on the high notes, it's a brief note but stable and presumably tunable (though of course a different pitch from the bowed note at the same position).   If you think of the "jaw harp" there is harmonic instability from the twanging but is there pitch instability too?  My background is not with reeds so these are new questions.  

 

 

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Twanging consistency is not a pitch issue, you just need to get a long and loud enough twang for your tuner to register. The mic from the tuner needs to be very close, close enough to interfere with your pluck; touching the frame on the tuner may help. Once you get the pluck movement right you get repeatable results. There are much easier methods of tuning! 

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That's a good point. Well I might try one of those clip on contact mics that come with guitar tuners, maybe it can clip right on the zinc reed frame. Or an a resonant box. 

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The free reed is exited by air flow, experienced tuners know that the pitch changes as the reed excites and then settles to it's playing note. They also know that you can vary the pitch considerably by increasing or reducing air flow on the rig. Sounding a reed by sucking is sort of OK if you can ensure the air flow is consistent and the reed is relativity small, but you need massive lung capacity for the larger reed, even on a treble.

If you twang the reed, you are probably starting the reed at a point of amplitude typical of when the reed is being seriously over pressurised, when it would be sounding a bit (a lot)  flat in the instrument. The reed's oscillation  will quickly decay to nothing. Some electronic tuners will register a note, but you dont know if it is at the flat end of the playing pressure spectrum, or not. 

 

Don't forget you are looking for a tuning precision of +/- 3 cents as barely acceptable, and preferably half that to be sure. I don't think that mic sensitivity, clip on or otherwise is the issue. It is the reeds's lack of time at a stabilised frequency of oscillation, when that oscillation is at a playing amplitude. This can only be achieved by airflow, however created.

 

Dave

 

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On 12/24/2019 at 5:39 PM, d.elliott said:

The free reed is exited by air flow, experienced tuners know that the pitch changes as the reed excites and then settles to it's playing note. They also know that you can vary the pitch considerably by increasing or reducing air flow on the rig. Sounding a reed by sucking is sort of OK if you can ensure the air flow is consistent and the reed is relativity small, but you need massive lung capacity for the larger reed, even on a treble.

If you twang the reed, you are probably starting the reed at a point of amplitude typical of when the reed is being seriously over pressurised, when it would be sounding a bit (a lot)  flat in the instrument. The reed's oscillation  will quickly decay to nothing. Some electronic tuners will register a note, but you dont know if it is at the flat end of the playing pressure spectrum, or not. 

 

Don't forget you are looking for a tuning precision of +/- 3 cents as barely acceptable, and preferably half that to be sure. I don't think that mic sensitivity, clip on or otherwise is the issue. It is the reeds's lack of time at a stabilised frequency of oscillation, when that oscillation is at a playing amplitude. This can only be achieved by airflow, however created.

 

Dave

 

Dave,

because tuning is relative outside the instrument it seems not to matter if you are tuning to the initiation pitch or the steady state pitch as long as you get the reading at the same point. With twanging and an electronic tuner you do only get a momentary reading,  but presumably it occurs fairly consistently at the same stage in the cycle as it is easy to reproduce the pitch by twanging again and also easy to alter it by tuning. 
 

I don’t want to sound as if I’m advocating people do tuning jobs by plucking. Just saying it can be done. And when making a reed by hand from scratch it is the only practical method of gauging the point in the process at which putting the reed into the frame for final tuning is desirable. When I do this I always expect a shift in the pitch because it is difficult to hold the reed while tuning it in exactly the same place as the clamp will hold it. The pitch will drop if the reed is clamped further from the tip and raise if it is closer and a few thou makes a lot of difference. 
 

I see the biggest issue with twang-tuning as getting enough amplitude for the microphone and with the mic I use I tend to make contact between the reed holder and the mic as it seems to work more consistently. I also give it a very big pluck. I wonder if a contact tuner might be better for a reed in a frame? I’ll try it when I get home. I’m a long way from home right now and much of my gear is packed and stored 50 kms away as a precaution against losing it in a bushfire while I’m here.  My house is surrounded by enormous fires at the moment. The one to the north is 15 kms away and bigger than Greater London, the one a similar distance to the south is only a third of that. Hoping for easterly and westerly winds only until the rains come in a few months time. 

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Hello Chris, I have been sorry to hear about about the tragic fires in Australia, we have the converse in terms of never ending rain, flooding in the lower laying areas of the county. At least you can pump water, and you can clear river ways, as has been done through our village. There is so little that can be done about bush and forest fires when they get hold, especially in your climate. 

 

I have tried trailing mics and a contact mic in the past, the best I could have done is use them to confirm a reeds approximate pitch, given tuning tolerance expectations it would not be enough to work on an instrument.

 

Dave

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On 12/16/2019 at 12:27 PM, paulbrennan said:

Thank you all for these interesting opinions! It sounds like consistency might be an issue with twanging, which is interesting in itself, does anybody know technically why this would be the case?  I'm thinking of the violin played pizzicato on the high notes, it's a brief note but stable and presumably tunable (though of course a different pitch from the bowed note at the same position).   If you think of the "jaw harp" there is harmonic instability from the twanging but is there pitch instability too?  My background is not with reeds so these are new questions.  

 

 

There are a couple details I didn't see mentioned yet in this interesting thread that might be of interest.  One is that our perception of pitch is very complicated.  It's influenced by the number and nature of the overtones we hear, becoming more difficult as the number and relative intensity of overtones increases.  It's also influenced by the duration of the tone, becoming more difficult as the duration shortens.  Also, loudness of the tone also affects our perception of pitch.  In some sense, it's a wonder we can make sense out of most sounds.  

In psychological experiments on tone perception with varying duration and loudness, researchers usually use a pure tone, a single sinusoidal wave, because that eliminates complications due to overtones.  So plucking a free reed with its attendant overtones is not such an easy job for the brain/ear system.

When you pluck a tongue, you cause "transient" sound, meaning that it's not a steady, periodic sound in the main.  However, the frequency of vibration can be much quicker than the rate at which the overall sound (its amplitude and overtone structure) changes, and thus, you can often pick up enough cycles of tongue vibration to sense a pitch.  But not always.  

As some have pointed out here, the musical tone of a free reed is an air driven phenomenon and most musical notes have relatively long enough duration so that the vibration can be called "steady," or periodic.  But the physics of steady vibration is much different from that of plucking.  With plucking, if it has long enough duration, the vibration frequency is called the natural frequency of vibration, whereas with air driving, there are enough physical phenomenon at play to produce a calculable - and often perceptual - difference.  

The physical phenomenon I refer to involve so-called viscous and aerodynamic forces and are due to the air motion instigated by the vibration.  These forces are also at play during plucking, but not precisely in the same way.  In other words, if you would pluck a tongue by some mechanical means while the reed is in an evacuated bell jar, those air effects would not be present and there, the tongue vibrates truly at its natural frequency, a much simpler arrangement.  It would be very interesting to do an experiment measuring the true natural vibration frequency of a reed in a vacuum chamber and comparing it both with the frequencies obtained by plucking and by steady air flow.  The three are different.  You can very accurately measure the true natural frequency by exciting the vibration in an evacuated bell jar by say a periodically changing magnetic field, acting on a steel tongue.  

With my own calculations, based on a fluid mechanics study of free reed physics, I find that there's a discontinuity in the frequency of vibration as the tip vibration amplitude becomes large enough to cause it to emerge below the slot (vent).  While vibrating within the slot, the vibrational frequency is extremely close to the true natural frequency - even with all that air motion -, but at the transition, the frequency suddenly drops a small amount, perhaps too little for the ear to detect.  Then, as bellows pressure is increased, the frequency slightly rises.  

From practice, I believe that the vibrational frequency with steady air flow drops when bellows pressure increases to a certain level.  I'm not talking about any effect cause by a continual receding of the leather valve as air flow increases.  I'm currently trying to understand all this from a theoretical point of view.  

 

Regards,

Tom

www.bluesbox.biz

 

PS.  Chris, I'm disheartened to learn  of your difficulties because of the Australian fires, a world class disaster.  I fear things will get worse most everywhere because of Global Warming, and we as a world community will slowly adjust to it, but not without much death and destruction, with some people more unlucky than others.  Here in Eastern USA, our weather last Summer was unusual in that we had over all much less sunny days.  A lot of overcast and cooler weather, but I don't think much more rain.  So far this Winter, we haven't had much cold nor precipitation.  Very  mild and pleasant.  

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