Effect of pressure bar on string vibrations and tone?

Posted by: Withindale

Effect of pressure bar on string vibrations and tone? - 02/05/12 05:53 AM

I had been wondering about the effects of the "non-speaking lengths" on tone before I revived the D7 note mentioned in a recent thread about a possibly faulty hammer.

If running the plastic tip of a biro up the speaking length to the v-bar could affect the tone, what about the string on the other side of the v-bar? Would tweaking the strings at the pressure bar have an effect?

Well, you guessed it, it did. There was more of a singing quality and the sustain was longer, more pronounced. Just to make sure it was not just a figment of my imagination, I repeated the test on the other 58 notes along the pressure bar - by gently moving the strings above the bar sideways. The bells in "Oranges and Lemons", the nursery rhyme on London churches, bloomed and rang out.

Del has said (post #909400, 10/25/05 12:42 PM) that effective termination in vertical pianos depends on the pressure bar as well as the v-bar. Based on what Giordano and Korty wrote about their experiments on string and bridge movements, I guess the longitudinal vibrations in the string were being damped in some way, diminishing the power of the higher partials and overtones.

What should I do now? The piano is 86 years old and, maybe, the strings behind the pressure bar had never been disturbed before. The bar has a half moon profile and the curved surface feels slightly rough.

Should I release the pressure and clean up the bar, spray it with a cleaner or lubricant (which ones?), or leave well alone and carry on tweaking as necessary?
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/06/12 07:50 AM

Continue to "pitch woo" with this piano. You will feel your love returned to you.

That is what I really think is going on. I do the same thing with some of my possessions. smile
Posted by: rXd

Re: Effect of pressure bar on string vibrations and tone? - 02/06/12 08:16 AM

Although it is possible to flex the strings on a solidly tuned piano and not disturb your recent tuning, to move them around under the pressure bar will at least disturb unisons somewhat. It is difficult to ascertain what is causing what when so much changes. A slight change in the unison will create the appearance of more sustain and not necessarily address the main cause.

I would be inclined to leave cleaning abd dressing the upper termination points until restringing time. You can be certain that removing the pressure bar will totally destroy the tuning, pitch and stability, even if you manage to put it back at the same tensions, extra work is to be expected at the next tuning.

Unless it is actually buzzing or your tuner reports poor tuning qualities (I have never come across an Ibach where this is a problem) there is nothing to be gained from removing the pressure bar. Seating both sides of all the termination points before tuning will accomplish the same effect.

Maximum transference of power comes from adjusting the resistance (flexibility) of the hammer to the resistance (flexibility) of the string(s). Somebody please correct me if this is off base but it's the way I find helpful to look at it. this balance of resistance is very important for adjusting sustain qualities and, unless the string is physically impeded by excessive dust and debris in the bridge notch or something similar, the hammer and action are the adjustable factors. Usually a single long needle parallel to the moulding will be enough if the hammer shape and everything else is good.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/06/12 02:46 PM

Originally Posted By: UnrightTooner
Continue to "pitch woo" with this piano. You will feel your love returned to you. That is what I really think is going on. I do the same thing with some of my possessions. smile

For sure, Jeff, but don't let The Bells of St Clement's and my piano mislead you. The main point of the OP is the effect the non-speaking length between the tuning pin and the v-bar can have on string vibrations and tone, as implied in the title.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/06/12 08:26 PM

Thank you, rxd, for your advice. I imagine everyone will agree with you about leaving the pressure bar well alone.

Seating the strings at both sides of the pressure bar before tuning makes absolute sense but I have not seen anyone say that before, other than BDB who mentioned taking off some tension to loosen any rust and even out tension when tuning.

There is no doubt that this procedure produced a really significant change in the Schiedmayer. I am sure a before and after frequency power spectrum analysis would prove the point.

The interesting to me question is why this can happen, though I think you cover that, rxd, by the string being "physically impeded". In this case, presumably, that means the string cannot move easily at the pressure bar in response to vibrations. As I mentioned in the OP I guess the power in the longitudinal vibrations dissipates at the bar.

In consequence, if Giordano and Korty are correct in what they wrote in 1996, and if I understand it correctly, the upper partials will suffer. Their measurements showed, as may be well known, that the power of these partials in the transverse waves of the string is much less, by several orders of magnitude, than that in the soundboard and the sound. In fact they say the piano string itself sounds like a guitar string.

They also suggest that the power in the longitudinal wave creates the equivalent of transverse vibration at the bridge due to small bulges and depressions in the surface of the string created by the movement of the longitudinal wave.

So I guess that anything that interferes with the the movement and reflections of the longitudinal wave between the tuning pin and hitch pin, other than the bridge itself, will affect its power and therefore the tone. Sometimes critically perhaps.

My tuppence worth.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/07/12 07:36 AM

Originally Posted By: Withindale
Originally Posted By: UnrightTooner
Continue to "pitch woo" with this piano. You will feel your love returned to you. That is what I really think is going on. I do the same thing with some of my possessions. smile

For sure, Jeff, but don't let The Bells of St Clement's and my piano mislead you. The main point of the OP is the effect the non-speaking length between the tuning pin and the v-bar can have on string vibrations and tone, as implied in the title.


You are talking about the "speaking" of the "non-speaking length" (ahem...)

Maybe you could have someone ELSE randomly dampen and undampen these non-speaking lengths, without you knowing which ones, and then YOU play the notes and see if you can tell the difference.


I just looked up the synopsis of the paper you are referring to:

"Motion of a piano string: Longitudinal vibrations and the role of the bridge

N. Giordano and A. J. Korty Department of Physics

Using strain gauges as a tool, new results have been obtained for the amplitude of the vibrations of the bridge and soundboard of a piano. These results should prove valuable for the construction of theoretical models of the role of the soundboard in sound production. In addition, they indicate that longitudinal string vibrations, i.e., the stretching and compressing of the string, make a very important contribution to the initial sound signal. The amplitude of this longitudinal motion is a nonlinear function of the transverse vibration amplitude. The mechanism by which the longitudinal vibrations couple to the bridge motion has also been considered. © 1996 Acoustical Society of America."



This seems to be talking about the "non-speaking" lengths at the other end.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/07/12 08:45 AM

Originally Posted By: UnrightTooner
Maybe you could have someone ELSE randomly dampen and undampen these non-speaking lengths, without you knowing which ones, and then YOU play the notes and see if you can tell the difference.


How can anyone aurally tune or voice a piano if everything they hear is a figment of their imagination?

... and now a lady has just walked in out of the blue to give me a massage ... more later!
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/07/12 10:16 AM

Originally Posted By: Withindale
Originally Posted By: UnrightTooner
Maybe you could have someone ELSE randomly dampen and undampen these non-speaking lengths, without you knowing which ones, and then YOU play the notes and see if you can tell the difference.


How can anyone aurally tune or voice a piano if everything they hear is a figment their imagination?

... and now a lady has just walked in out of the blue to give me a massage ... more later!


If a tree falls in the forest and there is no one around to hear it, does it make a noise? See, we cannot get away from perception. (I am fascinated by the discovery of "dark matter.")

Doing what I suggested would determine if this IS a figment of your imagination.

But as far as tuning, that is what checks are all about. You can have progressive 3rds and not have an equal temperament. But if you have progressive 3rds and 6ths then it surely is ET.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/07/12 05:19 PM

Originally Posted By: UnrightTooner
You are talking about the "speaking" of the "non-speaking length" (ahem...)

The "speaking" of the "speaking length", actually, in response to the question "Interesting that X should have an effect, seeing that X is not even part of the speaking length?".

For X read non-speaking length, tuning pin, pressure bar, hitch pin, or whatever.

The non-speaking lengths can certainly have an effect. One post I read said re-seating the string at the hitch pin brought a dead note back to life.

Originally Posted By: UnrightTooner
Maybe you could have someone ELSE randomly dampen and undampen these non-speaking lengths, without you knowing which ones, and then YOU play the notes and see if you can tell the difference.

Yes, if damping were the issue, but the strings were re-seated at the pressure bar and took on a new lease of life. I hope the change is not reversible.

To illustrate the difference, let's bring back Jurgen's roomful of experts from the other thread for a before and after experiment. At the end, the presenter talks about bloom and sparkle and says he replaced the 80 year old strings. Ok we can hear that, the experts retort, but what about those unisons?

It would be interesting to know to what extent re-seating strings at the pressure bar is general practice. I can see it might usually be unnecessary.

Originally Posted By: UnrightTooner
I just looked up the synopsis of the paper you are referring to:

"Motion of a piano string: Longitudinal vibrations and the role of the bridge. N. Giordano and A. J. Korty. Department of Physics, Purdue University.

Using strain gauges as a tool, new results have been obtained for the amplitude of the vibrations of the bridge and soundboard of a piano. These results should prove valuable for the construction of theoretical models of the role of the soundboard in sound production. In addition, they indicate that longitudinal string vibrations, i.e., the stretching and compressing of the string, make a very important contribution to the initial sound signal. The amplitude of this longitudinal motion is a nonlinear function of the transverse vibration amplitude. The mechanism by which the longitudinal vibrations couple to the bridge motion has also been considered. © 1996 Acoustical Society of America."


This seems to be talking about the "non-speaking" lengths at the other end.

Giordano and Korty measured the transverse and longitudinal vibrations at the bridge because they were interested in how energy was transferred to the soundboard. As we are talking about wave motion, it's reasonable to assume measurements of the vibrations at the other end of the string would have been equivalent.

This brings us to Del's comments about the termination points:

The duplex string segment sings anytime its natural fundamental vibrating mode ends up close to the fundamental, or some partial of that fundamental, and the string deflection angle is small enough to constitute an inefficient termination to the speaking length.

To repeat energy transfer across the string termination is a function of the shape of the termination point (i.e., usually the V-bar), the string deflection angle, and the length of the duplex string segment.

With one or two notable exceptions, upright pianos rarely, if ever, have string noise problems through the tenor and treble sections even though their string deflection angles across the V-bar are often quite low. And despite the fact that vertical piano V-bars are often very poorly shaped. The reason they do not is because there is a pressure bar very close to that V-bar. The duplex string segment between the V-bar and the pressure bar is very short. Hence the string termination is very efficient despite the shallow string deflection angles typically found here.


Del's first paragraph shows he is talking about the transverse vibrations everyone associates with piano strings. The Giordano and Korty paper and the synopsis emphasise the importance of the longitudinal vibrations which store much of the potential energy. As the angle at the v-bar is likely to be quite low, according to Del, the longitudinal wave will tend to carry on while the transverse waves are largely reflected.

I guess it is this energy that will be lost if, as rxd puts it, the string is physically impeded. This could happen if there is some crud at the pressure bar that interferes with the stretching and compression of the string. Remove that and the longitudinal wave will carry on to the tuning pin where it will be reflected back towards the bridge.

So thank you to Mark for raising the question even though one or two of the unisons are howling a bit in protest at the answer.

That's most of the change in my pocket for now.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/08/12 07:50 AM

Ian:

I have no idea where to even start in response. I don't think you are interested in following what I or anyone else is saying. You seem to have a "Damn the torpedoes full speed ahead!" attitude.
Posted by: rXd

Re: Effect of pressure bar on string vibrations and tone? - 02/08/12 08:10 AM


Ian, I hope I'm not being pedantic but your statement that seating the string at the hitchpin will bring a dead string back to life is a bit Lazarus for me the way I read it!! It will, however be enough to get that bit extra from a note if necessary.

Jeffs' comment about seating round the pressure bar I agree with. While it does have its effect on the sound, I rarely find it necessary. I am never fighting for extra sound with an upright in most upright type circumstances. I will say, however, that a good 10 second scrub round there with a fairly stiff brush will approximate the results that Ian got with a biro pen and padded screwdriver but without disturbing the tuning. I have a plastic brush that I found in the road one day when I was riding my bike about 10 yrs ago. One of the most useful tools I ever had.

Seating strings to the extent practiced in the past 30-40 years was almost unheard of in regular piano service. I thInk it was Yamaha that first recommended it as a regular part of maintenance.

Although I use seating and leveling when the sound of the note demands it, I must admit to benign neglect in this area. I prefer the warmth that develops in the sound when strings haven't been seated for a few years. I always seem to have more than enough quality well defined sound without it.

I haven't seen much crud around pressure bars lately but I have seen some In the past that were turning green and fuzzy.

I often service pianos that are regularly tuned by someone else so I have an arsenal of techniques that don't disturb the tuning.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/09/12 05:01 PM

rxd, no you're not being pedantic, I can't find the post about the dead note and the hitch pin again. I came across it by chance and made a mental note when looking at what Ed Foote and others had to say about tightening coils.

People have described Schiedmayer & Soehne tone as mellow and, sure enough, a recording showed it was lacking higher partials compared with other pianos from Germany and Stuttgart. These have now returned adding an extra dimensions to the tone. That's worth a tuning.

The effects were quite consistent from note to note across the 59 keys. I am not a great believer in boasting about special ailments, or having any at all, but there it is.

I wish I too had picked up a stray brush marked "For cleaning pianos". Whether it would have done the trick I shall never know.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/09/12 06:01 PM

Jeff

Yes, damn the torpedoes; they all going off in the wrong direction, wasting a lot of energy. Treading on a landmine in a post is worse. Point taken.

I happened to see you chopped down a tree at the weekend. Did it make a satisfying noise?

Speaking of perception, the Christmas lectures for kids at the Royal Institution in London and on the BBC this year were about the workings of the brain. The presenter focused the audience's attention by asking them to count how many times some jugglers tossed Indian clubs at each other. No one noticed the gorilla walk across the back of the stage.

When it comes to pianos, one can take this perceptions and preconceptions thing a bit far. Jurgen had a good story about auto-suggestion, but rxd's party piece about fixing the weakest note scarcely comes into that category. I was reminded of it the other day and spot on the screw was not quite tight.

All this is a roundabout way of saying I thought you had jumped on that bandwagon with your proposal of having someone dampen some notes and see if I can tell the difference. There really is no need. Reshaping heads, tightening loose plate bolts, and increasing the blow distance had each had an immediately noticeable effect on the sound. The pressure bar effect was right up there with them.

Did you mean dark matter as an allusion to longitudinal waves in strings? There are some parallels; you can't see them but it seems they need to be there for a piano to sound like a piano. By the way, how do you dampen them?

PS Lisa, who interrupted my notorious post, is no figment of anyone's imagination.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/10/12 07:18 AM

Ian:

Just because you are convinced of something, does not mean the rest of us should believe it too, or believe it for the same reason. That is why I suggested the blind damping test: to provide more convincing info for all of us.

But I was just now thinking about what you actually did. You moved the strings sideways on the v-bar and pressure bar. There are times I have done the same thing. Usually it is to get the strings to work better with a damper, but it will also change the tone because of where on the hammer the strings strike! It is much like using the una corda pedal to use a softer part of the hammer to change the tone, or even addidng some shims to the una corda stop to soften the tone of the entire piano in an emergency.

What you are experiencing may have nothing at all to do with longitudinal waves. We did get diverted with the feather treatment when what was actually done physically moved the strings.

Oh, and there were many satisfying sounds involved with the raccoons. smile
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/10/12 09:15 AM

Jeff:

Yes, quite right, but does not mean you should disbelieve it either. I'd agree it's generally a good idea to add an "or something else" to any list of possible explanations.

To recap and clarify, I did two separate things for different reasons. The first to restore sound to D7 which was "dead", a light touch to start with and moving the strings at the v-bar to make sure (D7 only). Days later, the second was triggered by Del's post about the pressure bar being an essential part of the termination in vertical pianos, and associating that with the question about coils and the non-speaking length affecting tone.

To start with I just wanted to see whether re-seating D7 at the pressure bar would make any difference. Rather to my surprise it did, D7 sustain compared to C7, E7 and the sharps. For all the other notes I flexed the strings only on the non-speaking side above the pressure bar, as little as possible to have an effect, as assessed by comparing each note with its tweaked and untweaked neighbours.

I can't see how I can do a blind damping test now. Instead I'll record some notes over the weekend and see how the before and after frequency spectra compare.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/10/12 10:00 AM

Ian:

Maybe I am misunderstanding what you think is going on.

Is this what you think: that the non-speaking portions above the v-bar are affecting the tone? If so a blind test, by randomly damping and undampening a non-speaking length, can still be done.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/10/12 10:52 AM

Originally Posted By: UnrightTooner
Is this what you think: that the non-speaking portions above the v-bar are affecting the tone?


Jeff

Good point.

I think a change in the nature of the contact between the strings and the pressure bar has affected the tone.

I do not think the portions of the string between the v-bar and the pressure bar and between the pressure bar and the coil have any discernible effect. My attempts to dampen them have made no difference.
Posted by: Emmery

Re: Effect of pressure bar on string vibrations and tone? - 02/10/12 11:05 AM

Well I just did a test on a single F2 string on a spinet with Pianalyser. For the first stike I had the weave removed between the tuning pin and the v bar. The second strike, I completely dampened off that portion between the 2 with a rubber mute and my finger pressed against it. You can see from the two readings, nothing unusual happens. No partials removed or lessened, or changes in amplitude. The backstring going down to the hitch pin may be a whole different story but those short lengths of string running from the v bar or the pressure bar to the tuning pin do nothing with the tone other than maybe play with ones mind, if they are inclined to presume something is happening.

First strike with no dampening or cloth weave...


Second strike with string dampened....
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/11/12 03:49 PM

Originally Posted By: Withindale
I can't see how I can do a blind damping test now. Instead I'll record some notes over the weekend and see how the before and after frequency spectra compare.


Thank you, Emmery, for taking the time and trouble to do the experiment. The result was, I imagine, no surprise to you. One of those devices would have saved me some time! Had to make recordings instead, then used Sonic Visualiser to compare them with those I made last May after I had reshaped the hammers.

Jeff, here are the results for one note. Nothing to do with the hammers or the strings changed until a week ago when I flexed the strings above the pressure bar.

E4 Before


E4 After

Note: the noise below the fundamental, on the left side of the spectrum chart, may be from the fan in my laptop.

The $64 question is what might account for the increased power of partials 6 to 10 and the higher frequencies evident in the lower screen showing the analysis of today's recording?
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/13/12 07:52 AM

Originally Posted By: Withindale
.....

The $64 question is what might account for the increased power of partials 6 to 10 and the higher frequencies evident in the lower screen showing the analysis of today's recording?


Knocking crud away from the termination point?
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/13/12 07:56 PM

Originally Posted By: UnrightTooner
Knocking crud away from the termination point?

Some microscopic crud, it seems, Jeff.

Originally Posted By: Emmery
The backstring going down to the hitch pin may be a whole different story

I wonder, Emmery. Your test certainly ruled out transverse vibrations, so it's a good bet changes in longitudinal vibrations explain the improved tone of my piano. As one can work out from the spectrograms, the relative power of the partials, before and after, increased by an average factor of about 5 for the lower ones rising to 12.5 at the tenth. For this to have happened the longitudinal waves must have been well reflected at each end of the strings, at the tuning pins and the hitch pins.
Posted by: Emmery

Re: Effect of pressure bar on string vibrations and tone? - 02/13/12 11:33 PM

I think the waves are reflected at the termination point on a v bar or or aggraffe and the section between that and the tuning pin are pretty well null for sound. An additional pressure bare would even deaden it some more if there is something extremely faint going on there. In fact, only some pianos have a weave of cloth in there, and often I just think its for looks. With that string length being so short, even the faintest sounds would be on a high order of frequency magnitude, something nearer to the same speaking string lengths we see in the top treble notes.

The back string from the bridge to the hitch pin is a whole different thing. The bridge is not as rigid or solid as the steel termination points at the other end and will allow vibrations to leak through to the other side.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/15/12 05:29 PM

Originally Posted By: Emmery
I think the waves are reflected at the termination point on a v bar or or aggraffe and the section between that and the tuning pin are pretty well null for sound. An additional pressure bar would even deaden it some more if there is something extremely faint going on there.

What you say, Emmery, is what I would have said; that is until a question came up about the effect on tone of the non-speaking lengths. It turns out there is more to the full story - piano hammers produce longitudinal waves in the strings as well the tranverse waves you describe.

If transverse waves were the only game in town, there is no way tweaking the strings at the pressure bar would have had anything other than your extremely faint effect on my piano. There had to be something else to explain the release of energy I heard.

The obvious candidate was longitudinal waves, by which I mean waves within the strings themselves. These waves will not stop at the v-bar but will carry on behind the pressure bar until reaching the end of the line at the tuning pin and coil. As there is nowhere else for them to go they will turn round and go back along the string towards the hitch pin, and so on back and forth. From my experience it appears that a good proportion of the energy will leak into the pressure bar if the string is stuck to it with some amalgam of dust, must and rust.

A search of this forum yields no mention of longitudinal waves at all, other than a crop of posts about the (sometimes objectionable) longitudinal mode in bass strings. As Giordano and Korty's paper, Motion of a piano string: Longitudinal vibrations and the role of the bridge, is not mentioned either, it may be helpful if I summarise some of its key points:
  • Longitudinal vibrations are generated by the hammer blow stretching the string.
  • Transverse waves show no sign of longitudinal vibrations but the motion of the bridge and soundboard does.
  • The relative power of the higher partials in the string spectrum [i.e. the transverse waves] is much less than their power in the sound spectrum [i.e. produced by the soundboard].
My answer to the $64 question, in my recent post showing the before and after frequencies, was that much of the energy stored in the longitudinal waves was being dissipated in the pressure bar. When this energy was released, as a result of flexing the strings, the motion of the bridge and soundboard increased in a way that boosted the power of the partials.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 07:18 AM

Originally Posted By: Withindale
.....

The obvious candidate was longitudinal waves, by which I mean waves within the strings themselves. These waves will not stop at the v-bar but will carry on behind the pressure bar until reaching the end of the line at the tuning pin and coil.

.....


Uh, why wouldn't the waves stop at the pressure bar? I think they would.

My understanding of longitudinal waves is different. It is the same as tone bars. The frequency depends on the length and thickness. The tension (stretching) has nothing to do with it.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 08:28 AM

Originally Posted By: UnrightTooner
Originally Posted By: Withindale
.....

The obvious candidate was longitudinal waves, by which I mean waves within the strings themselves. These waves will not stop at the v-bar but will carry on behind the pressure bar until reaching the end of the line at the tuning pin and coil.

.....


Uh, why wouldn't the waves stop at the pressure bar? I think they would.

My understanding of longitudinal waves is different. It is the same as tone bars. The frequency depends on the length and thickness. The tension (stretching) has nothing to do with it.


Longitudinal waves in strings are sequences of stretching and compression along their length. Click here for an animation; don't worry about the time axis. As the stretching and compression movements are within the string there is nothing to stop them at the pressure bar. Think about liquid flowing through a pipe curving round the pressure bar. There must be reflection at either end, at the coil and tuning pin and at the hitch pin; otherwise energy would disappear and the note would be deadened, as sometimes happens.

By tone bar do you mean as in percussion instruments such as a xylophone, as described here, for example? The vibrations shown in the diagram on that page are transverse.

Giordano and Korty have quite a bit to say about stretching and longitudinal waves but the fundamental point is that you can't have transverse waves in piano strings without inducing longitudinal waves as well.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 09:25 AM

Ian:

I would think that the bearing friction would stop the longitudinal waves.

But let's say it didn't. Would goofing around with the "non-speaking" part change anything if the high friction bearing points do not have an effect either?
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 02:03 PM

Jeff:

OK, let's follow up on your suppositions and look at two imaginary pianos.

In the first, the longitudinal waves stop at the v-bar and the bridge. The non-speaking lengths are just guy-wires at constant tension.
  • FFF blows never knock any notes out of tune
  • beckets never slip out
  • tightening coils never has an effect on tone
  • flexing strings at the pressure bar affects unisons but not tone
  • re-seating hitch pin loops has little or no effect on tone.

In the second, longitudinal waves carry on past the v-bar and the bridge pins. They are reflected at the tuning pin and and the hitch pin.
  • FFF blows can affect the tuning by the end of a concert
  • beckets sometimes slip out or break
  • tightening coils improves the tone of some notes
  • flexing strings at the pressure bar may liven up some notes
  • moving strings back and forth at at the v-bar may improve a note
  • re-seating hitch-pin loops sometimes improves the sound; badly made loops can break.

Which is more like a real piano?

Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 02:10 PM

Ian:

I do not play poker with someone that stacks the deck.

You are completely ignoring sloppy stringing and poor pin setting.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 02:31 PM

Jeff:

The pianos were strung, tuned and serviced by the experts on this forum. What happens is what they describe.

You might say the first piano is one that is not played and the second one that is.
Posted by: Emmery

Re: Effect of pressure bar on string vibrations and tone? - 02/16/12 03:10 PM

Originally Posted By: Withindale
Jeff:

The pianos were strung, tuned and serviced by the experts on this forum. What happens is what they describe.

You might say the first piano is one that is not played and the second one that is.


Ian, many of those changes occur because of the speaking length being effected. Sometimes grooves form in v bars and pressure bars and moving a string off them effects the speaking length tone. Also, part of a piano going out of tune during heavy playing is simply uneven string tensions equalizing out as they slip over the bearing points, something that we try to avoid with a good pounding in but nobody gets it perfectly even I think.
Posted by: UnrightTooner

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 11:42 AM

Ian:

So the "imaginary" pianos are actually anecdotal pianos. And you picked and chose them for your own purposes. One is an ace for your hand and one is a duece for my hand.

I am finished with this Topic. Maybe things will go better with your next one.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 01:25 PM

Jeff:

All the things that happen in the second piano, and don't happen in the first, depend on the transfer of energy between the speaking and non-speaking lengths.

I have one torpedo left - marked TONE - and it's down to your invaluable persistence that it's almost ready to go. We'll see if it's an ace or a deuce.
Posted by: Supply

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 01:47 PM

Go for it Jeff. thumb
Tilting at windmills is a thankless task, but someone has to do it! eek
Posted by: Emmery

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 02:15 PM

Originally Posted By: Supply
Go for it Jeff. thumb
Tilting at windmills is a thankless task, but someone has to do it! eek


Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 03:20 PM

Did George Brown College invite the Star Trek crew to a piano recital?
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/17/12 07:43 PM

Returning to reality; Harold Conklin measured longitudinal vibrations at the hitch pin over 15 years ago. See Generation of partials due to nonlinear mixing in a stringed instrument J. Acoust. Soc. Am., Vol. 105, No. 1, January 1999.


Experimental set up
the sensor marked FS measured the tension end force.




Longitudinal wave frequency spectrum




Longitudinal wave spectrum - higher partials




In summary of this thread, longitudinal waves are the reason it makes sense to seat strings really well at all bearing points, from the tuning coil to the hitch pin. Loss of energy at any of those points will likely have a disproportionate effect on the power of the higher partials and tone.

Ironically, Harold Conklin was looking for "phantom" partials.
Posted by: Chris Storch

Re: Effect of pressure bar on string vibrations and tone? - 02/18/12 07:58 PM

Ian,

I'm going to start by saying that I believe that you're probably right about the pressure bar having something to do with tone. Something interesting is going on there, for sure.

However, I think you've misapplied the conclusions of the ASA paper you've cited. The ASA paper actually contradicts what you showed in your spectrograms in your earlier posting.

From what I gather, you ran an informal experiment, and recorded two spectrograms before and after a treatment to the strings at the pressure bar. Your spectrograms show an increase in the relative amplitudes of the upper partials, yes. All of the partials that were enhanced appear to be (roughly) integer multiples of the fundamental.

Look closely at the ASA paper again. The authors are showing that the longitudinal vibration components are creating what they call "phantom partials". Correct me if I'm wrong, but the phantom partials don't fall in line with the partial ladder normally seen created by the transverse vibrational energy. That's in direct contradictions to what you show in your spectrograms.

Longitudinal vibrations along a string can be and often are different than the partial series set up by the transverse vibrations. Give a listen to any of the sound files at the following site, and you'll hear how this can be true.
http://www.speech.kth.se/music/5_lectures/conklin/longitudinal.html

Sure, the longitudinal vibrational energy may have had something to do with the results of your experiment. Again, I agree that there's SOMETHING interesting going on in the nonspeaking portion of string under the pressure bar - to be investigated further. But I think some of the assertions you've made in this thread cannot be substantiated on the references you've cited.

Correlation does not equal causation.

Chris S.
Acoustician by Day, Piano Tech by Night.
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/19/12 07:55 AM

Hallo Chris, I agree with your point about spurious correlation.

I was once asked to analyse the socio-economic factors stunting the growth of children from deprived backgrounds around Manchester in England. Some doctors had sponsored a study and the data, collected at some expense, showed two distinct groups. They had not noticed Group A were all boys and Group B were all girls.

I hope I haven't fallen into the same trap with my interpretation of Harold Conklin's paper on the generation of partials due to non-linear mixing.

Chris, I do not believe I have misapplied the results of the ASA paper or that my spectrograms are conflict with them. As I see it all the overtones we hear result from the non-linear response of the bridge to the vibrations in the string. You have to consider the longitudinal wave component at all frequencies, not only for longitudinal modes.

To my mind you have a reasonably complete model of a piano string when you include both longitudinal and transverse waves. I am not so sure you need look much farther unless you want to understand the mechanisms which dissipate energy or, more importantly, generate sound.

Please say if you think this wrong or if there are still some unwarranted assertions. In the hope of avoiding any further misunderstandings I think I'd better go back to square 1 and run through the whole argument.

My "experiment" was to see if anything would happen if I reseated the strings of D7 at the pressure bar. To my (untrained) ear, the note had livened up considerably in comparison with its neighbours. This was after I had revived that note by reseating them at the v-bar. I then repeated the exercise on the other 58 notes under the pressure bar, with similar results.

The question was why this occurred. As Del said, and as Emmery observed, the v-bar and the pressure bar are very effective at terminating the transverse vibrations of the string. It follows that what I did could not have directly affected the transverse vibrations in the speaking length.

Conventional wisdom is that what happens in the speaking length determines the sound. This is true of course for most practical purposes associated with tuning and voicing. Wave diagrams and formulae are about the speaking length. The non-speaking lengths do not feature in the Five Lectures on the Acoustics of the Piano you mention.

Earlier in this thread rxd mentioned transference of power from the hammer to the string; it is that energy which is at issue. Once the hammer has left the string its energy can either be dissipated or turn into sound. When you enliven a note by seating its strings you increase its energy by reducing the amount of energy being dissipated at one or more of the bearing and end points.

This is where conventional wisdom comes into the argument. This appears to be that all waves are in the speaking length and virtually none of their energy transfers across the termination points. Therefore making adjustments at the pressure bar, tuning coil and hitch pin can have no effect.

The elephant in the room had better come out into the open now; all the vibrational energy in the string is in the longitudinal vibrations. These waves are periodic stretchings and compressions along the string itself. They result from the hammer hitting the string and the transverse movements of the string that follow. Rather than take my word for it, read this short article on Longitudinal Waves quoted from "Physical Audio Signal Processing" by Julius O. Smith III.

By the way, longitudinal modes are only one aspect of longitudinal waves.

The mathematics associated with waves in taut strings are moderately complex and have been the subject of some debate over the years. However a paper published in 2011 may have resolved the matter, "The potential energy density in transverse string waves depends critically on longitudinal motion", David R Rowland 2011 Eur. J. Phys. 32 1475, see the abstract here. I do not have a copy of this paper but the general conclusion makes sense. This is that [to derive] the correct formula for the potential energy density in transverse waves on a taut string ... the longitudinal motion of elements of the string needs to be taken into account, even though such motion can be neglected when deriving the linear transverse wave equation.

In other words, when you are talking about the behaviour of the speaking length, i.e. the transverse wave equation, you do not need to worry about the longitudinal waves. On the other hand, you should when talking about the behaviour of the piano, especially the bridge and the soundboard, as Giordano and Korty's work showed.

The next question is whether the longitudinal waves are mainly reflected at (a) the v-bar and bridge pin or (b) the tuning pin and the hitch pin. It seem obvious to me that much of their energy must reach the tuning pin ...

Originally Posted By: beethoven986 Re: How long should a tuning last?
It honestly depends. For a normal person, probably three months. For a professional pianist, maybe a month. For a concert at Carnegie Hall, probably until intermission.

... and the hitch pin but in this forum such it seems such a conjecture needs solid proof.

Giordano and Korty measured the longitudinal motion at the bridge and then Conklin went one step further and measured the force of the longitudinal waves at the hitch pin. His experiment shows that a significant amount of longitudinal wave energy, if not all, is travelling along the whole length of the string. This is key point from Conklin's paper for this thread.

Once one knows that there is wave energy in the non-speaking lengths one can see why adjusting strings at the tuning coil, hitch pin, pressure bar, and bridge pins might affect the tone. As might any other bearing points where energy can be lost.

The point that extra longitudinal wave energy enlivens a note by increasing the power of the higher partials originally came from Giordano and Korty. This was a general observation they made without mentioning phantom partials. Conklin says he spotted the phantom partials in Giordano and Korty's spectrograms and, in that sense, his work extends theirs.

You will see in the following diagrams that there is longitudinal wave energy in the partials as well as the phantom partials. In fact he goes on to discuss how the phantoms smear the partials, see Fig 18 in the paper. I am including the diagrams here because it will be easier to compare them one above the other than spread about in the pdf.

Sound



Transverse waves at the bridge



Longitudinal waves at the bridge




Posted by: Johnkie

Re: Effect of pressure bar on string vibrations and tone? - 02/19/12 08:15 AM

Can someone wake me up when this is over please ............ I know you're keen ... but ........ my glass eye has gone to sleep!! sleep crazy sleep
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/19/12 12:12 PM

Johnkie, taking a leaf from your book, there was no need for that ... or for most of this thread! As The Honourable Mr Justice Lindsay once said, after listening to the contrary opinions of experts and leading counsel about mental disorders caused by stress at work, it becomes necessary to spell things out.
Posted by: Johnkie

Re: Effect of pressure bar on string vibrations and tone? - 02/19/12 01:54 PM

Sorry if I offended you Ian - That was not intended. It was meant to be tongue in cheek, and something to lighten this thread. wink
Posted by: Withindale

Re: Effect of pressure bar on string vibrations and tone? - 02/19/12 02:15 PM

Not at all, Johnkie.