An Interesting Dulcimer Experiment

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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Sat Sep 19, 2015 9:40 am

Notes on the Effect of Top and Back Bracing in Mountain Dulcimers

In another post a few of months ago Robin Clark in Wales asked why the mountain dulcimers of George Orthey in Newport Pa. sounded so loud. He posted some pictures and dimensions of his Orthey dulcimer, so I made myself a replica, as best I could.
Orthey Dulcimer Replica 01.jpg

It turned out to be a very nice sounding instrument – I would be pleased if my own designs sounded as good. I can’t say whether it sounds like a typical Orthey (never seen or heard one), or how much the design vs the materials was responsible, or if I just got lucky with the combination of everything. But the instrument has a clarity of sound and ease of tuning and playing that is very pleasing. There was nothing I disliked about the sound.

So I thought “Why not try to improve it?” The essence of this dulcimer is a light and stiff fretboard/top assembly with a light and flexible back. The fretboard is hollowed and arched, and the hollow extends to just in front of the bridge, which sits above the endblock. Originally there were no top or back braces. I was interested to see the effect of adding bracing to an already good instrument. In the past I have added/removed bracing from my plywood test dulcimer with no real change in tone – but I never thought it was a fine sounding instrument to start with.

So I sawed the back off the Orthey replica and installed five full width braces on the top and back plates, and glued the back back on. There was about 1mm reduction in the side height afterwards, caused by the saw cut and sanding.
Orthey_Before and After_Braces.jpg
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Sat Sep 19, 2015 9:56 am

Notes on the Effect of Top and Back Bracing in Mountain Dulcimers (Continued)

Installing top and back bracing did not result in a better sound. It was generally the same but there was an additional tone which had a boominess about it, and several frets had what I would call “wolf notes” – a clear difference in tone compred to adjacent frets. Overall, adding the bracing was not an improvement.

After a while I decided to go back to the original state of no bracing. Sharp flat steel rod, some splintering of wood, some scratches of sound hole edges and the two major bout braces from the top and the bottom were removed. The waist braces and the two minor bout braces were still in place on the top and the back – only the major bout braces were removed. I made some measurements as each brace was removed and listened for any tonal changes. The order of removal was 1st top brace (closest to bridge); 2nd top brace; 1st back brace and finally, 2nd back brace.

Removal of major bout top braces did not audibly change the sound. The wolf notes were still there and the general tone was the same.

Removal of the 1st major bout back brace did reduce the wolf notes, but overall the tone seemed thinner.

Removal of the 2nd major bout back brace sounded very like the original no-brace configuration.

There are a number of things to note here:
1. This is a clear case of top/back bracing degrading the sound of a mountain dulcimer.
2. The bracing seemed to be responsible for the introduction of wolf notes to the instrument.
3. The top bracing seemed to have much less of an effect on the tonal changes than the back braces.
4. The bracing at the waist and minor bout seemed to have little effect on the sound.

The audible changes were reflected in the tap resonances of the dulcimer box, and the air resonances of the cavity. The bridge-tap spectra give some clues as to why the sound degraded with bracing. The following picture shows the box resonances from zero to 800Hz for the various stages of brace removal.
Orthey_BridgeTapResonances_Braced_UnBraced.jpg

In the picture above, the first panel is of the original unbraced dulcimer. It’s unusual in that the first two resonant peaks on the left represent the first air resonance at about 180Hz, and a combined 1st Bar resonance and 2nd Air resonance at about 274Hz. (Disregard the small peak at 50Hz – it’s an artifact of the measurement) So either by a quirk of the selected materials, or possibly because of the design, I don't know, the usually separated second and third resonances both fall at the same frequency. Vibrational mode testing with a loudspeaker shows this:
Orthey - Combined 1st Bar and 2nd Air Modes.jpg

I haven’t seen this in a dulcimer before, although I suspect it’s not really unusual. The pattern on the back results from the second air resonance vibrating the flexible back wood in the usual circular mode, and on the top is a hybrid bar/circle pattern. Two resonances falling at the same frequency is normally a recipe for dead notes or wolfnotes but in this dulcimer the other resonances happen to be spaced such that the overall sound is very good.

The second panel shows a resonance too far. The full bracing of the top and back have stiffened the dulcimer box to the extent that the first air resonance (the “Helmholtz”) has been raised in frequency and is nearly the same as the combined 1st Bar and 2nd Air resonances – three resonances all falling at the same frequency. That’s too much to get away with and the result is the boominess and wolf notes.

The third panel is after the removal of the two major bout braces on the top. I didn’t hear much change in tone with their removal, and the spectrum is much the same as the full-brace setup – three superimposed resonances.

The fourth panel is after the removal of the back brace closest to the bridge. The back has regained some of its former flexibility and the 1st Air resonance has fallen in frequency and moved away from the 1st Bar and 2nd Air resonances. The wolf notes were improved, but overall the tone seemed thinner to me. Hard to decide if it was an improvement or not.

The final panel is the removal of the second back brace. The spectral pattern has reverted to be much the same as without bracing. There were then no major bout braces on the top or back, however the waist and minor bout braces were still in place – three each on the top and back. To my mind the tone was the same as I remembered with no bracing – no vices or wolf notes, clear and resonant.

Conclusions
So is there a take-home lesson here? There might well be.

Bracing the back of a lightweight wood mountain dulcimer can have a significant effect on the tone of the instrument compared to an unbraced back.

This is neatly consistent with the previous post that indicated the significance of back bracing stiffness in the bright/mellow tone divide. It’s an area of construction that I hadn’t considered of much importance, but it seems like it might be after all. Ken Bloom is at least partly vindicated – the stiffness parameters of the back of a mountain dulcimer can affect the tone.

On the other hand, the addition and removal of the major bout top braces seemed to have no audible effect on the tone, and neither did the presence of the waist and minor bout braces. I’m sure there are changes in the spectrum with this other bracing, but I couldn’t hear it. This reinforces the dominant effect of the fretboard on the way the top vibrates – the addition of top bracing doesn’t seem to modify the overall top stiffness enough to alter the tone because of the over-riding stiffness of the fretboard. It also reinforces the principal role of the major bout over the minor bout in producing much of the sound, and may partly explain why teardrop dulcimers don’t sound characteristically different from hour-glass dulcimers.

But of course there’s no free lunch here. We pay for a light and flexible back with more effect on the tone by knee damping. Possum boards and table feet become more necessary. After comparing this dulcimer, with removed braces, on my lap and played on a table (with three small feet), it's clear that knee damping negates nearly all the gains made by removing the bracing. It's basically fairly ordinary played on the knee. This might well be a reason to have a double back - not because an outer back somehow makes the instrument louder or better, but just to reduce the effect of knee damping on the inner back and by proxy, maintaining the inner back's coupling to the top plate which then can radiate more sound. It would be important that the outer back did not acoustically load up the inner (vibrating) back by completely enclosing it, or by having a very narrow open air gap between the two. The outer back might be perforated over its surface, for example, so that the air between the two did not act as a cushion and modify the inner backs vibration. A possum board that only touched at the edges of the two bouts would be an alternative as long as the back was prevented from touching the knee at all - even a light touch is often enough to disable the first air resonance coupling between the back and the top.

Richard T
Last edited by rtroughear on Thu Oct 08, 2015 11:31 pm, edited 2 times in total.
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Re: An Interesting Dulcimer Experiment

Postby Robin the Busker » Tue Sep 22, 2015 8:25 pm

Another great practical experiment Richard 8) Very interesting to note the effect that bracing has on timbre! To me, there doesn't seem any point in following the example of the guitar in terms of bracing when building a dulcimer. A guitar need bracing to stop the string pressure twisting the bridge and tearing the instrument apart. Even then, guitar makers try and get rid of as much bracing as they dare. Dulcimers do not have that issue - you can make the box the fretboard sits on pretty much as light as you like and only brace where absolutely necessary. I have a number of pre-revival dulcimers without braces or kerfing that are still going strong!
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Wed Sep 23, 2015 8:54 am

Robin, I always install bracing on the top and back of my dulcimers. In experiments on my test dulcimer they have not made any real difference to the sound one way or the other, so I put them on mainly for structural strength reasons. For the top there are a couple of reasons that seem valid to me. Firstly for an arched fretboard the long term stability of the top is more likely if a supporting cross brace is under each foot - up to say four or five arches. Not so necessary for Orthey-type short arches. The other is that if there is no top bracing then thin softwood tops are more likely to be damaged by squeezing between fingers and thumb. The generally harder backs are not so vulnerable, nor would more traditional hardwood tops.

But in the light of this test I might revise the nature of back bracing in my dulcimers. Like everything its not necessarily a bad thing, or a good thing. Heavy back bracing coupled with a lighter top/fretboard might be a very good combination, with reduced knee damping. On the other hand it might result in a coincidence of multiple resonances as is this case, with poorer outcomes. It's all a bit of a lottery. Having no bracing doesn't guarantee good outcome either. The best dulcimer I've ever made, to my mind, has medium-stiff back bracing and medium top bracing.

Richard T
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Thu Oct 08, 2015 11:24 pm

I came across an interesting Master's thesis from Finland. It concerns the modelling of the vibratory behaviour of the Kantele, Finland's national instrument. It's not exactly a mountain dulcimer, but is the nearest thing I've seen that's been analysed thoroughly. The thesis is 64 pages, and can be freely downloaded as a .pdf file from:

http://www.researchgate.net/publication ... ed_kantele

There is necessarily a moderate amount of mathemetics, but also a good summary of stringed instrument acoustics, a lot of which is as relevant to mountain dulcimers as to other instruments.
(Note: in the text, you could replace the word "eigen" with "resonance")

Richard T
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Fri Jan 01, 2016 5:04 am

Mountain Dulcimer Bridge-end Fretboard Undercut – Test Results on One Instrument

I did these tests some time ago but haven’t reported them because they are not particularly enlightening - I report them now for two reasons. Firstly I spent a good deal of time getting them and someone might be interested; and secondly, I want to undo the modification involved and use the dulcimer for something else.

The Experiment

A number of makers cut a slot under the fretboard of their dulcimers up to nearly the bridge position. The strings are then terminated on this cantilevered portion of the fretboard end. The reason for doing this is usually about removing the influence of the relatively heavy end block on the way the bridge can transfer sound energy into the dulcimer box, and therefore making the sound somehow better.

So I did this on my test dulcimer to see what I could see. The result was a noticeable tonal change, that I did like, but was not clearly “better” than the original.. More “different” than “better”. As usual there are all sorts of qualifications to the results, and no free lunch – there seem downsides to me. And the usual caveat applies – this is the test result of one dulcimer, others may not follow the same pattern.

The Tests

Some measurements were made on my test dulcimer and then a horizontal slot was cut in the fretboard from its end and almost up to the bridge position. Several configurations were tested of where the strings were attached and whether there was a downforce on the fretboard end or not. The Six test cases were:

Case #1 Fretboard End glued to end block; strings attached to end block
Case #2 Fretboard End glued to end block; strings attached to fretboard end
Case #3 Fretboard End undercut; strings attached to fretboard end. This pulled the end of the fretboard UP about 1mm.
Case #4 Fretboard End undercut; strings attached to fretboard end; slot closed with a light wedge
Case #5 Fretboard End undercut; strings attached to end block; strings over a saddle to remove pressure on fretboard end (unloaded)
Case #6 Fretboard End undercut; strings attached to end block; strings pressing down on fretboard end (loaded). This pressed the end of the fretboard DOWN about 1mm to touch the end block.

The following picture and diagram shows the setups.
FretboardUndercut_Test Cases_Photo_01.jpg

FretboardUndercut_Test Cases_Diagram.jpg


For each test case the same data was collected under constant conditions:
1. String strike recordings using a rod-pendulum-plectrum for adequate repeatability.
2. Standard test tune recordings
3. Frequency spectra of the sound recordings of bridge taps with a rubber hammer
4. Vibration mode measurements (Chladni patterns) – loudspeaker excited

From this data additional sound spectrographs were generated and sound pressure levels calculated.
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Fri Jan 01, 2016 5:09 am

Mountain Dulcimer Bridge-end Fretboard Undercut – Test Results on One Instrument (Continued)

Results

Sound Pressure Levels
Results_StringStrikes_and_Tune.jpg


The sound pressure levels were calculated using the PRAAT signal analysis software from the recordings of strings strikes and test tunes. There were three test tunes for each test case, and thirty string strikes. The frequency spectrum of each recording was also used to calculate the Spectral Centre of Gravity – the frequency at which there was as much sound energy below as above. High C-of-G might point to a brighter tone; lower C-of-G might indicate a more mellow tone.

Overall Case #6, loaded-undercut might be a little louder, but there’s not much in it for all the test cases. So undercutting a fretboard end might not lead to much change in loudness.

.Case #3; the “standard” undercut configuration did have a consistently lower spectral C-of-G which might point to it having a warmer tone.

Listening Preferences

The test tunes were randomised and the names anonymised and then ranked according to my listening preference (yours might well be different) – three test tunes for each case. I did it twice on widely separated occasions.

The result was a consistent preference for Case #6- Undercut-Loaded, followed by equal preference for Case#1 – Standard dulcimer; and Case #3 – Standard undercut. The preference for Case#6 might have been because it was a little louder than the others – maybe just variation in the recording.

I consistently didn’t like Case #5 – Undercut-unloaded.

Bridge Tap Frequency Spectra

The bridge tap spectra for the various test cases showed some consistent similarities, and some differences associated with the undercut slot.
FretboardUndercut_Bridge_Tap_Spectra_Comparfisons.jpg


When the undercut end of the fretboard was free to vibrate (Case #3 and Case #5) there was an additional strong resonant peak at about 430Hz. Otherwise the frequencies of the resonant peak series was fairly consistent over all test cases – but their amplitudes varied considerably.
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Fri Jan 01, 2016 5:12 am

Mountain Dulcimer Bridge-end Fretboard Undercut – Test Results on One Instrument (Continued)

Vibration Modes
For each peak in the bridge tap spectra we might expect to find an actual area of the dulcimer that can be made to vibrate easily at that frequency if excited by a loudspeaker. The vibration mode series for the all test cases were fairly consistent in pattern and frequency. Here’s an example of one vibration mode:
FretboardUndercut_Vibration Mode- Comparison_Lo.jpg


The frequencies of the various resonances for both the bridge-tap spectra, and the vibration mode analysis are in the following table:
Resonant Frequencies_ Comparisons.jpg

The undercut doesn’t seem to have changed the basic resonance characteristics of the dulcimer box..
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Fri Jan 01, 2016 5:14 am

Mountain Dulcimer Bridge-end Fretboard Undercut – Test Results on One Instrument (Continued)

Spectrograms
Sound spectrograms of lots of string strikes were analysed in PRAAT to see if there was any characteristic difference between the open slotted Case #3 and wedged Case #4. There was a clear tonal change when the open end of the fretboard was either lightly wedged, or had weights added to it. At frequencies around 430Hz there were differences in the two cases. Here’s a picture of spectrograms of string strikes :
Spectrogram_3rd String_C#3_Open_then_Wedged_and_Weighted.jpg

Each side shows six string strikes, firstly with the undercut slot open (Case #3), and then with the slot lightly wedged (Case #4). Vertical axis is frequency, horizontal is time. The series of little “flags” are the overtones of the fundamental, darkness indicates loudness of the overtone, and length of the “flag” represents sustain. The right hand side has a weight attached to the end of the fretboard as well as being alternatively open then wedged.
FretboardUndercut_Wedged_and_Weighted.jpg

It was consistently the case that any overtone occurring at about 430Hz was both loud and short if the undercut slot was open. However, if the fretboard end was constrained from vibrating by touching or weighting, the overtone was unaffected by additional wedging.
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Fri Jan 01, 2016 5:18 am

Mountain Dulcimer Bridge-end Fretboard Undercut – Test Results on One Instrument (Continued)
Fretboard End Vibration
The tap spectra, vibration mode testing and the sound spectrograms all point to an additional resonant frequency in the fretboard end at about 430Hz.. It’s flapping in the breeze like a harmonica reed. To see if this was actually the case I recorded open slot and wedged bridge tap spectra with both an air microphone and a small piezo transducer stuck onto the end of the fretboard.
BridgeTapSpectra_Air Mic_and_FB End Contact Piezo_Lo.jpg

When the undercut is open (and unweighted) there is a dominant resonance at 407 Hz, about 10dB stronger than the 1st Bar resonance at about 220Hz (which also shows up at the free end of the fretboard for some reason, but not the 1st air resonance).


Conclusions

Undercutting the fretboard end of a mountain dulcimer can produce a significant change in overall tone. It may sound “better” to some listeners, others may prefer the original tone. Hard to say. In this one case it made the dulcimer no louder.

The reason for the tonal change does not appear related to the fretboard being glued to the mass of the end block, but more likely to the presence of an additional fretboard resonance in the unconstrained end of the undercut fretboard. In this case it fell at about 430Hz – a frequency region that enhances the perception of warmth in a sound.

This fretboard-end resonance might be acting as an initial filter to the string-bridge vibrations, altering the strength and duration of overtones that are nearby in frequency, and hence modifying the overall tone of the dulcimer.

Anything that damps this fretboard-end vibration, such as touching, weighting or wedging, seems to revert the arrangement towards the tone of the original glued-down fretboard end.

The fact that very small weights or light wedging can modify the overall tone in this case might mean that in different dulcimers with fretboards of different density and dimension it might be possible to “tune” the sound in some respects, particularly for specific string tunings.

The undercut did not seem to modify the way the whole dulcimer box vibrated – it appears to be an initial filtering of the string frequencies into the box.

An additional contributor to the tonal change, which is only speculation, is that with the open slot I thought that the endblock of the dulcimer vibrated more strongly than without it. This was just a subjective judgement on my part by feeling the dulcimer end with my fingers – I should have made some measurements, but didn’t. If it is the case, then the cutting of the fretboard slot has allowed the 1st bar vibration mode to become stronger. The 1st bar mode is an important component of the overall dulcimer sound. So it’s ironic that the removal of the fretboard from the endblock to allow the endblock to vibrate more might be a factor in the tonal change, rather than the other way around.


Is there a price to pay for this tonal change? There might be in the long term. The string pull on this fretboard end for an open slot totals about 6kg (13lb). That’s the weight of two housebricks pulling upwards on the unsupported fretboard end. Depending on the materials and design, over time that stress might become significant.

Richard T
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Re: An Interesting Dulcimer Experiment

Postby Truckjohn » Fri Jan 01, 2016 3:41 pm

Love the work you are doing here and that you are publishing all of it here for folks like me to poke through.

I think my biggest take away from this whole series of experiments and posts is that the "fretboard" is the main determinant of the "voice" of the dulcimer. It makes sense as this one single piece is the stiffest and heaviest "brace" on the whole instrument as well as the main string attachment point.

A lot of the experiments make general sense to me. More mass generally kills trebles faster than it kills bass - which ends up making a mellower sound. More stiffness generally kills bass more than trebles - so an instrument tends to sound "brighter" or it has more "cut".

The part with shorter sides and a better sounding instrument kinda makes sense also - as it would tend to more strongly couple the top and back as well as raise the main air resonance higher.

I wonder if the amount of bracing that we see in some of the historical instruments were more about trying to stop cracks caused by poor humidity control during construction and air dried unseasoned wood. Many of those luthiers didn't have real good humidity control and they didn't really have access to kiln dried lumber.

Now, I just gotta try some of this out.

Thanks
John
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Re: An Interesting Dulcimer Experiment

Postby rtroughear » Sun Jan 03, 2016 5:26 am

Vibrations of End Block and Fretboard End

In the previous post it was suggested that the end block of a mountain dulcimer might actually vibrate more if the end of the fretboard is undercut.

Using a small button piezo to directly measure the vibrations, this does not seem to be the case. For this dulcimer, at both the resonant frequency of the end fretboard (~430Hz) and off that frequency, the end block remains basically unaffected by the undercut fretboard. Except that a harmonic at the resonant frequency (430Hz) is short an sharp at the end block, whilst flailing away at the end of the fretboard. Not surprising – the energy is being spent moving the fretboard end rather than transferring into the dulcimer body.

Here’s a picture of six notes with the piezo recorded at the base of the end block, then stuck to the end of the fretboard – alternately with the slot open, then wedged – three of each. The corresponding spectograms are to the right.
Piezo Vibrations_End Block_End Fretboard_Open_and_Wedged.jpg

The series of harmonics for the wedged condition is pretty much the same whether recorded from the base of the end block, or from the end of the fretboard – they are hard-connected by the wooden wedge.

Other than the damping of endblock harmonics at the fretboard resonant frequency, the picture is the same at other string tunings.

Richard T


And TJ, thanks for the comments, but I'm at the stage now where I suspect I know hardly anything. So if you have any revelations, wouldja let me know ....
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