Emir
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Experimental modal analysis of oud top
I've been working on the establishment of a new musical instrument's acoustics and vibrations laboratory for a long time. Finally, I got the first
results. This video is about natural vibrations of a real oud top, by using impact hammer and accelerometer.
https://youtu.be/Fv3H_fHKz6o
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suz_i_dil
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Interesting approach.
Are you going to test all the frequencies of the human earing in this study ? I mean approx 30 Hz -> 20000 Hz
it's difficult imagining a track to use those results in building. But I was suprise of the very specific area around the small soundholes at 371
Hz
Thanks for sharing !
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Emir
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Modal analysis is a way of visualizing of vibration's eigenmodes which are very important to determine sound quality of musical instruments. Of course
there are a lot of vibration modes possessing high frequencies. I usually measure activity of higher modes by using sound analysis is similar to
tapping test ( I will send FRF graph of this test).
Nevertheless sound analysis metod gives us very usefull information about resonance profile of instrument in the audable range, I also need to know
mode shapes. By using these shapes of lower modes (shapes of upper modes are very complicated), I decide where braces will be glued and how thickness
graduate of top will be done. Lower modes are very important. Since they have large moving area, they produce more sound power than upper modes. For
example, the first four modes of guitar top produce 80 percent of its sound level. Upper modes mostly have effect on timbre than power.
Actually, This is my first modal analysis by using new equipments. Although I used almost 100 FRF measurement point, test took all day. Since upper
modes shapes are smaller than lower ones, I think 500-600 point necessary to measure up to 2khz (20 khz is not imposible). It will take very long time
but I hope, I will make it soon. 
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suz_i_dil
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Interesting, but I feel I should get back to my physic science lesson to understand well the aproach.
those lessons begin to be far away from my actual field of work
I ask recently about a possible positioning of the braces depending on overtones :
http://www.mikeouds.com/messageboard/viewthread.php?tid=15903
Do you think this is consistent with positioning depending them on your method of analysis, on frequncies ?
Would be glad to read some feedbacks about. Thanks for sharing your approach with us.
best regards
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Emir
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I know, these vibration modes, modal analysis, FRFs etc. seem very complicated but we need them to understand how a oud produce sound. I attached
image showing relation between string and oud body easily. (Full text of my work including this image and others is here, but only Turkish now ; https://www.academia.edu/17198067/Telli_%C3%87alg%C4%B1larda_Ses_Tab... )
Actually vibrating string itself radiates an insignificant acoustic energy. It has to transfer its vibrations to the much larger area, such as top,
body and inside air. this interaction is called coupling (image). If coupling of your oud is strong, your instruments sound is strong, rich, have
specific timbre and easy playable.
first of all; activity(magnitude), damping factor and frequency of Vibration modes determine this coupling (we measure these by modal analysis).
Later its important that how to excite these modes. that is, If region glued bride is rigit, symmetric, thin, close to braces etc (we measure these
by sound analysis).
finally, we have to test your idea about the way of mapping bracing according to strings upper partials. But, Actually I'm not sure that there is
strong interaction which we can use. I shift braces only to adjust vibration region area, activity of modes of top and stiffness of region near
bridge. Not only fundamental but also all harmonics of string are affected if they coincide one of them. thus I think we have to focus on mode shapes,
activity and exciting to determine brace location.
Best.
[file]37798[/file]
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