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 Post subject: Physics of the Didgeridoo
PostPosted: 02.12.2006, 16:51 
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Frank Geipel of "Didgeridoo Phenomenon" fame kindly provided this very interesting article for our forum. Frank is an occasional visitor to our forum but hasn't found the time to contribute regularly. I'm really happy to have this here as it provides a lot of insight but will undoubtly create a lot of additional interesting question as well.

So here it is:

Physics of the Didgeridoo

by Dr. Frank Geipel

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About physical basics that are responsible for the typical sound
characteristics of the didgeridoo and lead to its specific playing
style.


From the physical point of view the didgeridoo is a more or less
irregular shaped pipe which determines the shape of the interior
vibrating air column.
According to whether the mouthpiece end is open or closed with the
mouth, we find an irregular sequence of resonating frequencies of that
air column (intrinsic resonances). The positions and distribution of
these intrinsic resonant frequencies depends fundamentally on the shape
of the air column.
Slapping with a flat hand on the mouthpiece and keep it closed, you
obtain the sound of the intrinsic resonances of the one-end-closed pipe.
Opening the mouthpiece right after slapping on it, you obtain those of
the both-end-open pipe.
Every intrinsic resonance of the one-end-closed pipe (closed with the
mouth) is playable with the vibrating lips (basic drone and the series
of overblows or toots). Playing the lowest resonance produces the basic
drone, playing the next higher resonances produces the toots.

This row of the playable intrinsic resonances is not comparable with the
nature scale (e.g. row of the playable overtones (flageolett) on a
guitar string) because by the irregularity of the inside form at the
didgeridoos in comparison to other instruments these are not harmonic.
The sounding harmonic overtones during playing the basic drone
correspond to the nature scale. But generally these harmonic frequencies
don't fit with the frequencies of the intrinsic resonances.

When playing an intrinsic resonance (at the didgeridoo the lowest to get
the basic drone) the harmonic spectrum of the overtones of this
intrinsic resonance resounds.
That means the 2, 3, 4, ..., nfold of the fundamental frequency.
In case harmonic frequencies fit with intrinsic resonance frequencies of
the air column, these will be amplified and become audible as a singing
tone (see below). Harmonics lying between intrinsic resonances are not
amplified and are audible only (if at all) indirectly through the sound
characteristics evolving. That means, the spectrum of the intrinsic
resonances (depending on the interior form of the air column) filters
(amplifies or dampens) the spectrum of the resulting harmonic overtones.

If one sings a note with the voice which falls on an intrinsic
resonance, this sung note is also amplified. Additional the mixture
frequencies (sums and difference frequencies) still arise from basic
drone and sung note which can partly lie under the fundamental
frequency. In typical traditional playing techniques of NW-Arnhemland
the interaction of the intrinsic resonances with the mixture frequencies
(sums and difference frequencies) of the fundamental and the voice are
very important.

The stronger an intrinsic resonance and the more harmonic overtones fit
with other intrinsic resonances the higher is the acoustical impedance
it can feel as backpressure.
The resulting harmonic spectrum (sound spectrum) is partly transfer onto
the wall of the didgeridoo and comes into resonance with it.
The typical sound is transferred to the surroundings directly through
the open end by the air column and a little part indirectly by the wall
of the didgeridoo.
Additionally to the passive interior form depend intrinsic resonances,
in the typical western way of playing the overtones are also influenced
(filters, amplifies, dampens) by the variable resonating spaces in the
vocal tract.
In typical traditional playing techniques of NE-Arnhemland the dynamic
movements of the tongue activates especially the sound of the complete
spectrum of intrinsic resonances of the “one-end-closed-pipe” (e.g. with
the “cut”) and temporarily of the “both-end-opend-pipe” (e.g. with
low-pressure-effects during the fast retroflexing tongue).
This leads to additional percussive drum-like effects and to typical
whirring harmonic overtones, amplified alternately by different
intrinsic resonances (“wobbling” or “ringing” overtones, see below).
The main sound and playing characteristics are determined by the
complete contour of internal cross-sections of the air column in the
didgeridoo and are often influenced most distinctly by the interior form
of the first third of length beyond the mouthpiece.
Many naturally grown eucalyptus-didgeridoos with interesting
sound-features chosen by Aboriginal makers have interesting structures
in that area.
That is one reason why sound characteristics of slide didgeridoos or
self-made simple drilled instruments are often very restricted because
the most interesting area is dominated by the less interesting
cylindrical shape.

Oversized bells
(inner diameter >12 cm) often result in a loud instrument. Mostly this
volume hides the lack of an interesting sound characteristic but
nevertheless is rather interpreted or perceived as a positive sound
quality by untrained listeners.

Harmonic wobbles
Quickly alternating amplification (5 – 10 times per second) of different
overtones in instruments with a defined alignment between intrinsic
resonances and harmonics, obtained by using certain traditional playing
techniques.

Singing harmonic
Distinctly perceivable harmonic, amplified by an intrinsic resonance of
the vibrating air column.

28.3.2006 - Frank Geipel
www.didgeridoo-physik.de


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 Post subject:
PostPosted: 03.12.2006, 11:46 
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Cheers for that Christian. Unfortunately I find myself unsure as to what I understand and what I think I understand :oops:

Rather than using it to explain why certin styles have/haven't certain harmonics can we apply it to play with the harmonics of a particular didge to create more audible and combination harmonics.

Other than just expertimenting and seeing what sounds are amplified/created by siting and playing around is there a way we can tell what the possible harmonics of a didge are and then specifically find the vocal pitch to act with them for a given effect?

Or have I just lost the plot entirely :roll:

Kev :lol:


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PostPosted: 03.12.2006, 14:12 
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Joined: 12.05.2006, 14:34
Posts: 83
Location: Rome,Italy
Thank you Christian for this page....
I have the book as you refer "didjeridoo phenomenon". Good book....especially the chapter of Djalu and Charlie Mc Mahon.


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 Post subject:
PostPosted: 04.12.2006, 00:01 
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Joined: 24.08.2006, 00:30
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Location: dacusville, south carolina
I agree with long dog
I found this http://www.phys.unsw.edu.au/jw/yidakididjeridu.html that sheds light on some questions that I had conserning passive voice and stuff, check it out.
I hope I'm not being too abstract
high tongue low tongue
Vocal tract resonances


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PostPosted: 04.12.2006, 08:34 
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Joined: 24.03.2006, 11:02
Posts: 11
Location: 07545 Gera - Germany
john bylund wrote:
I agree with long dog
I found this http://www.phys.unsw.edu.au/jw/yidakididjeridu.html


I think that this page is good for looking at the lips while playing the didjeridu (who can normally do that). The other fact of this side is a little bit disillusioning. The physic backround, Mr. Hollenberg based on are too old I think. Frank Geipel is far ahead this work. I would be pleased if these researchers could cooperate with each other to melt their knowledge. In my eyes, now the australian fraction of researchers is doing nearly the same work Dr. Geipel has done in former years. Thats not neccessary...
But the Videos are awesome, not only for beeing so large :-p

Mario


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 Post subject:
PostPosted: 04.12.2006, 10:21 
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Joined: 22.03.2006, 21:12
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Location: Italy
Hi all,

The only real difference between the work of Lloyd Hollenberg, and that of Frank Geipel, is that one is a serious academic analysis of the detailed acoustics of the instrument and that the other is a more popular rendition of the same acoustics. (with some additional more "modern" an interesting theories).

The other interesting difference is the focus of Frank geipel in this article in the explanation of effects that arise in traditional playing, something that the australian group has not yet got to. In these aspects it would indeed be interesting if these groups could collaborate a bit more.

However, if you really believe that the work of australian researchers is so old fashioned could you explain perhaps why?

If you read carefully through the archives, you will also find that much of the discussion in the first two thirds of Frank Geipels article is already present in discussions that we have had here over the last six months or so and as such reflects the general modern perception of the acoustics of the didgeridoo in particular in our discussion of the passive voice. The first
reference to heterodyne type effects and there contribution to the complex sound of the passive voice was in the article John Bylund refers to by the australian group.

The didge is an instrument in which there are at least four different sets of frequencies interacting.

- The intrinisic resonances of the tube (that coincide with the toots).
- The natural harmonic series of the played note, (frequencies all integer multiples of the fundamental frequency).
- The resonant frequencies of the oral cavity (that can be modified by shaping the oral cavity and moving the tongue about)
- The voice and resonance of the vocal tract.

These all interact in simple and complicated ways to provide the overall sound. Frank' s article posted above is a nicely written account of the simpler effects.
The research of Lloyd and his colleagues is based on a deeper analysis of a simplified model of these effects and in particular the coupling between throat, oral cavity and didgeridoo - this is not old-fashioned but rather due to the fact that the rigours of scientific investigation require one to make simple models with which one can investigate various hypothesis.
If you read the paper carefully however, you will basically find the same discussion of acoustics in that paper as in the article by Frank Geipel posted above.

The two newer things mentioned in Franks article are the harmonic wobbles, and the both end open effect created by the retroflexed tongue. Some more details on these effects would be quite interesting to read, pity that frank does participate more actively so that we could have a bit of a discussion about this.


cheers,
martin


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PostPosted: 04.12.2006, 11:26 
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Hi Martin

Can you or someone answer my question:

Quote:
Other than just expertimenting and seeing what sounds are amplified/created by siting and playing around is there a way we can tell what the possible harmonics of a didge are and then specifically find the vocal pitch to act with them for a given effect?


So can your average musical numpty (me) with out a physics lab find out what the peak harmonic fequencies of their didges are inorder to incorporate them or elliminate them? Or even identify desirable ones that are quiet and find a vocal harmonic to enhance them?

By the way, I have read Franks article where he built a didge to his desired harmonics according to the formulas and it worked which I thought was pretty impressive and useful for a didge builder to be able to do.

Kev :?:


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PostPosted: 04.12.2006, 12:02 
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:wink: :wink: hi kev,
sorry, i missed the question..

just play all the toots you can.
these are the intrinsic resonances.

cheers,
martin


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PostPosted: 04.12.2006, 12:13 
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Oh Ok :lol: Told you I was a musical numpty!

I thought I'd be able to analyse a drone spectrum and pick out the strong harmonics in the drone and then play around vocally to specifically enhance any of interest or try to create the combination harmonics :oops:

Kev :(


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PostPosted: 04.12.2006, 12:53 
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you are no numpty, certainly practical.
your idea is good just remember that the spectrum is a mixture of the natural harmonics of the fundamental note and the intrinsic harmonics of the didge.

Maybe there are some other tricks though, ...

cheers


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 Post subject: some answers
PostPosted: 16.12.2006, 16:06 
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Hi all serious guys,

with this I try to answer some questions from the discussion. Hope my relatively poor English is understandable. It is not so effective for me to write in English. Therefore the english part of our website isn´t ready. In the german part you can find more information.

to Longdog:
With my simulation-tools I am able to find out what the possible intrinsic resonances, the possible harmonics and the amplified mixed frequencies of the voice and intrinsic resonances of a given interior form. Also where is a singing harmonic or better a harmonic wobble pattern. But the effort to do that with a lot of simulations is only usefull (for me) for the creation of new instruments with especially play- and sound characteristics.

When you have a given instrument (a realy hardware in your hand) the experimental way during playing is the most effective way to explore what is possible.

To create a didgeridoo with special characteristics (fundamental, the overblows, special harmonics singing or wobbles, the backpressure, the resonances for voice mixed frequencies, …) is not an easy way. My way is to create interior forms and simulate all the usefull spectra. Then I am work like an optimizer to find better interior forms step by step. The most “know how” is the interpretation of the simulated spectra. Therefore it was possible to do a lot of didgeridoo playing with FFT-analysis and learn about the interpretation. When I see a FFT-analysis (in the right scale) from a didgeridoo sound I have good conception of the possible sounds. Also the perception of special harmonic effects requires a good training of the hearing. Some characteristics about resonances of the wood and the “radiation” of the sound I solve with empiric know how. The more “dynamic” a didgeridoo is played, the more information about the interior form can found in it. The most information I can extract when playing the typical NE-Arnhemland techniques. Tuning or changing the distribution of the intrinsic resonances in a wished way is often a time consuming thing.
1) not all combinations are physically possible
2) the complete contour of internal cross-sections of the instrument determines the distribution and strongness of the intrinsic resonances, playable sound characteristics and backpressure. Tuning a given instrument, I have to measure the cross-section contour and analyse the impedance and sound spectra by FFT. Then with simulations I can find out, if a tuning makes sense or not. If yes, on witch locations in the instrument I have to change the cross-section. About tuning and creation of didgeridoos I could write an own book, if I had enough time.


To Mario and Martin:
Thanks for the discussion. I know the work of the australian group. I had also contacts to Lloyd Hollenberg and other scientists. They do a typical academic scientific and good work to explain a lot of effects during didgeridoo playing in a scientific group. It would be a nice thing for me to work in such a group, but for my person I am a biotech. It is my private hobby and I have no group. I am very impressed about the native players and the used traditional playing techniques. My motivation is not to generate scientific publications in scientific journals (I did this during my university time in the field of biotech). My motivaton was to find methods to create special instruments they support special traditional playing techniques. Because I was not able to find enough usefull scientific work about that in 2002-2003 I had started my own research. In my opinion the passive acoustics of the instrument itselv was often neglected. I do my research only on topics they support the practical self creation of interesting sounds and feelings. The article in the “Didgeridoo Phenomenon” I have written to support the understanding of the complex stuff to an average didgeridoo player. I wonder a little because I havn´t got a reaction from the australian group.

Now I have very more know how than written in the little paper and I use it to create very special instruments by simulation experiments. This instruments I build by myself (with some tricky methods) or with some talented woodworkers in Europe. At the moment I work with Bongossi (an african ironwood). My favourite field is to create yidaki-like instruments with several harmonic wobble characteristics. But to play this effects you need a special “traditional” technique.

(Listen to the sound example from Sven)

The creation of good mago-like instruments is also very interesting. You have to create an interior form where the sum frequencies of the fundamental of the instrument and the special voice frequencies is amplified by an intrinsic resonance between two harmonics. And for higher pitched instruments the difference frequencies of the both is deeper than the fundamental and audible.

Happy droning and wobbling

cheers
Frank


Attachments:
File comment: playing technique induced harmonic wobbles with a special Yidaki
SM-E-short.mp3 [701.63 KiB]
Downloaded 515 times

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 Post subject: Re: Physics of the Didgeridoo
PostPosted: 14.09.2008, 16:32 
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WOW
your good!

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 Post subject: Re: Physics of the Didgeridoo
PostPosted: 04.10.2008, 09:51 
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After 6 years of private theoretical and experimental (low budget) research driven by curiosity and the longing after individually playable desired instruments a highlight of the common Didjeridu research project (Geipel / Reimer) is reached now.

Many Didjeridu players from all over the world want to know whether it is possible to create an instrument after its individual desired conceptions. Since some years that is possible with our Computer-Aided-Didjeridu-Sound-Design-tools (CADSD), but required much experience and simulation time, since many interior forms had to be adjusted and varied by hand, to get the desired relevant impedance and sound spectra.

So far it was not possible to find the interior forms for desired spectra in a more efficient and automatically way. In order to reach this, I started at the beginning of 2008 the project „Didge(R)Evolution“- the application and advancement of nature modelled evolution algorithms for the generation of desired forms from practically immense, infinite large variety of possible forms.

The current project highlight is a simulation system, which is able to generate desired instruments on basis of an advanced high performance simulation model of the Didjeridu acoustics and using new Didjeridu specific directed-evolution-methods. It runs like a “living evolution system” in which special instruments with many specific parameters can quasi “breed”. The so created interior forms often looks similar the cross-sectional contours of good termite-carved instruments, with the difference, that the method is able to generate also forms which do probability not occur in nature.

http://www.didgeridoo-physik.de
ComputerAidedDidgeridooSoundDesign
Didge(R)Evolution

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