Playing the Didgeridoo: Newsletter Challenge (09/25/07)

The tube acts as a simpler oscillator, the breath only acts to "pump" the oscillation...adding enough energy to maintain the standing wave. Hence the volume of air passing into the tube from the player has very little to do with the nature of the wave or the sound which is a characteristic of the pipe.

My daughter made a set of "paddle pipes" hollow tubes struck on the end with flat rubber coated wooden paddles. The pipes were tuneable purely by altering the length of the resonator......striking the tube harder made the sound louder....but with no alteration in pitch, although some aquisition of higher order resonant frequencies.

For you to hear the sound, air must be displaced......but there was no air input or energy input following the initial strike with a paddle pipe hence the standing wave decayed and became silent.

As mentioned earlier there is a long wavelength and high air wave velocity although low frequency....hence quite large air mass movement at the end port of the tube.

If it is accepted that the volume of air displaced past the end port position is many magnitudes greater than that introduced by the player to maintain the oscillation...then an overall vacuum in the tube is created if the equalisation can't take place at the exit port. Subsequently anything blocking the tube will be exposed to increasingly greater external air pressure as the high pressure "peaks" leak past the seal created by the leaf....rather like a flutter valve in a pump.

This process will only continue until the internal and external pressures on average equalise...then the leaf remains stationary. Driving the oscillator harder....increasing the energy or magnitude of the standing wave will increase both peak pressure and trough vacuum pressure and the external pressure will drive the leaf further into the tube....stop driving the oscillator and the pressure will equalise from either end through the player's mouth, or through leakage past the leaf which doesan't make a perfect seal, and it will drop off the tube.

This response offered by bonesrus.....a non engineering but physical science background type person

Hi my name is Bruce Copley and if you would like to see more about what is possible with the didgeridoo check out Bruce Copley didgeridoo on You Tube

yours in rhythmic breath

bruce copley

One last (hopefully) point:

Some have claimed that the standing wave will hold the leaf on with no air flow, citing the example of a "flutter valve", which opens and closes. However, unlike the flutter valve, which must have some sort of hinge or pin holding one (or two) side or edge (or even center) point(s) to be fixed, the leaf has no such fixed attachment to the musical instrument, which must then negate this as an example.

OK, one more LAST point (I really mean it this time! ):

Maybe my understanding of the terms falls short here, but wouldn't the regions or peaks of high and low pressure in a standing wave be considered "anti-nodes" rather than nodes? I thought a node in a wave was the point that did NOT change, having a value of zero, as the wave travels and the areas between nodes flip from positive to negative peaks (anti-nodes). Wouldn't this be the same definition in a standing wave, i.e. the node being the point of equalization between the two anti-node extremes?

On the subject of nodes and antinodes, I'll follow your lead elsewhere and say "that depends". In this case on whether you are referring to levels of pressure variation or velocity variation. In general, the pressure node (just outside the open end of the didj) corresponds to a velocity antinode, and vice versa (actually effectively just outside the closure at the mouthpiece end). For them others: peaks of pressure variation are pressure antinodes, etc.

I'm being a little pedantic here referring to peaks/troughs of "pressure variation" rather than of "pressure", because loose terminology in this area seems to have caused so much confustication* in this thread
*A portmaneau word denoting (totally?) confused explanation.

Fyz

For them others: peaks of pressure variation are pressure antinodes, etc.

Right, that is what I thought. Of course a "peak" of pressure variation (maximum of absolute value) can be either a "valley" of pressure (negative) or a "peak" of pressure (positive), and so are anti-nodes.

Many people posting here wrote of positive and negative pressure "nodes", which made no sense to me, since they were referring to areas of maximum magnitude, and so should have been more rightly called antinodes. I don't think anyone called them velocity "nodes", except yourself, which I understand, since they are 90 degrees out of phase. A good analogy is voltage and current in a purely resistive (no inductive or capacitive reactance) or a tuned (inductive and capacitance reactances being balanced) AC circuit.

Uh, oh.....my "ped-antics" are showing again!

For them others again - over time, the pressure at any position in the didj varies from a minimum to a maximum - but (at least with steady-state sound) the average pressure at any point cannot vary with position - except by a small amount associated with the variable flow velocity of the exhaled air.

Pedestrantic Fyz

I understand your troblem!

Just to throw a different areo-dynamic principle into the mix.. The leaf was most likely curved. I suspect a COANDA effect on the leaf itself.

I suspect a COANDA effect on the leaf itself.

Hmm, perhaps, being in Australia, it was the KOALA effect. That is, a KOALA bear was chasing a eucalyptus leaf and while swiping at it created air currents that pushed the leaf against the end of the didgeridoo! By playing harder, the musicion cause the KOALA to grow more excited and increase the intensity of his waving arms (legs?) and therefore the force of the air current against the leaf. The KOALA then relaxed when the playing stopped allowing the leaf to fall.

The next question is, what was a KOALA and a eucalyptus leaf doing in a music shop? Perhaps John and his new-found friend, the shopkeeper, were having their music lesson outside in the Outback, or perhaps at the Sydney Zoo (do they have Koalas in the petting zoo?).

ROFLMAO!

I suspect you are correct - that the leaf is initially attracted into the curved region of the (acoustically-driven alternating) flow around the end of the pipe. Thereafter, the closeness of the leaf to the pipe generates a region between the leaf and the edge of the pipe where the air naturally flows faster, and we see a combination of Coanda and Bernoulli.

Fyz

Hey Kids this is my first ever Blog entry but your discussion has been so interesting I just had to get involved.

I think that the leaf gets caught in a low pressure area between two high pressure 'nodes' and if the leaf moves into the tube it would be due to a change in frequency.

Swirls by air keeps it attracted towards did.. even sound is created inside this instrument when membranes vibrate ...emm already answered

i would like to design something around this theory what............

HI i am a seasoned didgeridoo player and as a gig leaving / party trick, i walk round a room playing the didge whilst it holds onto a £5 note at the end, when i stop playing the note falls away.

I am not a scientific person but when i play the didgeridoo it creates a vaccuum, the wood ressonates and you can feel air on the way out at the end of the instrument, but as above it creates a vaccuum also.

Why this happens i have no idea, i just know the trick works well everytime.

You do need usually an E key didgeridoo with an approx 90mm opening though!!!

Enjoy

Didgeridoo Pete

www.idoo.info

Read above, its not a vacuum as such, its standing waves.

If it was a vacuum, it would suck the note in, not allow it to pulsate on the end.