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

I must admit that, given the diameter of the base of a didgeridoo I find it a bit surprising that there is enough Venturi effect to suck a leaf into the pipe. Perhaps this particular instrument had a shape that resulted in unusual flow at the end?

Fyz

Not Venturi, but Bernoulli!

If the horn had a hole somewhere midway between mouth and opposite (open end), then a leaf might stick to it by the Venturi principle. Air flow across an opening causes a decrease in pressure, causing air to flow into the hole from the side opposite the air flow.

Not applicable in this case.

If you want to out-pedant ER, that's your prerogative. However, the Venturi principle refers to a narrowing relative to the bulk of the flow giving a higher velocity and related lower pressure. As the end of the didgeridoo is open to the atmosphere, a higher velocity around the pipe side of the leaf than on the open side is the only mechanism I can see that would attract the leaf into the pipe. I doubt the continuous air flow would be anything like enough; however, it is conceivable that the acoustic resonance (highest velocities at the open end of the pipe, and a very low acoustic impedance due to the small size of the orifice relative to the wavelength) might just produce enough cyclic velocity to hold the leaf in place. However, I think I'd need to see it for myself and to try it with and without the sound. For what it is worth, I couldn't replicate the effect with any of the large-orificed pipes sitting around my garden.

You are correct, for the Venturi effect to take place a constriction is necessary, but unless the leaf is held in place by some outside force (in which case the problem is moot) the Venturi effect will not happen. I was thinking of certain equipment where a vaccuum is created by porting a Venturi tube at its constriction. It is still an application of the Bernoulli principle. However, I believe that what we have here is a simple application of the Bernoulli principle using air flow to create an area of low pressure across the entire surface of the leaf including the center where the flow must split. If the Venturi effect was exhibited, wouldn't the leaf only pull in at the outside edge of the end of the instrument, which would be the constriction, as that would be the area of low pressure? Maybe this (Bernoulli vs. Venturi) is just semantics (or is that pedantics? ), I am not sure, but I still believe that the cause is air flow and NOT standing waves from vibration.

I think it is the constriction to the flow that causes the effect - whatever you choose to call it, and the effect is observed throughout the area adjacent to the higher speed of flow. Whether correctly or incorrectly, I have habitually used either Bernoulli or Venturi when the higher speed of flow was induced by a constriction to a pipe or at the output of a pipe, but exclusively Bernoulli when the flow speed was induced in the open - as by an aerofoil or similar. Whatever, you are the only person (so far) to pick me up on it. If/when I can work out a uniquely correct usage, I'll try to stick with it...

As I said, I don't know either, but it's hard to imagine a high continuous flow-velocity being achievable at the open end of a didgeridoo (particularly not if played "correctly" using recirculatory breathing* - though it can be surprising what skilled practitioners of apparently unpromising techniques can achieve). However, I do know that vibratory velocities in wind instruments commonly exceed the continuous velocities by a wide margin - so the bigger question in my mind would be whether the leaf would redirect the vibratory air flow at didgeridoo frequencies or simply move with it
*I say this on the basis that the only classical instrument that I know where recirculatory breathing is commonly used is the oboe, for which air flow rates are notoriously slow - it's a instrument where you commonly need to breathe out at the end of a phrase)

I think the only way to agree a conclusion would be via a suitable experiment - using a genuine didgeridoo, and fixed duration of exhalation, and with and without sound.

Fyz

I was in Australia a year ago watching an aborigine playing the didgeridoo. You can use thinwall PVC pipe as well (as he showed us). there is no unusual shape. I think it is a majical leaf from the Wheels of Time (ter 'angreal).

Interesting, but how big is the instrument and how is the end shaped. If the end had a strange shape, I suppose that the effect could have been a lot like that of an air foil. This might be another of those puzzles with insufficient information, otherwise I am not sure. Have we any Aussies out there in cyberspace who can shed some light on this.

A didgeridoo. This particular instrument is more ornate than most. Taken from Wikipedia. They are between 1m and 2m long and conical in shape. how a venturi is developed I dont Know, maybe its a gum leaf, and it might be a bit sticky?

Gummed up? But the leaf dropped of when John stopped blowing. BTW, what's the leaf doing blowing around inside the music store?

Maybe it was a "leaf" of paper, as in "loose leaf notebook"?

I see by your Avatar that you are in Australia. Have you ever played one of these devices? Is the sound that they use in the "Outback" commercials, that which provides background audio as a low rummmable, from this device? Of course maybe Outback Steak Houses are not popular in Australia although I have been a couple in Korea. In all my travels I have never been to Australia although I came close to it when the company I worked for was doing a project for Goro Nickel. In fact I have never been south of the equator.

No I have never played a didgeridoo, But as I got into bed last night after watching and old steam train video, I thought smoke rings? maybe the effect is the same. So if there are some smokers out there, then roll up a piece of paper, take a lung full of smoke and pulse the smoke out through the paper tube. A bit of R&D might help?

Regards JD.

The frequencies and variations of the sounds are created by cyclic (if thats a word) ie: inhaling and exhaling. He may have been on the inhale cycle.

That's not what is meant by "circular breathing". This is is a technique by which you blow out through your mouth all the time. You breathe in only through the nose, and while you are exhaling from the lungs you allow your cheeks to fill with air, which you then use to blow from your mouth whilst you are next breathing in through your nose. It's ..... difficult, and I can't imagine that John would manage this on his first lesson, unless he's already mastered the technique for some other reason. I expect that's why he had to stop blowing and the leaf fell of.

Fyz

Thanks for the info. Since I'm not musical, the only thing I play is the radio! (pounding on drums only counts for taking out my frustration.) I'll settle for the low pressure created by the harmonic frequencies.

The dimensions of the didgeridoo will probably be the determining factor here. We can see from this illustration that the instrument covers the player's mouth.

So, the player's breath does not cause the didgeridoo to vibrate as in most wind instruments, the player's lips do. Blowing harder increases the vibration of the lips and that makes the instrument louder.

Because the didgridoo is 1 to 2 meters (3 to 6 feet) in length and any stream of air caused by blowing tends to decay after a few feet, air probably does not come out the end. The vibration of the instrument might create small eddies or vacuums that draw air in.

Or, what I think is most likely, what happens is that the air comes out from only one side of the end, and this stream of air pulls in air from the other side.

I don't know for sure, but that is the best explanation I can produce right now.

I assume that you mean that the air-flow at the output will not depend on the configuration of the flow near the lips (in which case I agree).

Fyz

I suppose if the instrument sets up a standing wave there could actually be a low pressure zone just at the outlet of the tube. This would cause the leaf to stick, but that is a bit far fetched and the chances that you are going to get the perfect combination of factors in an instrument to meet this condition are going to be rare.

Its is just a hollowed out log... one end for breahe to enter the other end air comes out!! Yes am from Australia!!!

I will hazard a guess at the harmonic vibrations set up in the didgeridoo! When John blows, a low pitched brrrrrrrrrrrbbbbbrrrrrrrrrr sound comes out of the other end which has a frequency just in the right zone to keep the leaf in its place. when John blows a little harder, the frequency is the same but the amplitude is higher, thus vibrating the leaf up the shoot so to speak! (Think of a ram pipe on an air filter box!)

Taking a stab at it, I would say because the didgeridoo is conical in shape, it acts as a nozzle in reverse? The velocity and pressure is established at the mouth piece, as it travels along the didgeridoo the velocity drops and the pressure increases to that which was in the player mouth before blowing into the instrument, and if the volume of air is just enough to fill the inside of the didgeridoo, between pulses. Then the inertia of the expanding air can possibly create a partial vacuum momentary. And the gum leaf which is not really sticky is held there till he stops? He's not in the music store but under a coolabah tree.

These 2 posts (9&10) seem to come closest to expressing the first recollection that comes to my mind.

(Being an ex French Horn player adds no insight whatsoever!)

"Mr. Wizard", "Bill Nye the Science Guy" and others I am sure have demonstrated the principle by sticking a pin thru a small piece of stiff paper, inserting the pin into a thread bobbin, and challenging kids to blow the pin & paper out (via the open end).

The harder you blow, the lower the air pressure (due to "You-Know-Whose" theorem/principle) on the bobbin-side of the paper... thus, the higher pressure on the opposite side ("free-air") presses the paper ever harder against the bobbin.

"Duh?", or "No Duh"...?

I've performed this experiment countless times (on a much smaller scale) using lycopodium powder to investigate the vibrational structure of various resonators. Powder near to maxima of the vibrational velocities is loosened from whatever might tend to hold it in place, so all the powder eventually comes to rest in positions where there is little movement (the nodes of the vibration). Unfortuately for this as an explanation, the open end would be where vibrational velocities are greatest.

BTW, the distance between the pressure zones depends on the frequency of the vibration, not on its amplitude.

I believe the low pitch sound wave in the outlet end of the instrument creates a reverse impact force on the leave cousing the leave to stick to the inner wall of the instrument.

My guess is that there is negligible wind flowing through the pipe, but there is a higher pressure in the center of the pipe than at the open end and the pressure differential depends on the loudness of play.

When the loudness level is increased ("When John plays louder") there may be a transient of air flowing into the pipe from the open end.

Jorrie

bernoulli principal

MDaubert

Could this be the result of the low frequency vibration that is holding it in place.

I believe that several posters had elements that were correct, but none had a complete answer. However, ndt-tom was probably closest when he gave the Bill Nye example, assuming 'You-Know-Whose" refers to Bernoulli. Guest #17 also said "bernoulli principle" but did not elaborate. Interesting that jdretired started in the right direction describing the air flow and calling it a "reverse nozzle" but then went on a tangent.

Here is my take: When the player blows into the instrument he begins air flow down the horn. Initially this is high pressure, and low volume flow, because he purses his lips to make them vibrate, similar to a brass instrument, not a woodwind/reed instrument. However, the shape of the horn, encompassing his whole mouth and flaring out somewhat towards the open end (here is the "reverse nozzle" effect) reduces the pressure, and increases the volumetric flow somewhat. At the open end there is still not enough pressure and air flow to cause a blowing effect that might deflect the falling leaf. As the leaf falls against (or is somehow by chance blown against by external air currents) the open end, suddenly the cross-sectional area of flow decreases dramatically, causing an increase in the velocity of the air across the leaf on the horn side. Since the outside of the leaf remains in relatively still air at ambient pressure, the Bernoulli principle takes effect, creating an area of partial relative vacuum, or negative pressure differential between the horn side and the other side, keeping the leaf in place and partially "sucking" it into the horn.

Bernoulli said that if air flows over two opposite surfaces on an object (the leaf), the side with greater velocity of air will have a lower pressure relative to the slower air, and so a force is exerted by the higher pressure against the lower pressure side, the horn in this case. Since the ambient air is relatively still, its relative velocity is near zero, while the horn side has a positive velocity, causing lower pressure on the horn side.

Blowing harder increases the air flow and velocity and increases the pressure differential, sucking the paper in further. When the player stops blowing, air flow stops and pressure is neutralized, allowing the paper to fall off.

An experiment is required here:

Place a piece of paper over the end, hold it there with tape at the top to form a hinged flapper valve. Blow into the other end, slowly increasing the air flow and noting if the paper is blown open at low air flow, and then held against the end when the sound ( and standing waves) begin.

The wind from the instrument had the same effect on the leaf as the air moving over the wing of a plane giving the wing lift, in this case a leaf.

Well, we seem to have a healthy difference of opinion as to whether the leaf effect is caused by air flow and the Bernoulli principle, and has nothing to do with the frequency and standing waves (my view, seemingly shared by others as well) or by the more exotic (I am not saying incorrect, because I just don't know for sure) case where the vibration of air is the causal effect and standing (pressure) waves set up areas of negative and positive pressure, with a negative pressure zone being at the end of the tube where the leaf is held on. I believe I have stated the other case succinctly. If not, someone please correct me.

A simple experiment could prove this. If a long PVC pipe similar in size (length and inside diameter) to the didgeridoo were to be constructed (one could probably play it the same way and get a similar sound and I am thinking it has nothing to do with the slight taper inside) and a small speaker or other type of audio transducer were placed at one end, fixed in place, then sealed up with only the connecting wires protruding, the wires could be connected to an audio oscillator to drive the speaker/transducer. This would produce sound with zero air flow, but setting up the standing waves and varying the frequency until just exactly the right pitch was found. A small piece of paper somewhat larger than the opening could easily substitute for the leaf. Would a frequency be found that would "suck" in the paper? Presumably increasing the amplitude (power) would then cause the paper to suck in more, which would result in visibly increasing the flexing of the paper. Turning off the vibrations should then allow the paper to drop.

On the other hand, take the same tube, seal up one end, introduce a small air tube or pipe (no vibrations, air pressure and flow only) through a hole in the sealed end, and using a pressure regulator and appropriate valving vary the air flow and pressure and see if we can't get Bernoulli's principle working at the other end of the tube on the small paper disc.

My money is on Bernoulli!

Either way it's Bernoulli - the energy in the standing wave at the open end of a pipe being primarily kinetic energy (velocity) and not pressure. Oh, and the velocity differential between the side of the leaf inside the pipe and that outside is caused by the constriction of the pipe versus the lack of constraint outside - so it's the Venturi effect as well

Fyz

So which is the primary factor working here, the standing wave from vibrations or the continuous air flow through a narrow opening? I think it is the air flow.

I also think you are stretching things (and being pedantic yourself) just a bit to call a standing wave effect "primarily kinetic energy (velocity) and not pressure". If you want to get really technical, air pressure is just a kinetic effect of the energy of the air molecules whizzing about (O₂, N₂, CO₂, Ar, etc.)!!! Now how is that for being PEDANTIC!

I'll give you that Venturi thing anyway. It's just a special case of Bernoulli.

Do you think that vibrations alone without continous air flow (as in the transducer set up described above) would cause the "leaf" or paper to get "sucked in"? I don't.

But hey, prove me wrong!

On the other hand, we have the Ping Pong ball and funnel experiment. Much like the Bill Nye spool experiment, place a ping pong ball in the center of a funnel. Hold it there, even upside down where gravity should allow it to drop when you let go. Start blowing into the funnel through its orifice, or better yet attach a low pressure continuous air flow source and watch as the ball not only does not fall out, but actually get sucked in as you blow! In this case, the ping pong ball does much the same as the leaf on the end of the instrument, restricts the air flow so that the air must move faster. No vibrations here, just good continuous air flow.

Vielen danke, Herr Doktor Bernoulli.

I think they demonstrated a loudspeaker driving a quarter-wave resonant tube with a hole in the side near the open end at high-school around the same time as the lycopodium powder, and that it sucked (no slang intended). This may be false memory, but I'm almost certain that fluid pumps have been made on similar principles.

You mention the ping pong ball and funnel experiment. This is a nice thought but keep in mind the Didgeridoo is a straight tube, not at all conical and the funnel has a very wide conical shape in comparison.

I still lean toward the standing wave theory.

I cited the Ping Pong ball experiment not as proof but as an example of what is possible using only human breath and the Bernoulli principle. Obviously some people are better at controlled breathing or even "circular breathing" (I am quite sure I can NOT do that!) and could do a better job of exhaling for longer periods of time. That is why I suggested the continuous air supply as an alternative.

The Ping Pong ball illustrates how something light could be drawn in simply by blowing air around it on all sides, going against the normal idea that blowing down a tube at an object would simply cause it to blow away. Despite the differences in shape, this is very similar to the mechanism that is, or could be, at work in holding the leaf on.

I still lean away from the standing wave theory, if only for the fact that, if the theory is correct, the leaf is alternatively attracted and repulsed, or did I misunderstand how this theory was presented by its proponents? If it is alternatively attracted and repulsed, then the net effect should be zero, allowing gravity to carry our hapless leaf away, like feather in Forest Gump which alights momentarily, then is quickly blown away by the winds of change. (Pardon me for waxing poetic.)

However, it might be possible that both forces are at work. The only way to know for sure would be to try vibration without air flow and airflow without vibration.

Then the question arises, has this ever, REALLY happened with a didgeridoo? Or, is this just a flight of fancy, with the writer surmising what COULD possibly happen, more of a thought experiment than an actual, documented phenomena?

Hmmm...

According to the acoustics-plus-Bernoulli theory, the leaf is attracted whichever direction the air is flowing. Obviously, there is no net force due to flow when the air is stationary, so the leaf is never significantly repulsed. Other acoustically based theories may have been implied, but this is the one that seems plausible to me.

Fyz

Ah, I see. Much like an AC electromagnet, whose magnetic field reverses 60 times/second (or 50 times/second in Europe and other parts of the world), but still attracts ferrous metals 100% of the time.

I might just buy that one!

Yes, we're slower in Europe. Not so good for old-fashioned ballasted lighting or indeed for the amount of iron you need in a motor or transformer; and you need bigger capacitors to smooth a DC supply. But slightly safer for electrocution. Strange how long it was prior to the present safety-paranoid storm-trooper regulations that the systems were chosen.

I worked for the US division of a German Machine Tool builder who built custom metalcutting systems for US customers. Originally the machines were totally engineered, assembled and built in Germany, with only USA control panels (and so USA specs for starters and transformers, etc.) coming from our plant in St. Louis, then "married" to the mechanical equipment in Europe.

The machine was then demonstrated for the customer during a customer approval "run-off" in Germany, but of course the toolspindle and ballscrew drive motors (and so "Speed and Feed" rates for metalcutting) were slower on 50 Hz than they would be on 60 Hz. One of our customers had a hard time understanding that this would affect the cycle time of the machine, and insisted that the run-off could not be completed until the "correct" cycle time was achieved.

Solution: the Germans rented an AC power frequency converter just for the motors, and POOF, cycle time was correct. Although not the primary reason, the next machines built for this customer would be fabricated in Germany, but assembled and run-off in the US on 60Hz power!

Actually I was not thinking in the instantaneous frame of mind but looking at the steady state phenomenon. An example of this would be an RF transmission cable with standing waves where you can feel significant temperature differences as you run your hand along the cable. In this case you end up with nodes of high pressure and others of low pressure.

Later on in this blog a fellow posted a comment with a link to experiments on the didgeridoo.

You're possibly just being a bit sloppy with your descriptions - unfortunately, the result is that the only natural interpretation of what you write is nonsense.
The average pressure is normally the same at all locations of a sound wave. If it wasn't the wave would cause net air flow, not reciprocal motion. What changes between node and antinode is the amplitude of the variation of the pressure through the cycle. Just to confuse matters further, the pressure antinode is the velocity node and vice versa.

Ok! If we take your approach then we eleiminate standing waves as an explanation. Also we have eliminated Venturi since the air flow is way too small and the geometry of the tube can not support it. Also eliminated is the Bernoulli effect since again the air flow is non-existant.

So what does that leave us?

I don't think that my comments eliminate standing waves as a source of motion that can result in local pressure minima. What I intended them to say was that expressing the wave itself as pressure minima and maxima that is misleading. There is no doubt that the wave can indirectly generate local net pressure effects via the velocity of the air stream and the Bernoulli effect. The point is that the sound generates alternating velocities, and these are enhanced near an orifice that is small compared with the sound wavelength. The essential feature is that the pressure gradients and the dynamic effects can balance.

Fyz

Didges can be straight bored or conical bored, it all depends on the shape of the branch from which they're made and, for traditional "manufacturing" methods, what mood the termites were in when they bored it out.

Personally, I favour the so-called standing wave theory, but not had enough time to
a) read all the posts, or
b) think about it
this week.

I don't think it's Bernoulli (a lad a uni had a bear called Bernoulli; mine were Nusselt and Prantl), as there's very little flow and the leaf's flat on.

Good experiment design, although it will only determine which of sound and/or airflow are the
"prime movers" for the leaf. It won't actually give the mechanism. For example, if sound is the main mechanism, it won't prove that standing waves are the cause (and as someone correctly pointed out, particles collect in the velocity nodes, whereas the end of the pipe will be a velocity antinode). -- mind you, I can't offhand think of a better experiment!

Here's another possible sound-based mechanism that nobody seems to have mentioned:

Perhaps it's the same mechanism as laser tweezers, where strongly focused laser beams are used to trap small particles and move them about in a controlled way. The particles need to have a refractive index greater than that of the surrounding medium for them to be trapped -- look up "laser tweezers" on wikipedia.

The sound coming out of the end of the pipe is strongly divergent, just like the laser beam in a laser tweezers setup. The only problem with this is that I'd expect the sound velocity in the leaf to exceed that in the air, so the "refractive index" of the leaf will be lower than that of the surrounding medium. As a result it should be repelled from the "focus", not attracted.

Any thoughts?

My money is on the standing waves and yes your experiment should work if someone can carry it out. The reason I think it is not Bernoulli in this case is that the amount of air flow produced by the didgeridoo player is extremely low. As someone else rightly pointed out the sound is produced by lip and indeed vocal chord vibrations produced by the player. Of course if someone is going to perform this experiment they could also introduce a small air pipe to simulate the breathing airflow and mount the speaker to simulate the standing waves. Gee, I wish I was a student back in college with nothing else to do :)

I think it's both sound and Bernoulli - due to the acoustic velocity of the vibrating air. Essentially, the local dimensions are so far below a wavelength that you should be able to consider the effect of the alternating velocity of the air just as if it was a quasi-continuous stream
.

when you blow into a didgeridoo you cause wind static electric so the leaf sticks till you stop. pretty cool huh.

Shocking!

This question is very interesting, and I do love some of the explanations, air velocity, Hmmm yes, could be, but try it yourself getting enough velocity down a 2" bore tube from breathing into it to suck a leaf into the end, yes, the explanations are plausible, but are they practicable?

As far as I can see, its the air pressure differences from a combination of standing waves and the resonance of the tube, as this is where the amplification of the sound is generated, the traveling sound wave as it exits the tube pushes out a pulse of air, then sucks air back in to the same tube, as the sound volume increases, this "breathing" will be more noticeable, similar to a idling car engine, and you place your hand on the end of the exhaust pipe, your hand will get sucked to the pipe between exhaust pulses.

If you try and test the air velocity one, all air will be traveling the same direction, if there is a hole in the tube, then that hole may cause a small vacuum at that point, but not at the end of the tube, this is how the SMC/Festo (and possibly others) generate a vacuum from a +ve air pressure line, using the venturi affect, but where the leaf is being sucked into is the exhaust, thus will not work.

The open end of any wind instrument which is resonating (ie producing a sound like the didgeridoo) is actually the antinode of a standing pressure wave. This antinode is actually an alternating +ve and -Ve air pressure at a frequency which equals the note being played which is why the leaf is staying at the exit - it is actually being blown out and sucked into the end of the didgeridoo at that frequency which to the human eye appears stationary or stuck at the end.

I don't know if this helps, but it's an interesting site. I'm still reading what everyone has posted so far.

Similar to a waveguide, the breath "current" will travel down from the mouth along the "walls" of the diggeridoo creating a bit of a vacuum in the center.

Upon exit the waves will drasticly expand outward taking the "current" with them so the leaf would of course have to be quite close to the exit to be insumed. [This is where the Venturi effect mentioned by others comes in to play...]

Notice, as he plays louder [Voltage increases], the breath "current" also increases, increasing the vacuum, but at a limited exponentially decreasing fraction less as the "Resistance" [the volume of the "waveguide" -minus- the diameter of the vacuum in the center] increases also. This also explains why diggeridoo's and waveguide's have a relatively narrow range of operating parameters.

And why diggeridoo's are only played in the outback, where there are no leaves!

tom

=-==

Perhaps someone can tell what wavelength ranges are played with the instrument. It may be that the leaf is affected by the pressure of a standing wave.

David

Cyclical breathing technique required to play didgeridoo means player is breathing in at the same time as expelling controlled air to cause lip vibration. Hence the leaf is pressed against the end of the instrument and moves inwards as he plays harder.

John is a technical genius to learn this in one lesson.

Surely the breathing in is done through the nose while the breathing out is done through the instrument while playing???

After work last night I decided to do some experimentation. I procured about 1.5 metres of plastic piping with about 75mm internal diameter. I carefully placed enough photocopy paper to cover the hole at one end but no matter how I blew - gentle soft hard or otherwise the paper did not stay on the end of the pipe - only a very gently suction held the paper in place.

This must surely negate the bernoulli principle explanation.

I was not able to create any didgeridoo-like sounds so I can't prove or disprove my own standing wave theory. Come on now - surely someone out there A, owns a didge, B can play it and C is interested enough to experiment for us :)

Forget about Bernoulli's law because:

1.Ther is not enough velocity in the air at the wide end of the instrument to create attraction force

2. Bernoulli's law needs tangential or parallel flow with the leaf or paper in order to create a holding force

I am 99 % sure that, as I mentioned before, the reason is the low frequency sound standing waves which creates reciprocal force at the end of the pipe causing the leaf to vibrate with the same frequency (around 100 to 200 Hz only a guess) letting the air to exit the instrument in a cyclic manner, and because the leaf is not a rigid body, and due to the dynamic properties of the leaf, some parts of it would not completely separate from the pipe, and the friction force keeps it in the same place i.e. some parts of the leaf acts as a vibrating valve letting the player's breath to exit the pipe in a cyclic manner without falling. Of course the process needs some imagination if we avoid to formulate it mathematically.

What size piece of paper did you use?

Size of paper was about 100mm X 100mm - not exactly as I tore it off the sheet {Leaves don't usually come in perfect squares :) } , but it was certainly enough to cover the hole completely. Again my presumption here is that the leaf was at least able to cover the hole???

All of the proposals so far require air flow past the leaf. My assumption has certainly been that the leaf was small compared with the diameter of the didgeridoo, and was "sucked" onto/into one edge of the pipe by the Bernoulli effect. If it was just acoustics without any air flow, it might just be possible for the leaf to cover the end of the pipe with a gap around the edge, but personally I have some doubts whether even this would work

Fyz

"While they're playing with the didgeridoo a leaf blows onto the end of the instrument and somehow gets stuck there".

I cannot see how you can interpret the leaf is smaller and is sticking to the inside wall of the instrument from the situation description! Surely the situation statement would use "into the end" and not "onto the end " if your interpretation was true?

Also for what it is worth to back up my own theory, I have read that when a high pressure pulse exits an open ended pipe due to the standing wave phenomenon it generates a suction effect on exiting the pipe. This suction zone. which is essentially a low pressure wave now travels back up the pipe. Again my understanding is that this pressure/suction switching is happening at somewhere between 42.5Hz and 85Hz (42.5Hz is the fundamental frequency for a 2 metre long pipe and 85Hz for a 1 metre pipe). Is this push-pull action good enough to keep the leaf essentially oscillating on the end of the pipe even though there doesn't seem to be any net pressure difference between the inside and outside of the leaf? Also this gives a chance for the continuous air flow coming from the player's circular breathing to exit the end of the pipe without actually knocking the leaf off.

Then you can take the next step and question the original article about "Leaves blowing around in a music shop"

This principle was used extensively in "tuned" exhaust systems to extract spent gases from the exit manifold of normally aspirated engines in the days before "blowers" were invented......in my teens I recall spending hours calculating the lengths required to find the sweet spot on the power band and then get the resonant vacuum exhaust pulse to arrive at the exact moment to suck that cylinder empty to optimise the gas and air loading. Also to tune base reflex speakers so that the range of resonant frequencies of the speaker driver matched and complimented the enclosure.

Tuned exhaust scavenging is not really the same, because the tuning causes the vacuum to appear at the time that the valve is open, and there is a pressure peak between whiles that is ineffective because the valve is closed at that moment. The principle is still relevant to the basic design of exhaust systems, regardless of whether the input gases are pressurised. BTW, for road cars the tuning is often optimised slightly below the revs for peak power.

Fyz

Been giving more though to the venturi effect? the didgeridoo I think is the only instrument that fully covers the mouth, and as the air passes through the lips do you think a venturi effect is generated here? at the input end and its effect carries down through the pipe, or is this another tangent?

Regards JD.

Prompted by this challenge question, I recently did a post about making a slide didgeridoo which is a didgeridoo which you can vary the length while you play it. As I can play the didg I decided to do a little experiment lastnight to help answer this question and it seems there are a few schools of thought.

For starters I used a single ply piece of toilet paper, and started with my fixed length bamboo didg. I put the toilet paper on the floor and played the didg while placing the end near the paper. The results were that when I played loud I could actually lift the didg off the floor and the TP would stay on the end and sort of vibrate and spin around, when I stopped playing it fell off.

I repeated this experiment with just blowing, and the paper just blew away no matter what I did.

I then ripped the TP paper into a small piece about the size of a bottle cap. This time while playing a drone it was easier to lift the paper up, even though the opening end of the didg was much larger than the paper.

I repeated this experiment with my slide didg. I discovered at some lengths the paper would dance around on the end, and even go inside a little if I played loud. At other lengths the paper wouldn't stay on the end, and at some lengths it appeared to be pushed away by my playing.

In general it was easier to see this effect with a very small piece of paper vs a whole sheet of TP. At no time was I able to make the paper do anything other than blow away by just blowing into my didg with out making a drone sound.

I took this experiment one step further and tried to see if I could lift the TP when the end of the didg was not in contact with it. I was thinking it would be cool if I could make it float in mid air. The result was that I was not able to do this, however at a distance of about 10 inches, the piece of TP did start to dance around on the floor, but I couldn't quite get lift off.

These results tell me that one I now have a cool trick to show others when I play, and two I am betting my money on standing waves.

Frank,

You convinced me. I too tried some experiments, but without vibration, because I really don't know how to do it properly. All I had was a paper towel roll tube and some Post-It notes (adhesive side away from the tube, no cheating!).

I tried blowing hard, soft, slow, fast, and holding the paper on the end (tube horizontal) then letting go, or tried picking up off desktop (tube vertical) and no joy in any configuration.

I am thinking that plain old vanilla Bernoulli (without standing wave pressure) requires a much larger surface area than the end of a tube provides. That is why the ping pong ball and funnel work well. That is also why the thread spool with is large flat end works well. However, a thin-wall cylinder, like a tube or didgeridoo won't work.

Glad to hear your experiment was a success, so we have more than just theory. Congratulations!

I guess I lost all my money and am now broke, since I bet on the wrong horse!

But you put your argument forward well! If we don't question things we will be left in the dark ages!

Thank you. That's really interesting. And I hadn't thought about resonances in the paper or the body of the tube - but I imagine that one of these must be the cause of the paper being repelled at certain frequencies.

Fyz

I am reposting this comment as it did not take the first time!

Thanks for the confirmation on the standing wave theory. Although I wonder if the TP or leaf is acting like a flapper type non-return valve where it allows the air to releave on the positive pressure but seals on the negative portions of the wave. Of course this assumes that the wave is just a bit off of standing, doesn't it?

bernoulli

SMC Corporation makes a "non-contact" vacuum pad that uses this principle to pick up a part without touching it and leaving a mark- works great for glass, etc.

I believe these pads are continuous flow rather than acoustics. I wonder if alternating flow (acoustics) would give any advantage.

Deary me - the 'official' answer started out rather well, but then rather went downhill.

Fyz

Deary me - the 'official' answer started out rather well, but then rather went downhill.

Yeah, I think we figgered out that dog won't walk, not by hisself anyway!

I like your idea with the standing wave creating a Bernoulli principle/Venturi effect low pressure area at the end of the tube.

Thanks. BTW, I've decided to start collecting plaudits (on the basis that they are never going to fill the tiny space I've got available for any sort of collection).

Fyz

Thanks. BTW, I've decided to start collecting plaudits

Hmmm...I have been considering collecting kudos. Some of them are extremely rare.

(And I don't mean the candy bar by the same name!)

ROFL

Hi Fyz. "the 'official' answer started out rather well, but then rather went downhill"

Now why would you say that (the downhill bit)?

I must obviously support the official answer, because I said it more or less like that in post ~#12. And no, I did not supply the challenge or the official answer!

Jorrie

...and I had the correct answer in post number 10.

However I do agree that the official answer is a bit shy on the real physics behind this phenomenon. I would have liked a slightly more elaborated answer. Perhaps it is just an observed phenomenon and although we know, as per Frankd20's experiment, that it must be standing waves that cause it, no one has actually delved into the exact physics behind it. Just a thought.

SFAIK, there is no way to sustain an averaged pressure gradient without averaged net flow. So, if there was an average pressure differential along the length of the pipe, the air would move until the gradient was neutralised. That would take at most a few cycles of the sound wave to stabilise, so it wouldn't hold the leaf in place. What I am trying to say is that you need to develop a region where there is net flow for a pressure differential to be sustained. I believe such a flow is developed by lateral discontinuities around the leaf.
Of course, I could be wrong, but in that case the explanation omits all the interesting bits of the effect.

Fyz

So typical of Engineers and Physisist! Over thinking and over analyzing the problem. Read the question again. "a leaf blows onto the end of the instrument and somehow gets stuck there" Does it specify which end? OR did you all assume it was the end opposite the mouth piece?

So typical of Engineers and Physisist! Over thinking and over analyzing the problem. Read the question again. "a leaf blows onto the end of the instrument and somehow gets stuck there" Does it specify which end? OR did you all assume it was the end opposite the mouth piece?

I guess you missed the parts that illustrate that the mouth piece covers the player's mouth!

BTW, I'm not an engineer or physicist, I am a drafter.