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yup. I first started using this ping-pong paddle example when I used to teach theoretical/classical mechanics to undergrad physics majors. It is also known as the Dzhanibekov effect after the Russian cosmonaut who observed it in space in 1985. en.wikipedia.org/wiki/Tennis_racket_theorem.

For a circular tube (as is most often used for an impedance tube as you describe), as long as the driving frequency is below the value f = 200/d where "d" is the diameter of the tube you should not have to worry about evanescent waves. The theory to determine the exponential decay is a little more mathematically difficult for circular tubes than for rectangular waveguides. But as long as you are well below the cutoff frequency (f = 200/d) for the first non-plane wave mode you won't have to worry about evanescent waves.

@@DanRussellPSU And what if we are above the driving frequency? My project is based on water filled impedance tube and my prof told to find out the length from where evanescent waves vanishes or only plane waves propogates.

who is Richard that you are referring to?

@@DanRussellPSU sorry, I think I confused you with another Dan Russell. He's a cartoon character, btw, the Dan Russell I was reffering to is a voice actor

@@SamueleCannella-tm7je there are quite a few people with the name "Dan Russell" (a rather large number, actually). I'm definitely not a voice actor for a cartoon. I'm a university professor in the field of acoustics (sound and vibration).

@@DanRussellPSU ok, thanks. and, also, sorry for the mix-up

Great. All I have e been able to find on any of his publications was “V”.

I'm not sure that I completely understand the physics reason why -- but when you run really hot water from a kitchen faucet that has an aerator in the tap you gets lots of really small bubbles (the water looks cloudy). You don't get the same bubble-infused water with cold water, or from hot water without an aerator.

yeah -- I know. Sorry. I wasn't trying to play to show any skill -- just trying to demonstrate how the spectrum for the oboe is different from that of a clarinet (which is roughly the same length and is also a closed-open reed-driven instrument). If it wasn't for COVID, I would have had one of the grad students who played oboe in high school and college (and who was taking the class) help with the demonstration.

Yup I understand. Sorry about that, thanks for clarifying that point. Great video for the purpose!

rough translation (Google Translate): "Children assemble quads and fly to Mars, and adults discover acoustic resonance"

To measure and display the frequency spectrum I used the iOS SpectrumViewPlus app (by Oxford Wave Research). I took a screenshot of the spectrum and edited it with with Preview app (on my Mac laptop) to insert the numbers identifying the odd harmonics (which show up most strongly on the spectrum)

Thankyou so much sir for sharing

it happens for brass players also -- trying to play in tune outdoors in cold weather is very difficult. Attempting to model the acoustics of wind instruments is also made difficult by the fact that the air exiting your lungs and mouth is warmer than the air in the room around the instrument, and there is a temperature gradient along the length of the instrument -- the air exiting your mouth and entering the instrument is warmer than the air that exits the other end of the instrument.

Perhaps I’m missing the point you are trying to make. In acoustics we treat air as an ideal gas for which the density and pressure are related to each other and to the temperature and molar mass through the ideal gas equations. You cannot change the density without also changing the pressure. The ideal gas relationships clearly show (backed up by a wealth of experimental data) that the speed of sound may be expressed directly in terms of the molar mass of the gas, the ratio of specific heats for the gas (both of which are constants) and the temperature. The equation relating speed of sound to density is c = sqrt(g*P/rho) where g is the specific heat ratio and P is the ambient pressure and rho is the density. The product g*P is called the Bulk (elastic) modulus of the gas and like the density ithe bulk modulus depends on temperature. For an adiabatic process like a sound wave, the pressure and density are related through the ideal gas law and that allows us to express the speed of sound as a function of the molar mass of the gas and the ratio of heat capacities and the temperature as c = sqrt(g*R*T/M) where at is the universal gas constant, M is the molar gas, g is the ratio of specific heats and T is temperature in Kelvin. Expanding that equation as a McLaren series allows the speed of sound in air to be expressed in terms of temperature as c = 331.6 + 0.61 Tc where Tc is now the temperature in degrees Celsius. So the speed of sound in an ideal gas can be shown to depend directly on the chemical composition of the gas and the temperature which is what this demonstration shows.

@@DanRussellPSU Nowhere did I say it didn't, I said there is more to it than just temperature, To quote you "" You cannot change the density without also changing the pressure."", this was my point. That at higher pressure the gas is denser thus sound travels faster through it, When dealing with The specific heat ratio of a gas is the ratio of the specific heat at """"""constant pressure, Cp,""""""""" to the specific heat at constant volume, Cv. But probably nowhere in the entire universe are there pressure variations? I was not saying that it was wrong just that there are cases where the ideal gas law is not usable because The pressure is not constant. to reiterate....."" however by ____> changing the pressure <________ enough and letting the air stabilize it can be modified, so using air temp. measure is only useful under certain conditions and is not an absolute that you can rely on."" A bottle of gas at 50atm and at 250K has a different value than a bottle of gas at 10atm at 250K, it is denser {sound travels faster through it ..... thus the specific heat ratio is modified not by the temp, but by the pressure change, I'm really not saying anything different just that when dealing with the real world sometimes you have to remember that the wording you use can limit the understanding of what you are trying to convey, by stating It is solely reliant on temperature, When it is really the To again quote your above statement " For an adiabatic process like a sound wave, the pressure and density are related through the ideal gas law" This is of course also ignoring {that in the real world the Volume is also not likely to be constant except in a non precise measure}. I'm wasn't trying to say you were wrong, only that by using an arbitrary value based on ideal conditions, you have " measure is only useful under certain conditions and is not an absolute that you can rely on." It represents the mean, and there are always, ALWAYS, outliers. IOW> Not all cows are spherical.

@@DanRussellPSU To condense... it's the ratio of pressure to temp. is my point, and not "solely defendant on temp".

​@@brianstevens3858 The speed of sound in a gas does not depend on the ratio of pressure to temp . . . the speed of sound in a gas depends on the ratio of pressure to density -- and that ratio of pressure to density is directly proportional to temperature. When it comes to the behavior of sound waves in a gas, there are very specific relationships between pressure, density, velocity, temperature, etc. . . . and those relationships (backed up by huge amounts of experimental data) show that the speed of sound in an ideal gas depends on the ratio of pressure to density, or in another form directly on the chemical composition of the gas and the temperature. Sure, there may be cases where the gas does not behave like an ideal gas, or where the specific acoustic process under study might not behave as an adiabatic process . . . but those situations don't apply to the demonstration I posted in this video.

@@brianstevens3858 if the only point you were trying to make is that there could be specific situations (under carefully controlled environmental conditions) where the air does not behave as an ideal gas and where sound waves can't be considered as an adiabatic process -- then fine. I just don't understand why you felt so obligated to make that kind of comment reply to a video demonstration that was definitely not trying to state that this was a fact that applied to every possible scenario. This demonstration and the homework assignment that goes along with it simply demonstrate that the speed of sound in air (under normal everyday conditions) changes when the temperature of the air changes, and that we can model this very accurately by treating the air as an ideal gas and sound waves as adiabatic processes -- with the result that measurements of the speed of sound in air (knowing the temperature) or measurements of the temperature (knowing the speed of sound indirectly through a measurement of the frequency of a resonator filled with air at a specific temperature). If you really want to get nit-picky about the full details -- there are a LOT of other things going on with this demonstration that are also being "ignored" through this simplified approach -- the simple equation for the frequency of the pipe ignores damping (due to thermo-viscous losses along the walls and radiation damping from the open end) and it ignores mass-loading effects at the open end due to impedance changes at the open end. It assumes that the air in side the pipe is all at the same temperature and that the temperature of that air does not change as the pipe is being excited through an aerodynamic flow (blowing over the opening). It assumes that the difference between the temperature of air inside the pipe and the air surrounding the pipe does not affect anything. The full specific details make this problem much more difficult than I described -- but to the level of accuracy required, the simple approach works very well. So, yes, I agree that not all cows are spherical. But many times you can get a very accurate measurement (or estimate if you prefer that word) by using some simplified assumptions, that do accurately match observations.

The external mic shown in this video is a i437L from MicW. If you do a Google search for "micw i437l omnidirectional lightning microphone" you'll get links where you can purchase this mic from Amazon, Sweetwater Sound, and B&H Photo Video, alf for about the same price. I believe this is a mic that works in the audio range (20 Hz - 20 kHz). I do not know of any mics that will work with an iPhone for infrasound or ultrasound. I'm also not sure that the internal electronics for an iPhone would support those frequency ranges.

The microphone I used in this video (i437L by MicW) was a 1/4-inch microphone and the calibrator I was using (Larson Davis CAL200) has a 1/2-inch mic opening, so the adapter was absolutely necessary. The calibrator must have a tight seal (with the correct volume) in order for the calibration to be accurate.

In the information (right below the video above) I have included the link to the website for Dr. Arai's lab where you can download the *.stl files to 3D print them.

The sacrifices we do for science - for SCIENCE!

I second this. Environmental noise is a problem.

This demonstration shows that a plane wave will always travel down the waveguide without decay, at any frequency. In contrast, however, a non-plane wave (when the two speaker sources are driven with opposite phase) will travel down the waveguide without decay only when the frequency is above the cutoff. When the frequency of the non-plane wave is below the cutoff, the non-plane wave will decay exponentially with distance as it attempts to travel (not a microphone artifact). If you are interested in reading more, here is a short conference paper I wrote about this demonstration: pubs.aip.org/asa/poma/article/21/1/025001/976143/Apparatus-for-demonstrating-evanescent-waves-in

@@DanRussellPSU How is this applied to light travel in a vacuum/non-vacuum?

@@MyM0RR0WIND -- the concept of evanescent waves (amplitude decaying exponentially with distance so that no energy is carried by the wave works for all types of waves. But, lightwaves are completely different types of waves. Light waves involve transverse oscillation of electric and magnetic fields. Acoustics waves involve longitudinal oscillation of particles in a mechanical medium (like air). An example of evanescent waves occuring with light when a plane wave of light reflects from a surface with a different material (having a different index of refraction -- like air-to-glass). When the direction of the incident light upon the boundary is greater than the critical angle (measured from the perpendicular to the surface), instead of penetrating into the second medium, the wave in the second medium is evanescent while being reflected from the surface. Here's the Wikipedia article about evanescent light waves: en.wikipedia.org/wiki/Evanescent_field

@@DanRussellPSU Yes I asked because I wanted to clarify that plane waves do not decay. So you are saying that at a certain threshold however two different plane waves of the same frequency but of opposite phases do decay over time?

@@MyM0RR0WIND Acoustic pressure plane waves in a waveguide do not decay over spatial distance (not time). The evanescent decay is a decay as the wave attempts to travel through a distance, not time. A plane wave means that the pressure is uniform across the cross-section of the waveguide. The situation where the two loudspeakers are driven with opposite phase does not create a plane wave, nor does it create "two plane waves of the same frequency but opposite phases." Instead, it creates a wave function which looks like a full-cycle sine wave (top part is positive while bottom part is negative) so that the pressure is not uniform across the cross-section of the waveguide (the wave is not a plane wave). Here's an animation showing what the wave patterns in the waveguide look like and how the non-plane wave (opposite phase pattern) either travels or evanesces depending on the frequency: www.acs.psu.edu/drussell/Demos/waveguide/waveguides.html Here'a another completely different example of evanescent waves . . . plane waves radiating from a vibrating plate when the wave speed of the wave on the plate is slower than the speed of sound in the fluid above the plate: www.acs.psu.edu/drussell/Demos/EvanescentWaves/EvanescentWaves.html

yes I do . . . current project comparing ash, maple, and birch MLB quality bats (same shape profile, same weight) to see how different "identical" bats really are. Some current work with metal bats to compare the effect of the shape and material of the endcap on the sound. Lots of non-bat work on golf clubs (drivers and putters and balls), pickleball paddles, tennis rackets, hockey sticks, etc. Just not many videos on those research topics to post. Maybe I need to change the name / description for my channel to be more general structural acoustics and vibration?

My iPad Pro shows a huge 30db quieter than my iPhone - both with decibel X. Not sure why 🤷‍♂️

then again with the air intake resonator?

and seeing the results describe if the latter is designed to complement former with respect to airflow pressure and if so at what airflow and at what frequency