Solving Striae in Kundt Tube Vibrations

[DaTario]

Hi All,

I have searched in some places but ´had no success in finding an explanation to the appearance of striae in the vibrations of the kundt tube. The powder vibrates as a response to an incoming sound wave front, and show a larger patern which is nicely explained by the theory of open tubes (or partially opened tubes). But in a smaller scale, the appearance of striae (typically with wave length of centimeters) has no well defined cause.

I appreciate the contributions on this subject,

Best wishes

DaTario

 

[DaTario]

Aditional comments:

some think these striae have to do with higher harmonics in the tube, but regarding the distance between adjacent striae these harmonics would have to have frequencies of around 33000 Hz, which seems unlikely.

best wishes

DaTario

 

[Meir Achuz]

The pure fundamental frequency is calculated negectling end effects.
End effects cause the wave in the tube to be not exactly sinusoidal.
Fourier analysis of the actual shape can show very high harmonics.
Which higher harmonic is dominant would depend on the R/L of the tube.

 

[GTrax]

How exactly is the sound generated?

As I remember (very long time ago), the tube was made to shriek by dragging a rosin cloth along it. In this case, the frictional excitation would have a very complex waveform that would make all sorts of local nodes, and the spacings involved would not be linked sound waves in the tube, but by which parts of the glass were mechanically stationary by bulk properties of the glass and the shape section. The larger scale repetition of these groups of striations would be related to the sound fundamental wavelength.

Analogous to this is when a centrally fixed steel square plate is bowed to resonate. The powder patterns revealing resonance modes show the parts of the plate that are mechanically stationary, not where sound waves in air might have a standing wave envelope.

OK - this is just my lame notion.. speculation if you like, but I have seen those happen in dust on top of corrugated steel sheeting where a piece is being dragged off a pile. The shriek is unpleasant, but the wavelength heard is no way associated with the spacings. Just because they happen when the sound is heard, does not mean one cause the other. Rather, both effects had a common excitation.

 

[DaTario]

thank you both.

I am wondering if it could have something to do with the velocity of sound in the glass. Could it be?

best wishes

DaTario