Re: [Phys-L] The resonance phenomenon in an open ended cylindrical tube

[Bernard Cleyet <bernard@cleyet.org>]

On 2015, Jan 12, , at 11:18, John Denker <jsd@av8n.com> wrote:

> You can choose what you like, but other folks are free
> to choose differently.  One can easily imagine detectors
> that respond to velocity rather than pressure ... basically
> a quick-acting anemometer.


That’s a ribbon mich.  

http://en.wikipedia.org/wiki/Ribbon_microphone  

Can be used in Blumlein pair configuration.  

http://en.wikipedia.org/wiki/Blumlein_Pair  

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On 2015, Jan 12, , at 10:31, John Denker <jsd@av8n.com> wrote:

> Reflection is controlled by the impedances, not by the
> speeds.  This is super-easy to demonstrate with electrical
> transmission lines.  You can have a piece of 50-ohm coax,
> a piece of 75-ohm coax, and 300-ohm twinlead all with the
> same speed of propagation.  There will be humongous
> reflections from the interface


Yes;  I discovered ThinNet cable is 50 ohms, because the std. termination is 50 and using a BNC one on the end of a cable resulted in no reflection. Inserting a length of RG-59 results as JD wrote.  Interesting results w/ both and no termination.   Also a series of decreasing amplitude pulses from an unterminated length and the pulse gen. and ‘scope end isolated w. a resistor.  

http://physics.gmu.edu/~rubinp/courses/407/coax.pdf  

Note the uses of the one k ohm resistor.

I discovered the aboves when I wanted to make a delay line w/ > 100m of ThinNet cable.  This was unsuccessful, because of the great attenuation.  So I made a macroscopic one using the battery winding of UPS P/Ss and fractional Farad caps.   The delay was about 700ms.  


bc  played w/ this stuff when he was lent a copy of Elmore and Sands back in 1956.  (Now has his Sands autographed copy.)