The teacher said it did not work .Some results were OK on the Open tube tests but it was useless on the closed tube tests .
At the time , I remembered realising that the speaker in it was really bad at low frequencies and next time , it was used with a better speaker but again , complaints that the results from it didn't match the expected results .
So here I am several years later at a different place and my wife (Physics teacher) brings home the Kundt's apperatus and begs me to see why it won't work .
So I methodically set it up as per the Manual provided by IEC .
Incidentally , it is a very sexy piece of gear .
I then proceed to test the open tube .
HINT: I found if the microphone was placed about 100mm in from the end ,the resonant peaks could be found more easily when watching the oscilloscope .
I also discovered that those little holes in the tube must be covered .There is a plastic clip with felt inside it that is ment to be slid over the holes to cover them .However I found the slightest movement of the clip would seriously upset the resonance .......obviously proved they do something .
In the end , I simply put some tape over the holes and slid the clips on . Surprisingly ,this actually increased the resonance as well .
I ended up with these results
Fundamental Freq 205Hz
1st overtone 410Hz
2nd overtone 618Hz
3rd overtone 822Hz
4th overetone 1030Hz
so we are now meant to divide the overtones by the Fundamental .
This gave these results .They were so close to text book answers I was totally amazed
1st overtone 2.00 ideally 2
2nd overtone 3.01 ideally 3
3rd overtone 4.00 ideally 4
4th overetone 5.02 ideally 5
Sliding the microphone in the tube found the Nodes and anti-nodes just like in the IEC Manual .....happy days
OK now it was time to setup for the closed tube tests .
This is where it all turned to muck.
I tried the Plunger at the end of the tube (80cm tube)
I could find resonances on the higher overtones , but I could not get a clear Fundamental resonance .There just seemed to be bogus peaks that made no sense .It seemed that the speaker was not making enough sound below 200hz .I tried another speaker , but it still gave me bad results .
I discovered that if I placed the Plunger about the middle of the tube , I could get some measurements that made sense but as soon as I pulled the plunger past half way , it was rubbish .
out of frustartion , I actually got a 800mm length of 90mm PVC pipe with an end cap .I cut a slot in it to slide in the microphone . It was better , but still dodgy . Thats when I made the discovery that if I held my finger on the end capright where the slot for the mic was , suddenly there was a dramatic change on the CRO .
I replaced the cap for a solid one and Bingo .....a fundamental resonance that made sense
I was wonering if that was the problem in the IEC plunger /Mic setup .
I played around with btis of tape over the slit and stuff , but nothing I tried seemed to help . In the end I stuck a rubber bung in the end and that fixed it . I now had a Low frequency fundamental that related to the over tones .
Now I had another problem .
How was supposed to slide the mic in to find the Nodes and Anti-nodes?
Because IEC put the speaker at one end , it was impossible to slide the mic in that end , so they decided to slide it in through the Plunger end and in doing so created a problem .
I used my external speaker and by placing it at 45 deg to the endof the tube , I could now slide the mic in from the speaker end .
I wondered if the IEC speaker could work like that even thoiugh it made very little sound at arround 100Hz and how to mount it on an angle ?
I pulled the speaker base appart and found a screw that I undid enough that I could twist the mount .
It worked .And now I could slide the mic in easily past the speaker .
