Design of IsoTube - microphone noise measuring enclosure

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kilmister

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Joined
Aug 22, 2008
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284
Location
Helsinki, Finland
Hello all!

I'm not sure should this be in Machine Shop or here. Please move if it's in wrong place.

I've started to design soundproof enclosure for microphone noise measuring purposes based on Neumann's isolationtube like one in the picture below.




I have ocassional access to great mic lab but I'd like to have one myself and due very simple construction of enclosure I decided to build it. I found source for steel tube with outer diameter 150mm and inner dia of 80mm which leaves 35mm of steel in walls -should stop pretty much everything. Top and bottom plates are solid 150mm round bar which is going into lathe for re-shaping. Bottom plate is fixed with reasonable amount of 6mm or 8mm screws, seal between plate and edge of tube. Top plate has quick access through 50mm hole or for bigger microphones quick-lock system made pair of toggle clamps.
Below is simple 3D pictures of parts I've designed this far.




For accurate A-weighted noise measures I also should have ability to use constant SPL source to measure microphones sensitivy, otherwise I can only measure millivolts coming out from circuit. So I thought it might be possible include constant SPL source inside of tube. I found Tang Band element small enough it fits in bottom plate and it's able to produce 94dBspl@1kHz with few watts and it dosen't cost fortune, like 30€ for pair.

But.

What happens inside of tube at 1kHz? Is it constant? I just can't figure it out. Something is telling me it might work but on the other hand it's after all solid steeltube full of reflections so there's good change for cancellation points. How much distance from element will effect?
I also thought to use bigger diameter tube - like 200mm - and fill it with pipe of rockwool but what happens when I measure side access mics?

All your thoughts are very much appreciated.

-Paavo
 
Do we know what the thing in the original photo actually is used for?

- Self-noise test would require very low-noise surroundings.

- Sensitivity test would require capsule to be mounted in a VERY well-defined position.

- Frequency response tests are not possible (at least not useable) in this small a space.

Jakob E.
 
gyraf said:
Do we know what the thing in the original photo actually is used for?

Thanks for your reply Jakob.

Yes, it is used for self-noise measurements. It is very thick and air thight steel tube. Tube only weights 60kg in my estimation.
I've made some self-noise measurements in the lab I mentioned above with exactly same kind of Neumann noise isolation chamber which is placed in anechoic room.

I'm not gonna to make anykind of freq response measurements in IsoTube, noise and sensitivity are my primary concerns. As I was wonderig in my first post, I'm not even nearby sure does little element inside of the tube work as constant SPL source. After all it's not expensive experiment so if it dosen't work I'll have anyway nice isolation chamber.

All the best,
Paavo
 
SENSITIVITY
Get an ECM8000 or other measurement mike calibrated against a 1/2" B&K by a reliable lab for response & sensitivity.  You need to do this anyway for serious mike work.

Trying to do precision SPL in a small chamber is difficult.  Look up old issues of B&K Technical Review (?) for how to do it, especially anything by Rasmussen.  The chamber must be VERY small.  Don't use a soft dome transducer or anything with Neo magnets.  Hard diaphragms & Alnico magnets are best for consistency, then Samarium Cobalt.

NOISE
The mike must be flexibly suspended within the IsoTube.  Your biggest leakage will be the hole for the cable.  This must be as small as possible and sealed with flexible damping compound.  The cable must be very flexible.

You will still need to put the IsoTube in a very quiet spot and it must not sit on a structure that will transmit vibration.

You don't need steel.  At Calrec, I used a thick chipboard box and put it on top of the wedges in my anechoic.
 
Thanks for your reply Ricardo.

Great remarks. I'll check B&K documents you mentioned.

I choosed steel tube cause 1) Neumann are using steel 2) I'm able to get it and it dosen't need very much machinery.

Cheers,
Paavo
 
I need to stress that the best & most convenient way to measure mike SENSITIVITY is to compare it with a calibrated measurement microphone using a quasi anechoic measurement method.  See
  • Clio Instruction Manual which has the best explanation of this I've found.  I highly recommend ClioQC for production testing and ClioLite as an inexpensive R&D tool.  Their measurement mikes are one of the few whose calibration I believe.
  • Professor Farina's Aurora plugins for Audition
If anyone has any experience of free, simple to use packages that will do this and allow a reference microphone curve to be used, please let us know.  I believe ARTA & REW are probably capable of doing these but its a PITA.  I use my own programs which are an even bigger PITA with Audacity but I know exactly what assumptions have been made.

Les Watts uses the full version of ARTA and likes it.

You could buy a B&K calibrator on eBay and if it isn't damaged would give good results for sensitivity ... but only for small omnis which you can somehow adapt to fit the calibrator.  Useless for cardioids etc.
 

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