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Erin's Audio Corner gets a Klippel NFS!

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hardisj

hardisj

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The biggest concern I had once I set up the NFS was the microphone boom as well as the cage. The cage is a safety feature. It isn't intended to be used on the microphone at all times (not my opinion; Christian Bellman verified this when I had as Skype session with him and asked about it specifically a few days ago).

For those who do not know what that is, here are a couple photos of the mic/cage:


DSC07831.png



DSC07829.png



This is what the mic looks like without the cage:

DSC07828.png






The obvious thing was to measure a speaker with and without the mic cage. I used a laser level and calipers to make sure the mic was placed in the same position in both cases.

DSC07823.png





And here are the results.
Blue = Mic with cage
Red = Mic without cage

Mic with and without Cage.png



As expected, there is a comb filter pattern high in frequency ~6kHz and above. The red response (without cage) is more smooth without large deviations, whereas the blue (mic with cage) shows +2dB variation at about 9kHz as well as other pattern issues.




**************
**************



So, yea, the mic cage creates some combing issues. But, the microphone without the cage isn't without its own issues. The simple fact that the microphone is hanging off a fixture which is attached to the boom creates a path length of about 5 inches from the mic to the mic boom which creates comb filtering as well. Thus, also diminishing the goal of "absolute accuracy". And, even if this weren't an experiment in trying to achieve a high level of accuracy, it's pretty obvious you don't want that reflection path because it would create response error/inaccuracy.

So, the next step was to create a new microphone holder, placing the microphone at the very end of the tube so there was no reflection path between the mic boom/holder and the microphone electet itself as shown below. Now, this isn't the *final* design but it gives you an idea for this discussion.

DSC07826.png




And here are the results.
Same as above but now with the "mic on new boom" in black.

OEM vs Updated Boom.png



Notice how much smoother the black line is now, and how it is devoid of any comb filtering? Yep. How much? In this particular case? About ±0.50dB. But this is a single axis measurement and I wouldn't be surprised if the error was higher off axis (especially in the far-field for those who don't use a NFS). Now the real question is: does the tester/engineer care about the deviation from the stock boom with ball joint (or otherwise) vs straight rod or not? Maybe some don't. But I know others do; not just industry but other DIY'rs. And if I am going to go for absolute accuracy, I might as well start with the easiest fix: the boom.

I will clean up the design a bit (as I discuss in the video below) and begin moving on to testing and reviewing speakers. :)
 

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hardisj

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piggybacking off the above, here is the video and the little description for it...



Before I begin the measurement journey with my new fangled Klippel Near Field Scanner, I wanted to take the time to improve the microphone assembly to achieve the highest level of accuracy in measurement I can possibly provide. Therefore, in this video I discuss how the microphone boom (of any measurement system; not just mine) can influence the response and a simple, cheap way to take measurements to the next level of accuracy.


 

restorer-john

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What amazes me is that the entire boom/cage/bracket structure is as large and unwieldy as it is for closeup to driver measurements. What were they thinking?

Surely Klippel should be mounting the electret element itself on the end of narrow, internally damped carbon fibre/graphite tube (like an old fishing rod bottom half) as far away from the rig/brackets as it can be and still be rigid. There should be no exposed wrapped cables either.
 
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hardisj

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I was asked on YouTube so I will answer here as well:
"Do you find it disappointing that Klippel did not do something like this out of the box?"

Well, the mic cage is a safety feature to keep someone from damaging the speaker while testing and they assume one will remove it for final testing. The mounting of the microphone hanging off the mic boom is pretty standard. Look around any store and you will find a pole with a mount attached to the end. In fact, I do not think I have ever seen more than a few people mount the microphone directly off the boom and those who have are DIY guys or some of the more well-respected measurement facilities (Warkwyn comes to mind). I don't think this is an oversight by Klippel at all. It's just something I'm sure they expect purchasers to understand and will deal with accordingly (if they even see fit). In my case, however, I'm going uber-nerd status and ... well... here we are. :)

Klippel's main concern here is protection. Thus the mic cage setup. I think that if I were to ask for a hollowed out tube for me to do this they would have been happy to provide that instead of the boom threaded for the mic adapter.
 

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Klippel's main concern here is protection. Thus the mic cage setup. I think that if I were to ask for a hollowed out tube for me to do this they would have been happy to provide that instead of the boom threaded for the mic adapter.

I think I have a few dozen brochures from the big manufacturers where speakers are positioned in anechoic chambers with a mic integrated into a very slim and long tube, far away from the rig. Some of the first "measurement" mics I made with Panasonic inserts back in the day were like that. (long brass tubes)

Sure, these days, everyone wants foolproof protection mechanisms, but this one defeats the purpose of the entire rig- it compromises and gives inaccurate results in the high end when it is installed. That's just plain dumb.

Surely the "protection" cage and microswitch triggers could have been avoided altogether in software with an initialization setup stage, where you walk the mike rig up to the 8 corners of a mounted speaker and it sets a 3 dimensional no-go zone, which is the surface of the entire speaker.
 

Hiten

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Newbie post.
1) Please be gentle 2) Dont ban me from forum :facepalm: 3) bash me up :p for suggesting mods to a brand new system

what if the Microphone boom is further improved by putting porous foam 'sleeve' or foam 'disc' as shown in the pic ? Probably would not make much difference but at high frequency measurements at loud levels it probably will. One can not be sure. maybe will provide protection to mic too.

Congratulations for acquiring the system ! Which would be the first lucky speaker to be tested ?

All the best. :)
Regards.
 

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Hiten

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addition :
Probably the 'robust' mic attaching boom was necessary as this would be rotating and need to be firmly secured. And probably minor deviation would result in inconsistancy.
regards.
 
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what if the Microphone boom is further improved by putting porous foam 'sleeve' or foam 'disc' as shown in the pic ? Probably would not make much difference but at high frequency measurements at loud levels it probably will. One can not be sure. maybe will provide protection to mic too.


As @jtwrace alluded to, I mentioned this in the video. BUT, I appreciate the thought... good looking out. :)
 

ctrl

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piggybacking off the above, here is the video and the little description for it...
Before I begin the measurement journey with my new fangled Klippel Near Field Scanner, I wanted to take the time to improve the microphone assembly to achieve the highest level of accuracy in measurement I can possibly provide. Therefore, in this video I discuss how the microphone boom (of any measurement system; not just mine) can influence the response and a simple, cheap way to take measurements to the next level of accuracy.
Thanks Erin! Finally, comb filter effect of Klippel NFS has been systematically studied - didn't expect to live to see it ;)
A measuring device for gozillions of cents only makes sense if you really push the precision to the limit.

With some sort of sleeve around the tube, it should even be possible to use the safety cage during preparation.
 

tktran303

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This is a very interesting find Erin!

I’m rather surprised it makes such a big difference!

I wonder how much of a difference a standard mic holder, on a tripod, affects the same top 3 octaves.

I mean, this is for people doing measurements out at 0.5-1M, using the old fashioned (non-robotic) manual system.
 

Nusse

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If you sometime in the future is going to measure the Kef R3;) and decides to disassemble the speaker, I have 3D-printed trim ring for the old SP1632 and the new SP 1753.
I have both drivers as well, but I think shipping to you and back (Sweden) will be ~$200
20160425_221747.png
sp1753_sp1632.jpeg
 

thewas

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What amazes me is that the entire boom/cage/bracket structure is as large and unwieldy as it is for closeup to driver measurements. What were they thinking?
I am also surprised and even use since years some gaffa tape on the mic holder I have for my quick and dirty hobby measurements to chamfer and smoothen out the transition from the mic to the holding tube.
 

restorer-john

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gaffa tape on the mic holder I have for my quick and dirty hobby measurements

Everyone knows gaffa tape has perfect sonic properties in terms of absorption and reflection. Incredible stuff. ;)
 

thewas

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Everyone knows gaffa tape has perfect sonic properties in terms of absorption and reflection. Incredible stuff. ;)
:p:D
Of course not in the reflection value itself but in changing the angle of reflection away from the mic capsule. ;)
 

Ericglo

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So I understand this. To get that last little bit of precision, you would like to modify the transition from the microphone to the end of the pipe?

I can't tell. Is that conduit or a metal pipe? Either way, you could probably grind around two inches of that pipe to taper it down a little better.

You might also be able to use something like neoprene rubber on the mic and trim/taper it down.

Maybe Frank Dernie could come up with a good solution.
 
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