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Old 5th March 2013, 10:00 PM   #31
Bazukaz is offline Bazukaz  Lithuania
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Hi,

I remember in the past I have tried to measure the effect of perforated sheet of metal by placing it between a cone speaker and microphone. The effect was zero with my measurement devices and open area of sheet around 65%. Perhaps it's not too difficult to test some specific stator design?

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Lukas.
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Old 6th March 2013, 03:12 PM   #32
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Quote:
Originally Posted by Marik View Post
I won't be able to give you any analysis of what difference the stators make. I'd think they do, as any obstacle is... an obstacle, and if anything, has its damping properties--after all, this is customary to put damping circles in front of dome tweeters (look at the ScanSpeak D2904, or Seas 27TAFNC/G).
I am familar with the use of plates placed in front of tweeter domes to produce a cavity resonance to boost/shape the HF response. My first experience(many moons ago) was with the Vifa D25AG and its triangular "precision phase lens". With it response was pretty much flat 20 kHz. Out of curiosity, I clipped it off and response started rolling off above 7kHz!

Quote:
Of course, the easiest and quickest way to know would've been just to measure with and without, but unfortunately, we cannot make the ESL to work without stators .
Your mention of tweeter resonator plates reminded me that I have a couple small neo tweeters that use perforated metal to hold a thin plastic resonator plate in front of the dome. I can take measurements:
1) With resonantor plate and perforated metal in place (OEM)
2) Resonator plate removed, but perforated metal in place
3) Resonator plate & perforated metal removed.

This should give us an idea how much cavity resonance we might expect from high % open area perforated metal.
I will have access to a B&K 4133 microphone tomorrow night, so will be able to get believeable data up to 40kHz.


Quote:
To put the things quite simply, this kind of resonator system is widely used in the microphone production and many commercial mics do use it for HF extension/correction.
Most of the response correction plates I've seen are solid or have very low % open area.
Just curious, what is the highest % open area correction devise you have seen?


Quote:
Originally Posted by Bazukaz View Post
I remember in the past I have tried to measure the effect of perforated sheet of metal by placing it between a cone speaker and microphone. The effect was zero with my measurement devices and open area of sheet around 65%. Perhaps it's not too difficult to test some specific stator design?
The cavity resonance that Marik is describing will probably only be noticeable with the perforated sheet metal very close to the diaphragm, which may not be possible with a woofer. Hopefully the tweeter experiment I described above will give us some insight.
Attached Images
File Type: gif D25AG.gif (120.6 KB, 104 views)
File Type: jpg NeoT_perf.jpg (26.2 KB, 101 views)
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Old 14th March 2013, 08:05 PM   #33
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Default Perforated plate Experimental Results

Quote:
Originally Posted by bolserst View Post
Your mention of tweeter resonator plates reminded me that I have a couple small neo tweeters that use perforated metal to hold a thin plastic resonator plate in front of the dome. I can take measurements:
1) With resonantor plate and perforated metal in place (OEM)
2) Resonator plate removed, but perforated metal in place
3) Resonator plate & perforated metal removed.

This should give us an idea how much cavity resonance we might expect from high % open area perforated metal.
My appologies for the delay on this...too many irons in the fire as of late.

Attachment #1: Shows the tweeter used and 3 different perforated plates tested

Attachment #2: Shows measurements for the "naked" tweeter along with the 3 test plates.

Tweeter was mounted on large board and impulse response windowed to remove edge diffraction contributions.
- Upper plot compares response at 1m on axis for the 4 different setups.
- Lower plot compares the increments for the 3 different test plates.

Results appear to support my experience that thin high % open area perforated plates do not have significant resonant behavior below 20kHz.
Attached Images
File Type: gif Perf_test_pics.gif (98.2 KB, 81 views)
File Type: gif Perf_test_results.gif (55.3 KB, 61 views)
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Old 14th March 2013, 08:14 PM   #34
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bolserst,
What was your conclusion from the tests? It looks like the tweeter with and without the screen but no phase plug has the flattest and widest bandwidth. Am I interpreting this correctly.
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Old 14th March 2013, 08:45 PM   #35
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Quote:
Originally Posted by Kindhornman View Post
bolserst,
What was your conclusion from the tests? It looks like the tweeter with and without the screen but no phase plug has the flattest and widest bandwidth. Am I interpreting this correctly.
The purpose of the test was to determine if the screen alone would significantly impact response.
My conclusion was that it did not.

I included the other configurations to illustrate the potential cavity resonance behavior that Marik had mentioned in post #30.


When deciding which configuration is best for this particular tweeter you would have to look at off-axis response as well.
When considering off axis, the OEM configuration provided the best/smoothest average response in a +/- 30 deg window.


*** Forget to mention, no data smoothing was used ***

Last edited by bolserst; 14th March 2013 at 09:03 PM.
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Old 14th March 2013, 08:47 PM   #36
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Thanks for the answer and I was going to bring up the off axis response but didn't want to bring that into the question. You are correct that is something to look at.
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Old 17th March 2013, 03:35 PM   #37
Bazukaz is offline Bazukaz  Lithuania
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Hi,

Recently I have decided to make several tests how an ESL element would impact propagating sound through it, by placing it between a dynamic system and a microphone. Measurement conditions:
Measurement type : MLS
Distance between mic. and dynamic system 25cm
All freq. response plots are calculated by using 3.5 ms window
10 averages
Neither microphone or dynamic system was moved or any settings changed during the measurements.

1) Freq. response of dynamic system only.
2) Freq. response after placing large stretched diaphragm in between, 6u thick, stretched to 1%
3) With bare ESL stator, assembled but without membrane
4) With complete wire ESL with silicone dots.

As it can be seen 3) and 4) shows a serious contribution to frequency response when impulse is coming from external excitations.

Regards,
Lukas.
Attached Images
File Type: png Freq. response 3 way system.png (9.4 KB, 42 views)
File Type: png Effect_membrane_1perc.png (9.6 KB, 15 views)
File Type: png Effect_stators.png (9.8 KB, 13 views)
File Type: png Effect_ESL.png (9.9 KB, 14 views)
File Type: jpg IMG_1419_small.jpg (69.2 KB, 19 views)
File Type: jpg IMG_1405_small.jpg (77.4 KB, 18 views)
File Type: jpg IMG_1394_small.jpg (73.4 KB, 17 views)

Last edited by Bazukaz; 17th March 2013 at 04:02 PM. Reason: Correct and extend
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Old 17th March 2013, 07:55 PM   #38
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Hello Bazukaz,

Trying to measure impact of structure placed in between a point source and a microphone can be problematic because of diffraction around the edges of the structure. For example, you might further investigate test 2) for the diaphragm and try taking measurement sets with diaphragm in place and then removed with the frame in the same location. My guess is that a lot of the wiggles added to the 2nd plot is reflection/diffraction from the frame. With both data sets, you should be able to subtract off the contribution from the frame and get a better estimate for that of the diaphragm.

Smoothing the data by eye, it does look like the HF roll off due to the mass of the 6um film is visible starting above 10kHz.
Theoretical calculations would put the -3dB point at about 17kHz.
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Old 17th March 2013, 08:43 PM   #39
Bazukaz is offline Bazukaz  Lithuania
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Hi,

At the moment I could not do the measurement of bare frame(no diaphragm) before it is glued on the next project . Maybe I will re-do the measurement some time after. It was just a rough evaluation experiment.
In fact I have done two measurements with membrane : low tension(similar to what can be obtained by heat shrinking) vs moderate 1% mechanical stretching. The purpose was to try observing any mid or high freq. resonance modes of the film. I could not draw any clear conclusions about this, effects should have been quite small.However in case of higher tension run the frequency response at top end looks a bit more ragged without smoothing. I am not confident about the results because the position of the frame was not carefully monitored, so it can be a contribution of reflections from it.

Your observation about HF roll off seem to be correct.
Below are two diagrams, with and without membrane, zoomed in and smoothed to 1/3.
Attached Images
File Type: png Smoothing, no membrane.png (9.0 KB, 19 views)
File Type: png Smoothing, with membrane.png (9.2 KB, 15 views)
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Old 17th March 2013, 09:32 PM   #40
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Bakukas,
I would think that a higher tension on the diaphragm would change the transfer of energy as the losses into the membrane would be lessened due to damping and the elastic properties of the membrane would also have to be considered. I know you wouldn't have any but a material such as Kapton would have a very high Young's modulus of elasticity. Thickness is also going to have an effect on transference of energy, the thinner the material is the more transparent it would be for the same material. It is an interesting experiment but what is your end goal? What exactly are you trying to accomplish with the experiment, is it only to look at the transparency of the material to sound passing through the membranes?
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