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Old 12th October 2012, 06:38 AM   #31
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I think the resonance is caused by the cavity between the two rows of magnets. It is very much like the cavity resonance of the Carver ribbon drivers (that are not true ribbons). The Carver's resonate at a higher frequency 7-8 kHz as the slot is narrower.

How did you measure the frequency response? It seems to be made at a low SPL. Are they really going that low in the bass? That will require very large excursions of the ribbon at higher SPL and probably "polute" higher frequencies.

Roger
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Old 13th October 2012, 12:47 AM   #32
Few is offline Few  United States
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I tend to agree with Roger but I'm surprised that the frequency shifts when the ribbon characteristics are changed if the resonance is a characteristic of the magnet gap. How much does it change? Small shifts might be explicable in terms of changes in the cavity but if it shifts by KHz I may have to rethink my assertion that the magnet cavity is the culprit.
Few

Last edited by Few; 13th October 2012 at 12:50 AM.
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Old 13th October 2012, 07:02 PM   #33
hanski is offline hanski  Finland
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The measurements are made with Room EQ Wizard software and the microphone is according to instructions at Linkwitzlab. Such a great site!

The measurements were performed with low volume, mic was 10cm away from ribbon surface.
I have learned that narrow ribbon without a baffle can't produce the needed SPL at bass frequencies, so the equalized acoustic response is first order high pass filter at 200Hz. (I have also a steep high pass filter at 80Hz to remove the ribbon excursion)

I can make and post frequency response measurements with different corrugation styles.
That should tell if my problems are caused by cavity resonances or ribbon resonances
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Old 24th October 2012, 05:15 AM   #34
hanski is offline hanski  Finland
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So here are two measurements (high pass filter used, measurement distance was 10cm). No other changes except ribbon tension is increased.
The normal tension measurement is performed with the pink noise test tuned ribbon tension.
And the max tension measurement is performed with even more increased tension so that the corrugations flatten out almost to a flat foil.

So it seems that the about 2KHz resonance is due to corrugation, not due to cavity resonances.
Another fact that supports this theory is that my ribbon headphones have exactly same phenomenon even though they have totally different construction and different materials.

Hopefully the book arrives soon and I will learn more..
Attached Images
File Type: jpg 40mm_ribbon_normal_tension.jpg (63.6 KB, 281 views)
File Type: jpg 40mm_ribbon_max_tension.jpg (66.4 KB, 267 views)
File Type: jpg both_graphs.jpg (69.8 KB, 268 views)
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Old 26th October 2012, 12:43 PM   #35
hanski is offline hanski  Finland
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Here are some more measurements. These are from my ribbon headphones with 6um thick capacitor foil.
Measurement distance is 1cm away from ribbon surface.

Measurement graphs show the same phenomenon as my previous post. The more ribbon tension is increased, the more corrugations will be flattened and the resonance diminishes.
Graphs also show the ribbon fundamental resonance frequency that is under 20Hz.

I guess this proves that the resonance is due to corrugation?
Attached Images
File Type: jpg headphone_norm_tension_ear_side.jpg (78.4 KB, 238 views)
File Type: jpg headphone_norm_tension_outer_side.jpg (80.4 KB, 235 views)
File Type: jpg headphone_flat_foil_ear_side.jpg (79.0 KB, 52 views)
File Type: jpg headphone_flat_foil_outer_side.jpg (78.0 KB, 61 views)
File Type: jpg headphones.jpg (50.4 KB, 113 views)
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Old 26th October 2012, 03:11 PM   #36
dacen is offline dacen  Norway
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I am in the process of finishing a DIY Apogee (Duetta Signature) like speaker. Right now I am making a corrugator tool for the bass ribbon. I have got hold of 25um Kapton film and the Aluminium foil is the Danish type 9um thickness. (Paper backed with wax as the adhesive) I have the test procedure for some Apogee's. They are showing no such resonance?
Could this be taken care of by the passive crossover possibly? Can not attach the procedure PDF format here (too big). Those interested send me a PM and I will mail it to you.

Last edited by dacen; 26th October 2012 at 03:17 PM.
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Old 26th October 2012, 06:01 PM   #37
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Quote:
Originally Posted by dacen View Post
I have got hold of 25um Kapton film and the Aluminium foil is the Danish type 9um thickness. (Paper backed with wax as the adhesive) I have the test procedure for some Apogee's. They are showing no such resonance?
Could this be taken care of by the passive crossover possibly? Can not attach the procedure PDF format here (too big). Those interested send me a PM and I will mail it to you.
There are laminated foil too, 12Ám polyester with 9Ám aluminium. You will then remove the unwanted aluminium by etching.
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Old 26th October 2012, 08:06 PM   #38
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Quote:
Originally Posted by RogerGustavsson View Post
There are laminated foil too, 12Ám polyester with 9Ám aluminium. You will then remove the unwanted aluminium by etching.
I┤ll second Rogers suggestion. I used the mentioned method some 20+ years ago. If you know the exact thickness of your alu-foil then you can design the membrane so that you will get exactly the impedance you want. This is very nice since you are able to design the width of the aluminum so that it is exactly the same as your magnets.

When done properly (be very patient and careful!) you will get a very nice, even and smooth membrane without rattling at buzzing. Now comes the difficult part: Tensioning of the membrane. You need to construct a tensioning system at two of the sides so that you are able to get a perfectly smooth membrane which has exactly the "same tightness" in all four directions. This is easy if the area is a square or similar, but if two of the sides are not parallel (like in the Apogee types) this tightening procedure can be a little tricky - but not impossible at all.

Best wishes

Karsten

PS! If I remember correctly we etched the aluminum with a strong solution of NaOH. This was extremely hard to the lungs so please wear rubber gloves, safety-glasses and a filter-mask - and open windows . My HiFi-buddy dropped a little of the NaOH on his carpet and there was a big hole in it immediately
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Old 26th October 2012, 10:20 PM   #39
dacen is offline dacen  Norway
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I will use the hand cutting method. Have done a prototype uncorrugated. Placing the sheet, aluminium side to aluminium side and draw the pattern on the paper backing. Cutting through both of them and taping with 3m tape afterwards to keep everything in place. I use diluted contact adhesive on both the Kapton and the aluminium side of the ready cut membrane. Afterwards I use White Spirit on the paper side of the membrane and after a few minutes the wax dissolves and the paper comes off quite easily. Clean away the rest of the wax with a soft cloth moistiured in White Spirit. Membrane ready.
Have to do a lot of test corrugations to adjust the lenght of the ready made corrugated membrane. That is, so I can stretch it to the correct size. The adjustable springs are ready waiting for the tensioning process.

Last edited by dacen; 26th October 2012 at 10:47 PM.
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Old 27th October 2012, 07:34 PM   #40
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Quote:
Originally Posted by hanski View Post
I could try to duplicate Apogee corrugation if I knew how many corrugations/pleats they used per inch or cm.
I own Scintillas and they have 15 per inch on the tweeter and midrange ribbons. I've had them completely apart if you need photos
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