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Old 4th August 2017, 01:19 PM   #21
lowmass is offline lowmass  United States
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Lupi, looks like your havin some fun.

Im curious about the HF drop and have two questions...

1- have you done FR tests between the 10um and the 80um ribbon using say a 4 ohm resistor in series? This to eliminate the inductance effect, give both basically the same load, and see if mass has ANY effect on HF performance?

2- Also what are you driving your ribbons with? direct? transformer?
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Old 4th August 2017, 06:09 PM   #22
JonasKarud is offline JonasKarud  Sweden
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Very nice! This is truly a Diy Project.
Whats the name of the magnet field simulation program?
__________________
Reality is just a word....
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Old 5th August 2017, 01:10 AM   #23
lupi is offline lupi  Italy
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Hi lowmass
It is true, for me it is a fun pastime.
Answer to step 1:
I made measurements in free air, with ribbons of either 10 or 80 um and without R in series, or with 1 ohm. I did not use higher Rs to not attenuate too much the signal. Unfortunately I did the 10 um ribbon measurements with a 0.6mm^2 1.2meters long connection cable, which has a not negligible inductance, so it increases the R series effect . The measurements on the 80 um ribbon I have instead made with a special connecting cable, with very low inductance. I send you both so you can judge yourself, taking into account the difference in the cables. Uncalibrated system, the lower curves are those with 1 ohm in series.
10um_Rserie.jpg
80um_Rserie.jpg
Answer to Step 2:
Direct driving with an amplifier I did for these tests
Attached Images
File Type: jpg 10um_Rserie.jpg (106.3 KB, 185 views)
File Type: jpg 80um_Rserie.jpg (114.3 KB, 179 views)
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Old 5th August 2017, 01:17 AM   #24
lupi is offline lupi  Italy
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Quote:
Originally Posted by JonasKarud View Post
Very nice! This is truly a Diy Project.
Whats the name of the magnet field simulation program?
Thanks JonasKarud
The name of the magnetic field simulation program is MagNet of the infolytica corporation
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Old 7th August 2017, 02:12 PM   #25
lowmass is offline lowmass  United States
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Quote:
Originally Posted by lupi View Post
Hi lowmass
It is true, for me it is a fun pastime.
Answer to step 1:
I made measurements in free air, with ribbons of either 10 or 80 um and without R in series, or with 1 ohm. I did not use higher Rs to not attenuate too much the signal. Unfortunately I did the 10 um ribbon measurements with a 0.6mm^2 1.2meters long connection cable, which has a not negligible inductance, so it increases the R series effect . The measurements on the 80 um ribbon I have instead made with a special connecting cable, with very low inductance. I send you both so you can judge yourself, taking into account the difference in the cables. Uncalibrated system, the lower curves are those with 1 ohm in series.
10um_Rserie.jpg
80um_Rserie.jpg
Answer to Step 2:
Direct driving with an amplifier I did for these tests
Ok yea not sure what effect direct drive from amp may be having here and theres a few variables that im not sure about.

The reason I ask is ether inductance OR ribbon mass should give you a drop in high frequency levels.
The last time I looked at this I think I got approximately 5-6 db drop at 20k by increasing ribbon mass about 3X. Cant remember the exact ribbon thicknesses but I think I went from 6 micron to about 18 micron thick foil. To get same load to amp I simply used a 4 ohm resistor in series with both ribbons and measured at low volumes up close.
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Old 7th August 2017, 09:06 PM   #26
bolserst is offline bolserst  United States
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Quote:
Originally Posted by lupi View Post
…I had to do the measurement with a mic at 20 cm because at higher distances the responses are altering too much. I have walls and close objects, and putting even 8 cm thick absorbents does not work…
Nice ribbon project.

REW provides the ability to window the impulse response to remove reflections from nearby objects and walls. The only drawback is that the length of the window limits how low in frequency the measurement is valid. If you have at least 3mS between direct impulse arrival and the first reflection, you will have a good measurement for frequencies above 500Hz. This is plenty to get a good idea of response and directivity at distances more representative of your listening position.

Attached is a guide on using the windowing technique with REW taken from:
https://www.minidsp.com/applications...d-measurements
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File Type: pdf REW_windowed_measurement.pdf (330.4 KB, 20 views)
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Old 7th August 2017, 10:43 PM   #27
bolserst is offline bolserst  United States
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Quote:
Originally Posted by lowmass View Post
Ok yea not sure what effect direct drive from amp may be having here and theres a few variables that im not sure about.
With direct drive the output impedance of the amplifier can have a large effect. As you mentioned previously, when driving ribbons without a transformer it is advantageous to add series resistance to ensure constant current drive no matter what the ribbon design or amplifier output impedance is. The only drawback is the need for a more powerful amplifier and the resulting power wasted heating the resistor.

Quote:
…inductance OR ribbon mass should give you a drop in high frequency levels...
This is exactly right. Most people are familiar with high frequency roll off due to inductance forming a 1st order electrical low pass filter with a resistive load. Similarly, the ribbon mass forms a 1st order low pass filter with the resistive part of the radiation impedance (airload on the ribbon)….the only difference being that this is a mechanical/acoustical filter.

Above resonance, most dynamic drivers are mass controlled (ie piston mass >> airload) and the motion of the piston is dictated by the piston mass. When driven with constant force, this results in piston velocity falling -6dB/oct with increasing frequency, and a flat on-axis response when mounted in an infinite baffle or enclosure. Most woofer box design programs actually ignore the airload all together. At the other extreme, if the piston is weightless, the motion will be completely dependent on the airload. At "low" frequencies the airload acts like a mass so piston velocity and on-axis response are just like for the mass controlled case above. But at "high" frequencies things are different…the radiation impedance is resistive so constant force results in the constant piston velocity and the on-axis response tilts up at +6dB/oct.

The determination of what is meant by "low" and "high" frequencies depends on the size of the piston. The transition between flat and rising response occurs around the frequency where the wavelength of sound is similar in size to the piston diameter or width. This is usually identified as ka=1 in normalized plots shown in acoustic texts. Since ESLs tend to be much wider than ribbons, their transition frequency is lower and helps to explain why the high frequency roll off with diaphragm mass is better known by ESL builders. BTW, you will also notice that this ka=1 point is the same point at which the piston starts to become directive and highs roll off as you move off-axis. It is not a coincidence. This “beaming” is actually what keeps the on-axis response flat for a mass-controlled piston even at high frequencies.

Practical ribbons tend to have mass in-between these two extremes; not small enough to be considered massless, but not heavy enough to be considered mass controlled. To illustrate, here are a few plots for a theoretical 40mm wide ribbon driven with constant current.
Attachment #1: Trends for increasing ribbon mass all driven with the same current. Note that the 80 μm is very nearly to the point of behaving like a mass-controlled piston.
Attachment #2: The same measurements, but level matched to more easily see the trends.
Attachment #3: The same measurements, but normalized relative to the massless response to highlight the 1st order low pass behavior.

Quote:
…The last time I looked at this I think I got approximately 5-6 db drop at 20k by increasing ribbon mass about 3X. Cant remember the exact ribbon thicknesses but I think I went from 6 micron to about 18 micron thick foil.
That sounds about right, assuming typical ribbon width in the 15mm – 40mm range.
Attachment #4 shows a level matched comparison from several years ago between 11 μm and 2 μm ribbons driven with constant current.
They were 20mm wide dipole ribbons with minimal baffle so the response slopes downward at low frequency, but the mass effect is still clearly seen.
Attached Images
File Type: png Ribbon_mass_trends_01.png (30.2 KB, 129 views)
File Type: png Ribbon_mass_trends_02.png (30.0 KB, 125 views)
File Type: png Ribbon_mass_trends_03.png (33.9 KB, 127 views)
File Type: png ribbon_mass_compare_11vs2.png (21.1 KB, 33 views)

Last edited by bolserst; 7th August 2017 at 10:52 PM.
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Old 8th August 2017, 12:42 AM   #28
lupi is offline lupi  Italy
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Hi Lowmass
Bolserst described the phenomenon better than I could, and in the light of what he said, we can understand both my case, in which I had a linearization on the highs by inserting a series resistance, and yours, as long as the width of your tape is less than 40mm as in my case
lupi
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Old 8th August 2017, 01:00 AM   #29
lupi is offline lupi  Italy
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Hi bolserst
Thank you for explanation and for attachement
Until now I did the pulse response with a pulse generator and the oscilloscope, with which I can limit the window to please, but certainly with REW will be more comfortable. I will also make, at the beginning, a comparison between the oscilloscope and REW responses
lupi
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Old 8th August 2017, 01:37 PM   #30
lowmass is offline lowmass  United States
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Thanks bolserst, excellent explanation.
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