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Old 1st June 2005, 11:15 PM   #11
soongsc is offline soongsc  Taiwan
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Since ferro fluid causes friction, at low levels, the hystersis this friction causes becomes a more significant content of the signal, thus there would be a change in measured resonant frequency because of this.
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Old 2nd June 2005, 07:02 AM   #12
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Didn't know that these drivers use ferrofluid.

According to M. Colloms also spiders, surrounds and stuffing (not in the free-air case of course) tend to cause hysteresis.

Since these are very high-quality drivers I assume that they don't show too much hysteresis and that the cause must be found elsewhere.

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Charles
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Old 2nd June 2005, 09:18 AM   #13
fiak is offline fiak  Europe
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Hi George.

The difference of 10Hz at speaker unit fs, in the range of 500mV looks serious.
If verified to happen in a variety of drivers, the excitation level sould be stated next to fs parameter at specifications.
Can you do the same measurement using a larger woofer?

My best. Anastasio.
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Old 2nd June 2005, 09:59 AM   #14
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by phase_accurate
Didn't know that these drivers use ferrofluid.

According to M. Colloms also spiders, surrounds and stuffing (not in the free-air case of course) tend to cause hysteresis.

Since these are very high-quality drivers I assume that they don't show too much hysteresis and that the cause must be found elsewhere.

Regards

Charles
Then maybe there is some other explanation. I accept the fact that different test methods would yield different results, but it seems that the plots George is showing uses the same method if I read his post correctly.
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Old 2nd June 2005, 04:24 PM   #15
soongsc is offline soongsc  Taiwan
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I checked the web site which mentions JX53 fluid cooled, but not in the JX92S. So maybe there is something else causing variation.
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Old 6th June 2005, 05:39 PM   #16
gpapag is offline gpapag  Greece
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Default Speaker's Fr variation with signal level. Further measurements.

High all
I managed to gather 10 woofers ("Fiak" kindly provided 5 of them and participated in the tests) of various diameters and brand names. I tested them for Fr variation as I do with the JX92S. Results on attachment.
As shown, the trend observed on JX92S (Fr decreasing for signal increasing) applies to all tested woofers as well, but with various slopes. Fr percentage change varies from 3.5% to 14% (16.4% for JX92S).
Only one woofer showed Fr decreasing up to a point and then increasing again.
I also tested 2 car full ranges and one TV full range.Same trend. Fr change varies little though (up to 3.5%). All three have very low Xmax (less than 1mm).
The power of the signal (VxI) at all measurements did not exceed 75mW RMS (110 mW RMS for JX92S)
Work, still in progress.

Regards
George
Attached Images
File Type: jpg various woofers.jpg (75.9 KB, 192 views)
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Old 7th June 2005, 12:25 AM   #17
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by phase_accurate
Didn't know that these drivers use ferrofluid.

According to M. Colloms also spiders, surrounds and stuffing (not in the free-air case of course) tend to cause hysteresis.

Since these are very high-quality drivers I assume that they don't show too much hysteresis and that the cause must be found elsewhere.

Regards

Charles
Given the new information, this statement seems correct. Even in free air, the VC cap and the space between that and the magnet structure would also play an important role. But I doubt there would be a trend directly related with the type of design.
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Old 7th June 2005, 12:42 AM   #18
soongsc is offline soongsc  Taiwan
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Default Re: Speaker's Fr variation with signal level. Further measurements.

Quote:
Originally posted by gpapag
High all
I managed to gather 10 woofers ("Fiak" kindly provided 5 of them and participated in the tests) of various diameters and brand names. I tested them for Fr variation as I do with the JX92S. Results on attachment.
As shown, the trend observed on JX92S (Fr decreasing for signal increasing) applies to all tested woofers as well, but with various slopes. Fr percentage change varies from 3.5% to 14% (16.4% for JX92S).
Only one woofer showed Fr decreasing up to a point and then increasing again.
I also tested 2 car full ranges and one TV full range.Same trend. Fr change varies little though (up to 3.5%). All three have very low Xmax (less than 1mm).
The power of the signal (VxI) at all measurements did not exceed 75mW RMS (110 mW RMS for JX92S)
Work, still in progress.

Regards
George
This is really quite new to a lot of DIYers. It seems that if we design acording to the flatter portion or the larger signals, it would be best for base design. If someone wants to optimize for different levels, then the middle or lower signal resonance could be used. I wonder whether there would be any audible difference though.

When the shape of an impedance curve varies based on signal level, thit would be a matter we would need to consider more.
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Old 8th June 2005, 08:49 PM   #19
gpapag is offline gpapag  Greece
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Default There is nothing new under the sun!

A few days ago I purchased the excellent book "TESTING Loudspeakers" of Joseph D' Appolito (1998 Old Colony Sound Lab): From this, I quote:

Section 2.6.3.2 DRIVER PARAMETERS AS A FUNCTION OF INPUT POWER
(refers to a 8" unit with +/- 7.5 mm stated linear throw))
TABLE 2.1
DRIVER PARAMETERS AS A FUNCTION OF VOICE COIL VOLTAGE Vvc
Vvc -fsa
0.5 -30.8
1 -29.9
1.5 -28.6
2 -28.2
2.5 -28
3 -28
3.5 -27.9
4 -27.9
5 -27.9

"...Notice that fsa decreases with driver level although it does stabilize 10% lower at drive levels of 3V and above..."

So it seems that Joseph D'Appolito is certain about such a behavior (while on fourth edition-1991 Old Colony Sound Lab-of the "The loudspeaker design cookbook" by Vance Dickason" section 8.30 it is stated:"...measurement should be made at the lowest nominal voltage level at which your equipment will function. Less than 1 V would be preferable. As the driver level increases especially when measuring small diameter drivers, the resonance frequency also moves upward. The measurements being made are for use with small-signal mathematical models, so the measurement voltage should be "small".)
So, what I have observed and reported so far is not unknown or queered. This is good. I don’t have to do any more measurements.
Anyway there is (at least) one more topic open. The difference in apparent Fr values of a driver obtained with “Speaker Workshop” (SW) and manual method. I had to do SW measurements by varying the excitation signal (through an external amplifier) to see if the discrepancy would disappear with higher signal. The results were not promising:

Exc. Voltage -Fr
mV RMS -Hz
53 -54.3
85 -54
112 -53.8
169 -53.5
208 -53.8
365 -52
422 -52.8

Measuring such a signal with an RMS voltmeter is not a sound practice, but for inter-comparison it is ok. Although three measurements were c/out for each signal level and results averaged, uncertainty is still high (+/- 1 Hz). Even so, chances are that apparent (indicated) Fr remains quite stable and 7-8 Hz higher than the Fr measured manually.
I still vote for the manual method, but I would be more that happy if proof for the opposite can be found.
Regards
George
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Old 9th June 2005, 12:05 AM   #20
soongsc is offline soongsc  Taiwan
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Due to the fact that normal manufacturing inspection allows for around 10% tolerance, I think it is more important to look at how much difference will cause significant effect.

The things related with this difference are:
1. Zobel network, if you use it. It is possible to just use SPICE to simulate the changes and effect.

2. Box design, it is possible to use Martin J. King's Mathcad Worksheets to simulate the effect. Note that temerature would also effect speed of sound, and thus efect the final result.

Another thing to remember is that 1W into an 8 ohm loud is 2.8 V, how taken the ear is less sensitive at low frequencies, maybe higher voltage values makes sense.

Also, don't forget the magnitude of impedance difference is also important. Do you measure that the same way?
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