TD-15M Resonant Frequency Question - AVS Forum
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post #1 of 32 Old 05-04-2013, 10:48 PM - Thread Starter
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Greetings all,

I have received my TD-15Ms and made an impedance measurement on the units as manufactured, without any break-in.



Unit_01



Unit_02

As you can be seen from the plots, the two units measure uniformly, but, the measured Fs = 42.5 Hz, instead of the advertised Fs = 34.7 Hz, which is a difference of 22%. I am currently running repeated sweeps from 20Hz to 50 Hz, to exercise the units and will re-measure them after about 10 hours.

I am wondering what difference between advertised and measured Fs is considered acceptable.

Kindest regards,

M
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post #2 of 32 Old 05-05-2013, 12:16 AM
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as you likely know, fs is set by moving mass and suspension compliance.

the suspension will loosen up and fs could drop about that much as they break in.

might even go lower than spec after years of hard use, so its kind of tough to set a specific % and say if it is "in spec" or not. measurement method and conditions can also affect the result.

i wouldn't worry about it. just be happy with that "flat as a board" impedance response. :-)

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post #3 of 32 Old 05-05-2013, 07:15 AM
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LTD is bang on, break in will move Fs down. Especially aggressive break in.

But also drive level while measuring impedance. If you do like I do and just measure with a WT3 or soemthing, the drive level isn't high enough to move Fs into the correct place. This has definetely been my experience. For tweeter and very small drivers, the drive level is ok. But on larger drivers, they need a little more grunt. You can see this on some Klippel reports that include T/S parameters for "driver warm" "driver cold" "high input" "low input". The "high input" will have a lower Fs.

Good luck.
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post #4 of 32 Old 05-05-2013, 09:55 AM - Thread Starter
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Hi LTD02, tuxedocivic,

first, thank you for the replies. I am not sure in what time should the break-in show a change, but 11 hours later with sweeps at cone amplitude about +- 5mm, the plots are exactly the same.

LTD02,

I am little confused about the comment to "be happy with that 'flat as a board' impedance response". It is my understanding that the Fs affects the Fb, i.e., the onset of mass control region where the speaker rolls off. It is modeled as Fb = 68 Hz with the advertised Fs and 83 Hz with the measured Fs. Thus, if I interpret the model correctly, there is a loss of SPL exactly where one does not want it. Am I misinterpreting?

tuxedocivic,

I am measuring with a bridge and network analyzer. Are you saying that the Fs is driving signal amplitude dependent? If so, I will ask John at which amplitude he is measuring.

Kindest regards,

M
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post #5 of 32 Old 05-05-2013, 10:42 AM
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"I am little confused about the comment..."

i was simply saying that you have a good driver there. don't worry about the highish fs. it will come down over time.

here are models of sealed (red and yellow) and ported (grey and green) using the factory spec t/s and the tigher compliance (high fs) specs.

pretty much identical performance. don't worry about it.


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post #6 of 32 Old 05-05-2013, 11:35 AM
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Ok so your drive voltage is high enough it sounds like 1 or 2V is good. Unlike most impedance test kits which measure around 0.1V.

No change after 11 hours of 5mm excursion! I'd start to get a little concerned. Talk to AE.
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post #7 of 32 Old 05-05-2013, 11:41 AM
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"the models cannot be taken too seriously"

the models are usually quite accurate if the inputs are accurate. just changing the fs will change things. i reduced the compliance of the driver to get the fs that you measured and let the rest of the t/s specs recalculate. that is much different than just changing the fs.

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post #8 of 32 Old 05-05-2013, 11:54 AM - Thread Starter
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Hi LTD02,

thank you for the simulation. It seems that there should not be much trust placed on the simulation. Hers is, what UniBox simulates (sorry, I have not find any way to overlay the responses as you did):



Fs= 34.7 Hz



Fs= 42.5 Hz

Hi tuxedocivic,

yes, I have an access to a very nice lab. ;-). Thank you for the suggestion, I will send John a report.

Kindest regards,

Pavel
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post #9 of 32 Old 05-05-2013, 04:10 PM
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15-20% variation is common for the parameter tolerances. It's not uncommon for the Fs to stay the same after what you did to drive the woofer to reasonable excursion levels. Simply changing the Fs in the box software without accounting for Vas (compliance) won't give you an accurate simulation. Vas and Fs are interelated and will factor out each other if there is any suspension break-in. Assuming no manufacturing changes were made in the spider / surround / cone you will probably measure a lower Vas since the Fs is higher.

This is a good example of why static measurements and simulations will only get you so far. The best thing to do is test it nearfield at a reasonable (real world) drive level. I have a TD15M on hand that I'll be doing that with soon.

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post #10 of 32 Old 05-05-2013, 07:18 PM
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The Fs of the driver, or more exactly the compliance of the driver is the hardest thing to control. The variation in spider and cloth surround stiffness from batch to batch can be quite different. Ordering small quantities over a long period of time means that the cloth used is likely from different batches. The amount of resin impregnated into the cloth is also a delicate process to control. As a result, a variance of around 20% is considered very normal. As far as break-in goes, these will soften up more over time. I had several TD15M's for Elite Audio that had suspension similar to these. Over several years of live sound use, they are right down in the specified range now.

The good thing is, the suspension stiffness is one of the least critical things to worry about in the system. The compliance of the suspension is only a small factor in terms of the overall system compliance. The system is really dominated by the enclosure and not the woofer suspension. Here are 3 different models showing the specified compliance vs the higher Fs driver. The first is with the woofers in a 6cf vented box tuned to 40hz. The second is a 3cf sealed box. The final is in an infinite baffle. Even in the infinite baffle you can see that the curves are nearly identical to 30hz. Below that point the woofer would not be used and it wouldn't be a factor.


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post #11 of 32 Old 05-05-2013, 08:55 PM - Thread Starter
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Hi Rick, John,

thank you very much for your responses.

In view of them, I carried out an experiment. I mounted one of the TD-15M (Unit_01) into a 107 liter enclosure and swept the system. The measured Fc was 66 Hz. Then I swept an Altec 515B, to determine its Fs = 22Hz. Then I mounted the Altec into the same enclosure and swept the system. The measured Fc was 64 Hz.

Does this look reasonable? If so, the conclusion is that the measurement confirms John's assertion that the enclosure dominates.

Kindest regards,

M
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post #12 of 32 Old 05-06-2013, 04:43 AM
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Quote:
Originally Posted by mefistofelez View Post

In view of them, I carried out an experiment. I mounted one of the TD-15M (Unit_01) into a 107 liter enclosure and swept the system. The measured Fc was 66 Hz. Then I swept an Altec 515B, to determine its Fs = 22Hz. Then I mounted the Altec into the same enclosure and swept the system. The measured Fc was 64 Hz.
Does this look reasonable? If so, the conclusion is that the measurement confirms John's assertion that the enclosure dominates.
The enclosure dominates because no matter what the Fs of the driver the enclosure Fb remains the same. Fb is determined solely by the net enclosure volume and port dimensions. However, while it's the enclosure that determines Fb it's not Fb that determines the final result; Fb is but one of many factors.
As for Fs variation from spec, it's seldom lower, and IME may be as much as 10% higher without severely affecting the result. But more than 10% higher is unacceptable IMO.

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post #13 of 32 Old 05-06-2013, 10:41 AM - Thread Starter
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Hi Bill,

thank you for the further education. If the enclosure dominates, why the interest in low Fs drivers?

As your Fs variation, without discounting your 10% number, John, Rick, and other posters do not think that 20% is unacceptable.

Kindest regards,

M
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post #14 of 32 Old 05-06-2013, 11:21 AM
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Quote:
Originally Posted by mefistofelez View Post

John, Rick, and other posters do not think that 20% is unacceptable.
It t

I thought it was a bit unacceptable. Not to the point of sending them back without exploring more. But needing some attention.
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post #15 of 32 Old 05-06-2013, 12:54 PM
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Originally Posted by mefistofelez View Post

Hi Bill,

thank you for the further education. If the enclosure dominates, why the interest in low Fs drivers?
Load them in a properly engineered low Fb cab and they will go lower than high Fs drivers. I believe that you might think that the box Fb is directly related to the cab F3, and it isn't. F3 is the product of a lot of variables, Fb being only one.
Quote:
As your Fs variation, without discounting your 10% number, John, Rick, and other posters do not think that 20% is unacceptable.
Plus/minus 20% would be OK, but you'll seldom see Fs coming in lower than spec, almost always higher. And if Fs is way off, what else? It would set off an alarm with me, at which point I'd check all the specs.

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post #16 of 32 Old 05-06-2013, 05:05 PM
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Does John still include measured t/s with the drivers?
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No he does not.
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post #18 of 32 Old 05-07-2013, 08:10 AM
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We do test and measure every woofer before it leaves to make sure things are correct. This avoids most all issues with woofers when they arrive to customers. The problem with printing parameters and including them is that it requires a full break-in procedure to get parameters that correlate with the published ones. Hooking up to the computer to run a sweep is a quick process. Depending on which spider or surround is used though, it could take only a few minutes to a full day or more to get the suspension to reach the end compliance. The time to do this is just not something that is available to do on every woofer. I had previously just sent parameters pre-break-in but I spent a significant amount of time explaining why the Fs was higher and Cms was lower than published. It just became too time consuming.

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post #19 of 32 Old 05-07-2013, 10:18 AM - Thread Starter
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Hi tuxedocivic,

I apologize for not including you in the same position with Bill, but it was your input that prompted me to e-mail John, who responded both privately to me and to the forum.

Hi Bill,

you are correct, I have believed, before reading your response, that Fb and F3 are related. Any reading material on this topic you could recommend so that I can " properly engineer[ed] low Fb [(sealed)] cab"?

Regarding checking all the specifications, I checked all those that I could determine from the frequency sweep, and indeed all of them are off: I used the following Application Note for the calculations. http://documents.jordan-usa.com/Famous-Articles/Struck-Dual-FFT-Measurement.pdf. However, if someone smarter that I could verify my calculations I would be very grateful; I can provide the impedance measurement files.

Unit_01:

Calculations of T/S

Measured
Rdc 6.65 [Ohm]
Zmax 1.00E+02 [Ohm]

Caculated
Rc 15.10737414 [Ohm]
Rf 25.84735679 [Ohm]
f1 30.04 [Hz]
f2 56.2 [Hz]

Thiele-Small Advertised
Fs 42 [Hz] 34.7 {Hz]
Qms 6.240308254 5.09
Qes 0.442343713 0.35
Qts 0.413063726 0.33

Unit_02

Measured
Rdc 6.95 [Ohm]
Zmax 95.45943041 [Ohm]

Caculated
Rc 13.73516984 [Ohm]
Rf 25.75738809 [Ohm]
f1 30.09 [Hz]
f2 56.7 [Hz]

Thiele-Small Advertised
Fs 42.5 [Hz] 34.7 [Hz]
Qms 5.919173535 5.09
Qes 0.464789524 0.35
Qts 0.430950152 0.33

Hi djarchow,

I see that John has already answered your question. Incidentally, it was my question also.

In that respect, I respectfully disagree that including the measurement after manufacturing would not be useful because it would give people as I the assurance that the measurements are not in error.

Kindest regards,

M
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post #20 of 32 Old 05-07-2013, 11:48 AM
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Quote:
Originally Posted by mefistofelez View Post

Hi Bill, you are correct, I have believed, before reading your response, that Fb and F3 are related. Any reading material on this topic you could recommend so that I can " properly engineer[ed] low Fb [(sealed)] cab"?
Model the driver of your choice in WinISD Alpha Pro. You can go with the alignment it recommends, which defaults to the flattest possible response, or choose your own. I highly recommend comparing both vented and sealed alignments. Contrary to semi-popular belief there is no inherent superiority of one over the other, what matters is the results. Some drivers do work better in VBs, some work better in IBs, it's all a matter of the driver specs. If it won't go as low as you want you need to use a different driver.

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post #21 of 32 Old 05-07-2013, 12:34 PM - Thread Starter
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Hi Bill,

thank you for the response. The outlined methodology is exactly what I did, except using an UniBox instead of a WinISD.

Kindest regards,

M
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post #22 of 32 Old 05-08-2013, 06:24 PM
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M, what temperature are you measuring at?

It has a big effect on suspension compliance.
Quote:
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... Simply changing the Fs in the box software without accounting for Vas (compliance) won't give you an accurate simulation.

Yes; the right way to do it is to enter Mms and change compliance until Fs matches the measured value.
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The enclosure dominates because no matter what the Fs of the driver the enclosure Fb remains the same. Fb is determined solely by the net enclosure volume and port dimensions. However, while it's the enclosure that determines Fb it's not Fb that determines the final result; Fb is but one of many factors.

All true, and also applies to sealed box resonance Fc in cases where, as already mentioned, the air spring stiffness usually is much greater than suspension stiffness.

Noah
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post #23 of 32 Old 05-09-2013, 10:51 AM - Thread Starter
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Hi Noah,
Quote:
M, what temperature are you measuring at?

I am not sure I understand the import of the question. I understand that there is a temperature dependence, but I have no idea how to measure the voice coil temperature. Can you please amplify?

In my setup, the drivers are in a room with a temperature of about 70 deg F. Despite the +-5mm excursion, both the motor assembly and the phase plug are at the same temperature.

Yesterday, after 60 hours of such a regime, the Fs on both dropped by about 5 Hz.

Kindest regards,

M
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post #24 of 32 Old 05-09-2013, 11:30 AM
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Quote:
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In my setup, the drivers are in a room with a temperature of about 70 deg F.
That's what matters, not the VC temperature. At colder ambient temperatures suspensions are stiffer, giving a higher Fs reading. Voice coil temperature does influence the electrical parameters, but not Fs, which is a mechanical parameter.

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post #25 of 32 Old 05-09-2013, 11:45 AM
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Quote:
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Yesterday, after 60 hours of such a regime, the Fs on both dropped by about 5 Hz.

Now you're pretty close to where it doesn't matter, IMO.
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post #26 of 32 Old 05-09-2013, 01:05 PM - Thread Starter
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Hi Bill,

thank you for the explanation. In regards to your previous instruction on how to engineer the enclosure, I have re-done the simulations (UniBox) for a sealed enclosure and here are the results:

For constant Qa=80 and Ql=30

Volume [l] Fb [Hz] F3 [Hz]
70 80 78
80 76 77
90 73 77
100 70 77
110 68 78

So the F3 is minimized (very shallow minimum) between 80 and 100 l; the Fb decreases as expected, and the response below Fb is up by estimating 1 dB at 40 Hz (low end design) between the 80 l and 110 l enclosure.

So can I just chose the enclosure size from the interval 80 l to 100 l. Or should I choose the 100 l since the SPL at the low design frequency is 1dB higher?

Hi tuxedocivic,

yes, the result is very encouraging.

Kindest regards,

M
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post #27 of 32 Old 05-09-2013, 02:40 PM
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Quote:
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So can I just chose the enclosure size from the interval 80 l to 100 l. Or should I choose the 100 l since the SPL at the low design frequency is 1dB higher?
Typically you use the smallest box that gives response that you can live with. Going larger to get 1dB probably isn't worthwhile. The thing with an IB is that no matter how large you make it you'll only go so low. With a VB you can get considerably lower by going larger and tuning lower, though it won't be as flat.

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post #28 of 32 Old 05-09-2013, 03:09 PM - Thread Starter
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Hi Bill,

thank you for the reply.

Kindest regards,

M
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post #29 of 32 Old 05-09-2013, 06:09 PM
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Quote:
Originally Posted by mefistofelez View Post

...

So the F3 is minimized (very shallow minimum) between 80 and 100 l; the Fb decreases as expected, and the response below Fb is up by estimating 1 dB at 40 Hz (low end design) between the 80 l and 110 l enclosure.

There is no Fb for a sealed box.

It's Fc, resonant frequency of the cone (coincidental, as the "c" is for "closed"), determined by Mms and the combined stiffness of the suspension and box springs.

Fb (which I believe would more appropriately be Fp, for port) is for a vented box, and is the resonant frequency of the air in the port, determined by its mass and the box air stiffness.

Noah
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post #30 of 32 Old 05-09-2013, 06:26 PM - Thread Starter
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Hi Noah,

with all due respect for your knowledge, here is a plot from UniBox, simulating sealed enclosure. Please note the upper right corner.



Furthermore: http://sound.westhost.com/tsp.htm recites: "Fb is the resonance frequency of speaker and box combined." Perhaps it is a matter of terminology?

Kindest regards,

M
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