Measuring T/S parameters

I'd like to hear from others about the methods used when measuring a driver's Fs and for calculation of Vas, specifically the mounting and orientation of the driver being tested. In the past, I've hung the driver from the ceiling for free air measurement, and a test box for Vas. Although my results are often fairly near published data, I've never been confident that I'm getting useful data. Below is some text from Speaker Workshop's help file on the subject, which doesn't seem entirely logical either;

"Impedance in Free Air
To measure driver impedance in free air, the driver should be suspended in air far from any acoustically interfering surfaces. Hanging it from the ceiling will NOT work - the driver must be kept physically stable or the shaking from the sound will affect the impedance measurements. The recommended approach is to take two heavy tables and two clamps then put the tables near each other and clamp the driver between them. "

This seems to imply the DUT is situated with the axis vertical, which brings gravity's effect into the equation (only there's no allowance in the real equation). Also, isn't being clamped between 2 heavy tables a little contrary to the "free air" stipulation?

"Impedance in Sealed Box- The Delta Compliance method
When measuring the impedance in a sealed box (for estimating T/S parameters) the critical value is the volume of the box. This measurement is only used for estimation of Vas."

Straightforward except no mention of axis orientation. Also, for best results, the test box needs to be fairly near the DUT's Vas in volume, which could mean having a variety of test boxes on hand.

"Add extra mass- The Delta Mass method
Add additional mass to the cone in such a way that the additional mass is known exactly and the mass won’t flex when the cone moves."

Again, implied vertical axis but no factor for gravity in the equation.

How do you guys do it, and how close to published data do you get?

I've tried to follow the steps outlined in this article, but the speakers I’ve measured don’t match the specs too closely.

http://sound.westhost.com/tsp.htm

Thanks, John, that's what I'd been using until recently too. It doesn't get very specific about the driver orientation either, though the illustrations show a driver "face-down" in the test box. I've checked the formulas in the spreadsheet, and there's no factoring of gravity there either.

I can't help but feel the driver should test horizontally, unless the calculations factored gravity into the picture. Some drivers would see little variance, but some would see a lot.

I've never used the delta mass method, but if the driver is positioned face-up on the bench, you'd have the bench as an interfering boundary as well.:dont-know

I measure Fs horizontal, face up on the concrete floor, with the driver sitting on stainless steel blocks so the vent is clear. I believe I get good results that way, and have been meaning to fabricate some stainless rings for better support.

WT2 website shows a "Bunje" suspension method; I've been wanting to try that. There's also ap notes about methodology there. WTPRO is a member here; check his prior posts for some discussion of these matters.

Thus far, Vas has been a crap shoot for me. The nickels don't seem to be reliably "one" with the cone, and I'm not about to stick stuff there....

Thanks, Zilch, good stuff on the WT2 site, including the first reference I've seen to justify my suspicions. Even a conversion from "free air" to "free space", and acknowledgement of sag-related issues too. Guess I just need to develop confidence in a method and go with it.

Had I never read Clark's paper ( http://jipihorn.free.fr/Projets%20en...s/dlcpaper.pdf ) on the ultimate measurement setup, stuff like this wouldn't bother me so much.;)

I've never done Delta Mass, but always thought a handful of tiny, flat magnets would be the ticket. Just symmetrically place pairs (one on each side of cone) to get the needed mass in a distributed area that'll handle the ride.

'Wonder if there's a standard for this procedure, or if it's as user-definable as the old 1W/1M. I have to admit having serious doubts about some of the published T/S specs (non-JBL), after doing a little testing of my own.

Quote:

---

Originally Posted by moldyoldy

I have to admit having serious doubts about some of the published T/S specs (non-JBL), after doing a little testing of my own.

---

Moin (*) Moldyoldy,

you are absolutely right, there are great discrepancies. On the other hand it must be considered that two speakers of the same model behave similar although their measured spec are (a bit?) different, there seems to be a sort of compensation. Many years ago I read a serie about TSP measuring and they stated this. And they made clear that the results depend on the method of measuring.
My concern has been more that pupils can handle it, acuracy was not at first place. ;)
___________
Peter

*) "Moin" means in northern Germany "hello", "how are you" and the like. First of all it is short and it can be said all day long, morning, midday, evening and at night. It is said that people in northern Germany do not talk so much, and in the country side it is a real fact. So, with "moin" all is said.
Sometimes it is said "Moin moin" :blink:
This mean that the one is in a good mood, the person seems to be talkative.

Moin, Peter! Always good to learn a new word, especially one that can replace a lengthy dialogue. Danke. :)

I use the original woofer tester and get good results. I tested three different drivers with it and then used manual methods. All three came within 1% of what the woofer tester produced. I use modeling clay wrapped in plastic for weight.

For the Vas computations I have a 1 cu ft test box with different cutouts. After I am done with a project I test the driver in the box for Vcb and Vfb and the results are alwys what I am expecting.

One thing I have noticed is that if two drivers are the same model but have different T/S specs, they seem to work the same when built into boxes. Differences in one parameter seem to be compensated with another. Doesn't seem to make sense but it does.

I also notice the FS drops 10 - 20% after warmup and after a breakin period. Run a new driver for a day with 25 Hz at 1 watt to loosen up the cone before testing

If you are wondering about Cms-vs-Drive level, the WT has a built in test to extract that information.

http://www.woofertester.com/cms_non_linearity.htm

Regarding how much of an effect Cms variation will have on response, consider how much stiffness the box air spring will provide. If the ratio favors the box, Cms variation wont matter much.

And how close can you get with good measurement and modeling? Here is another link. These kind of results are quite typical using nothing more than the delta mass test. And... I did not even have to enter the box volume or tuning. The software finds this automatically and aligns the simulator!

http://www.woofertester.com/wt2boxbuild.htm

Regarding the bouncing nickel problem.

1) Lower Idrive. The drivers being used on this forum have a lot of oomph and it is not uncommon for the cone acceleration to be higher than gravity can keep the mass firmly in place.

2) Glue them together in groups of 5 or so. This will add a teeny bit of mass, so you will want to measure the stack before using them.

Hope this helps,
Best regards,
WTPro

Thanks Keith! :)

I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.

Quote:

---

Originally Posted by Giskard

Thanks Keith! :)

I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.

---

Yea, thanks Keith. Very cool.

Sorry again to have missed your stand at CES. We were late; it was busy. Another time... :yes:

Quote:

---

Originally Posted by Zilch

I measure Fs horizontal, face up on the concrete floor, with the driver sitting on stainless steel blocks so the vent is clear.

---

I have measured Fs on a carpeted concrete floor raised on blocks and also with the woofer suspended and firing vertically in open air... here is a fairly significant difference... even hanging the driver near other objects will affect your results.

Quote:

---

Originally Posted by Giskard

I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.

---

I use double faced tape too... I use one that releases easily when you put a drop of denatured alcohol on it... I made an aluminum donut that I stick on the dustcap.


Widget

Thanks for all the input, guys! Since several of you are apparently getting satisfactory results with the driver in a face-up position while resting on a substantially solid surface, measurement error from a vertical axis orientation must be far less than I expected. The error does exist though, and will directly influence the resultant data as postulated by Clark;

"At and below a direct radiator's primary resonance frequency, suspension stiffness becomes equal to and surpasses the mass reaction to the driving force. While this is below the "flat" frequency range, it is important to downward response extension and the transition from fiat to roll-off. Thus, suspension stiffness variations with position become important below the "fiat" range. Diaphragm excursion is highest in this range making the importance of suspension linearity second only to force factor linearity.
However, in the Thiele-Small model suspension linearity becomes critically important because four of the five T-S parameters contain suspension stiffness as a factor in their definition."

He goes on to point out that compliance and Bl are the main parameters that are position-dependant in regards to T/S parameter measurement. An ideal test platform should therefore rigidly fix the driver in a position of horizontal axis with no nearby solids or boundaries, for both the Fs and Vas tests. I have yet to devise a practical and convenient way to satisfy these conditions at home.

From a hobbyist POV, it appears that the position-induced error of a face-up driver rigidly fixed on a solid surface is likely not as bad as the error obtained from a less-rigid suspended mount with a horizontal driver axis and no nearby boundaries.

Hmmm.....think I'll try one mounted to my fireplace mantel.....

Quote:

---

Originally Posted by moldyoldy

From a hobbyist POV, it appears that the position-induced error of a face-up driver rigidly fixed on a solid surface is likely not as bad as the error obtained from a less-rigid suspended mount with a horizontal driver axis and no nearby boundaries.

---

I believe that is true. Just buy yourself a punch down panel frame and bolt it to your floor. It is rigid enough.

Quote:

---

Originally Posted by Giskard

I believe that is true. Just buy yourself a punch down panel frame and bolt it to your floor. It is rigid enough.

---

Thanks, Giskard, in lieu of ever matching factory specs at least once with balls-on accuracy, I'll settle for the reassurance that I've done as good as I could with what I had. Kindy' along the same line as how the girls always get prettier at closing time.:cheers:

At low levels, many solid objects will work just fine as a test surface. The way you can tell if you have a problem or not is to examine the phase and impedance curves for anomalies. For myself, when I want to accurately know Fs I often use the edge of one of my rather heavy sub woofer cabinets. However, in some cases this does not work, and in my case utterly fails at high power.

The problem with 'solid' test rigs is that they are often not as solid as you may think when it comes to resonance control. The problem is that *any* spring and mass will resonate at the radian frequency Wms=sqrt(Kms/Mms) where Wms=2*PI*F, K is the spring stiffness (Kms=1/Cms) and M is the moving mass. Therefor even a very heavy mass can resonate at exactly the wrong place if K is just right.

My big problem came when I was working on our high power TS testing. Basically when a driver is moving +/-20mm you cant simply place it on a solid surface, nor is it practical to build an utterly massive test jig. The driver either walks all over the test surface, has a resonance none the less, or shakes the test rig to pieces. I was in a fix until I turned the problem back on itself. It is quite common for me to take an problem I am trying to eliminate and intentionally look for ways to make the problem worse. What this often leads to is a deeper understanding of the root causes, and quite often a fresh look at how to control the problem!

The wild thing about the Bunjiggy rig is that it exhibits none of these problems, costs practically nothing and Fs shifts upward a mere 1% (often less). What's more, you can suspend it from almost anything. The 1% Fs shift can even be improved by 'massing up' the driver frame. In this case envision a 100-lb sturdy metal frame to hold the driver that is suspended from bungee cables. At our CES booth I had a 12" driver doing the Xmax dance while hanging from the wire rack that also held our DLP projector. There was *no* jitter in the DLP image at 10 feet! Edgewound came by at one point, so maybe I showed him this.

A heavy hanging frame can also be suspended for side to side motion. It is way less convenient and requires a sticky mass (mess?), but there is no suspension droop. In my opinion this would be the ideal rig for the Q/FS and Vas using delta mass tests. Or, as I have eluded to in our web page... work the math backwards and all is revealed without needing the heavy rig!

One additional note: To avoid controversy, we adopted the conventional LDC wisdom (Ye' Old Resistor and Voltmeter) that the test mass should be ~44% of Mms causing Fs to shift down by about 25%. The WT2 is way more accurate so this mass can be quite a bit smaller. I suppose I should measure and post this at our web page.

Sorry for the long post. Hopefully this will be of interest. I also agree with the 'closing time' comment, though that is now a few years back!

Best regards,
WTPro

Bingo!

That's the kind of stuff I wanted to hear. Elaboration isn't wasted on this hillbilly, thank you WT.

Compared to other sciences, audio really jams me up sometimes. I mean, I know that the old 'equal but opposing force' rule applies to a 30g cone and a 30 lb basket, but at mW levels, it's hard to imagine any more than a shorthair on a gnat's ass. Then there's the infinite baffle...that isn't...anymore than an infinite horn is. I couldn't get either in my LR anyway, but I still have to design one to build some bedroom boxes. What really gets me though, is doing a Dr. Jekyll and getting chummy with all types of distortion as a musician, then doing the Hyde flip when I turn on the stereo and have to abhor all forms of it.

Or I might just have a dose of cabin fever...;)

Yeh, cranking up the signal amplitude was really informative for me. it was someone on this board that once mentioned using sawdust on a cabinet wall to also check for resonance's. Another good idea along the same line! I knew that just because I could not feel a low amplitude resonance it did not exist because I could easily measure it electrically.

The Dr Jekyll comment is well founded but depends a bit on the type of recording and playback method. When I consider pop/rock the fact that a studio technician follows the musician, I really begin to wonder just what the original sound was like. I mean, if you really listen to recordings they are all over the place! My opinion is that the reproduction path needs to be flat and smooth and after that it is a matter of personal taste using simple tone controls. BTW, you cant easily fix something that does not start out flat and smooth (along the lines of... You cant fix stupid!).

I also really like is this board because the folks here are into the science of what works. It is something we are trying to do at my our little 2 man company as well.

If you come up with a cure for cabin fever, send it my way. I am not much into this New England winter thing.

WTPro

Quote:

---

Originally Posted by WTPRO

If you come up with a cure for cabin fever, send it my way. I am not much into this New England winter thing.

---

As a scientist, you should take full advantage of this opportunity to determine empirically how long a six-pack of your favorite beer can be safely stored in snow without freezing.

Sample at regular intervals to assure it doesn't happen. (Destructive testing required.)

Repeat the experiment under varying conditions: sunny, overcast, snowing, nightime, etc.... :D

BTDT. I've got an electrical model of the process somewhere under all these cans and bottles.....I determined that while my average winter clime is sufficiently ugly to hinder my fishing, there's only 2 or 3 nights a season cold enough to make a (good) beer grow an ice leg, and the process of gathering data obviates the result.

Just think of those lucky folks in places like Minnesota that get to fish year round... 11 months of ice fishing interrupted by 1 month of bad ice fishing! Boston isn't so bad.

Moin,

when it still holds "Elaboration isn't wasted on this hillbilly, ... " here is a remark on gravity:

In an ideal approach it has no effect. Regarding the suspended cone as a spring, force and elongation are proportional (linear behaviour). This means that for a certain force there is a certain elongation. This is even true if there is already another force acting. When turning the cone facing upwards, there is the force of the cones weight, but the proportionality will not change.

Facing upwards the null position will change. It depends on the manufactorers recommendation whether a speaker may be used in this manner. A very high compliance driver may run into complications but there could be added a small amount of DC (I have never read about such an approach, but it might work because at full power the currents and forces are much bigger.)

"I mean, I know that the old 'equal but opposing force' rule applies to a 30g cone and a 30 lb basket, but at mW levels, it's hard to imagine any more than a shorthair on a gnat's ass."
Newtons law is always true. But for example when looking into atoms you need quantum physics as well , looking to the stars you need theorie of relativity as well. Resonances can be a real pain. (Once I rolled a speaker with a heavy top over an uneven ground. The top, ca. 30 kg, jumped down like a ball :(.)

"Then there's the infinite baffle...that isn't...anymore than an infinite horn is."
Yes, these are only theoretical simplifications to get a start. Electroacoustic is not that easy (- except we are "esoteric" and have our one and only golden rule - :o: ) although with todays standards we can easily be very successful :) . When turning into a practical design we always have to make a compromise, or at least we have to consider new effects or rules. We can approach the infinite baffle by a closed box. But all of sudden we have resonances as well, and the mass of the moving air will change too.
When tuning a closed box it is often done by changing the stuffing. A better way would be to choose another volume (rebuilding), as the fiber filling has ist own laws: "Fillings" by W. M. Leach, Jr.
http://users.ece.gatech.edu/~mleach/papers/Filling.pdf

Hope this has been useful.
___________
Peter

A belated "Thank You", Peter.

Thought I'd update this thread, as I found some new (to me) info.

(Note, this isn't a product endorsement, and isn't meant to detract from the good folks at Woofer Tester, whose input is greatly appreciated.)

I use DPC;

http://home1.stofanet.dk/cfuttrup/dpc.htm

but either due to bad links or my browser settings, was never able to view the documents referred to in the manual;

http://home1.stofanet.dk/cfuttrup/dpc/dpc.htm

In particular, the "Measure htm" document addresses the reason for this thread;

http://home1.stofanet.dk/cfuttrup/dpc/measure.htm

If I'm the only one that couldn't access the info, boy do I feel dumb! If not, the following Google search results provide working links to the bulk of the remaining docs;

http://www.google.com/search?sourcei...estofanet%2edk

Lots of material, easier to grok, valuable resource. :applaud:

OT update RE: "Cabin Fever";

I've caught several really nice walleye in the last 10 days, 2 were over 10 lbs each!

I'm feeling much better now...:p

This is where I am. I want to more accurately measure valid parameters.

Can anyone offer more "science" to this thread?

I totally get all the related math, but how do you get a decent "first" measurement of Fs that is irrefutable?

JBL has added Woofer Tester to their toolbox.

Saw that in one of the 1500AL tech pages... a bit surprised, actually.

Necro but why start a new thread?

I'm wanting to test Fb on some boxes I've built and am wondering if in lieu of a resistor I can sub in an L-pad set to about 8 ohm. I don't have any resistors handy.


Thanks,

Warren

http://www.cfuttrup.com/dpc_main.html