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!
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.
Sample at regular intervals to assure it doesn't happen. (Destructive testing required.)
Repeat the experiment under varying conditions: sunny, overcast, snowing, nightime, etc....
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.
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 - ) 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.
Hope this has been useful.
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;
but either due to bad links or my browser settings, was never able to view the documents referred to in the manual;
In particular, the "Measure htm" document addresses the reason for this thread;
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;
Lots of material, easier to grok, valuable resource.
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...
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?
Saw that in one of the 1500AL tech pages... a bit surprised, actually.
"No Walter, I don't think Larry was about to crack."
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