Hi just a guy,
Correct - although as soongsc has quite rightly pointed out, our discussion has only focussed on the stress due to differences in air pressure. The very significant contribution of the driver diaphragm and voice coil dynamic mechanical mass has not been considered.
The only reason I mentioned the formula was because I thought that in your reactance annulling experiments you were trying to equalise the pressure on each side of the diaphragm. The chart value would become zero in this case and could be easily seen.
Kind regards,
David
Correct - although as soongsc has quite rightly pointed out, our discussion has only focussed on the stress due to differences in air pressure. The very significant contribution of the driver diaphragm and voice coil dynamic mechanical mass has not been considered.
The only reason I mentioned the formula was because I thought that in your reactance annulling experiments you were trying to equalise the pressure on each side of the diaphragm. The chart value would become zero in this case and could be easily seen.
Kind regards,
David
Hi soongsc,
Thanks for pointing this out. You are quite right, normally the mass induced stress would be greater. The diaphragm acceleration chart in Hornresp together with the value of Mmd can be used to calculate the stress, if required.
Kind regards,
David
Mmd is not the correct mass to use to determine the worst case load or stress on the cone.
The accelerating force comes from the voice coil. That voice coil mass is accelerated directly by it's own force.
The coil former has some mass, that will load the glue joint between coil and former.
Both of these masses do not load the cone.
Mmd should have the former mass and the voice coil mass and the glue mass subtracted to leave the cone mass. The cone mass times the acceleration gives the cone loading due to moving mass.
Then add on the loading due to air load.
The accelerating force comes from the voice coil. That voice coil mass is accelerated directly by it's own force.
The coil former has some mass, that will load the glue joint between coil and former.
Both of these masses do not load the cone.
Mmd should have the former mass and the voice coil mass and the glue mass subtracted to leave the cone mass. The cone mass times the acceleration gives the cone loading due to moving mass.
Then add on the loading due to air load.
Strictly speaking this is correct.
But pretty much unobtainium for most of us mortals. Even when I am working on a driver design some of these numbers are educated guesses until you have a final product.
You can do a simpler force calculation including the Cms by calculating newtons per watt at the given efficiency. That is much more real world engineering.
@ Master McBean.
Totally awesome dude!
I'm sure we are going to coax you into tweaking this a bit, but this is getting exceptionally useful.
If we are going to assume the diaphragm is rigid body. First of all the net force is motor force subtracting the suspension force, BL and Km are not constant throughout the travel. The mass is the sum of VC+former+diaphragm+effective suspension mass. From this you can calculate acceleration, throughout the excursion, but what is the purpose of calculating stress on the surface?
I do analysis to predict behavior of diaphragm deformation, but if hornresp does not do this, then I cannot understand why calculate this? ABEC recently added capability to import diaphragm vibration deformation into it, so the radiation pattern would not be as a rigid surface. But that is 3D data. I am not even sure how good it is.
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To predict cone failure you need a whole other set of unobtainium specs. Numbers which you will never see.
The cone vendor I work with is world class, making cones for all the top brands and even they don't provide that kind of information. You have to do those types of measurements yourself. Rupture points, bending failures.
And many of these tests are specific to frequency, power input and type of enclosure. Because as we all know horns, sealed, and bass reflex enclosures all load the cone in a different manner. Throw in bandpass enclosures and you are really looking at some interesting testing methods.
It's not all that simple. And for the math to work there has to be simplifications. I think the direction that David is going is most useful. It gives us a starting point that we can pin some calculations to at the very least.
The cone vendor I work with is world class, making cones for all the top brands and even they don't provide that kind of information. You have to do those types of measurements yourself. Rupture points, bending failures.
And many of these tests are specific to frequency, power input and type of enclosure. Because as we all know horns, sealed, and bass reflex enclosures all load the cone in a different manner. Throw in bandpass enclosures and you are really looking at some interesting testing methods.
It's not all that simple. And for the math to work there has to be simplifications. I think the direction that David is going is most useful. It gives us a starting point that we can pin some calculations to at the very least.
"Fr1"? Where do I find that? On the Loudspeaker Wizard dialog box (I'm simming a TH BTW), all I'm seeing are S1, S3, S5, L12, L23, L34 and L45.
Never mind - just figured it out...
The HornResp help file refers to "Fr1" as the "absorbent filling material airflow resistivity (mks rayls/m)". Boosting it to a fairly high figure in HornResp and targeting just the S1 section of the horn seems to appreciably reduce out of band noise. I'm just wondering how this can realized in practice. What's the the "absorbent filling material airflow resistivity" of polyester fiberfill, for example?
I did a quick comparison to MJK's software in post 3796. http://www.diyaudio.com/forums/subwoofers/119854-hornresp-380.html#post3722926
MJK's software assumes polyfil stuffing. It's hard to tell how accurate this comparison is since the two programs have differences in the way they calculate some things, but this gives a decent idea of what Hornresp's stuffing numbers should be set at.
Maybe somebody has data sheets on different stuffing materials and can provide much more accurate results than this simple comparison, I haven't bothered to look for measured datasheet data yet.
Thank you sir.
The "Total Pressure" graph seems more than adequate for this purpose. In the picture I emailed you, you can clearly see the effect of reactance annulling on the total pressure.
Personally, I use this info to see what effect compression ratio has. (For example a 20:1 ratio vs the 3:1 ratio that is generally considered safe.)
This "Total Pressure" graph won't show EVERYTHING there is to know about pressure stress obviously but for my simple purposes it doesn't have to. Intuitively, we know that some cones are stronger than others. For example, assuming the same material and thickness, a 6 inch cone will be stronger than an 18 inch cone. And a very rigid modern cone that you could stand on without crumpling damage is stronger than an older style lightweight paper cone that could be punctured by a child sticking a finger through it. The dustcap size, location and shape (dome vs inverted) also have an effect that Hornresp will never (and shouldn't be expected to) predict.
Even a small amount of info is a lot better than nothing.
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Sound absorption properties of different materials:
http://www.bobgolds.com/AbsorptionCoefficients.htm
Unfortunately very few provide all the resistivity data. But there are a few a way down the list.
http://www.bobgolds.com/AbsorptionCoefficients.htm
Unfortunately very few provide all the resistivity data. But there are a few a way down the list.
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I admit I am not familiar with how driver compression ratios are defined, but in terms of volume compression, it would take a real strong motor to compress 20:1, what driver has that capability?
If we know a compression ratio, based in the area under pressure you can calculate a total pressure, but the real stress on the cone is bending caused by pressure distribution. If this is not calculated, I am wondering how useful the data can be. However, since it is already implemented...
Hi AndrewT,
You would have to agree though, using Mmd would definitely provide the worst case scenario
.Kind regards,
David