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Old 16th July 2011, 11:37 PM   #51
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Originally Posted by dlr View Post
This was my experience as well, though it was totally counter-intuitive. My thought was that it might "unbalance" the cone and introduce some rocking into the cone due to odd mass distribution.
One reason I opted for a symmetric layout - in theory the centre of mass of the cone should still be along the same axis, as every strip has an equal and opposite which is 180 degrees away on the far side of the cone and at the same distance from the centre. It also seems to work best from the point of view of damping the resonances.
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A three part arrangement initially seemed like the optimal location. The surprising thing was that no test of three points improved it without additional, more detrimental changes. Only later did I try just two.
So if I understand you right, you mean that initially you were using groups of 3 dots which were 120 degrees apart and the same distance from the centre ? And then another group of 3 dots at some other offset from the centre and with some angular offset from the first group ?
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At some point I recalled seeing the two "ears" on some Accuton drivers, so I focused on two only for any fixed distance from the former. The result was that just a very few of these pairs largely eliminated most of the higher frequency resonances.
Interesting that groups of 3 (per equal distance) didn't work without causing more problems than they fixed - I didn't try 3, but I did try 2, 4, and 8.

I first started off with 4 strips at 90 degree intervals near the edge of the cone (top/bottom/sides in the diagram) and there was significant improvement but it wasn't enough. I then added another 4 strips in between them at 45 degree intervals, making a total of 8, and this made a big improvement, especially around 3Khz, however it still left quite a big peak at 4Khz.

I then tried adding 2 strips further in opposite each other, then later 4 strips as shown in the diagram, which pulled down the peak at 4Khz a lot. (There is still about a 2dB peak there which I equalize in the network, along with the broad 3dB peak at 2Khz)

(On another pair of similar but different drivers, I only added the 8 strips around the perimeter and found I didn't need the extra 4 further in, and that all they did was put an unwanted dip in the response)

The interesting thing is that I found pairs of 2 strips didn't work very well for me. When going from only the 8 strips around the edge to adding 2 inner strips it reduced the resonance at 4Khz somewhat but it actually added a new low level resonance at another frequency nearby. I forget if it was higher or lower, but despite its low amplitude the newly added resonance had a very long decay time of several ms. It wasn't really noticeable on the frequency response plot, only on a CSD as a low level but long lasting ridge. I think I could just hear it too, on certain music.

Adding 2 more strips to make it a group of 4 spaced at 90 degrees eliminated the spurious new resonance, as well as further reducing the original 4Khz resonance. Staggering 2 of the opposite strips further in by half a block width improved the response at 4Khz slightly more, but if I staggered them any more than that (such as a full block width) the spuriously added low level resonance would return.

It seems that there must be at least 4 blocks equally spaced around the circumference at any given radius where there is a block, although I'm baffled as to why. This is what got me wondering about whether there is a secondary effect going on which is affecting bell modes - maybe the spurious low level resonance is a bell mode, and only adding damping/mass at two 180 degree intervals stimulates (or allows) a particular bell mode, while adding damping/mass at 90 degree intervals (or less) suppresses the same mode.

Whatever the reason was, 4 or 8 strips per radius seems to work well, 2 didn't for me. I'm unsure how 12 or 16 strips per radius might perform on a larger driver, I'll have to try it one day on a 12" woofer.
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Old 19th July 2011, 04:25 PM   #52
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I am not up on all the technical reasons things work or they don't but I have used alot of highend speakers PSB Platinum, Dynaudio, Triad....looking for certianly sound and output. I finally built a set with drivers from John at Acoustic Elegance and can tell you they are nothing short if amazing absolutly no noise from phase plugs or any measurable responce not supose to be there. He had tested several other mids for me and the dust cap breakup was so bad he went as far to change them out to different materials sizes shapes and dampenings added which all did help but as he explained just moved it around somewhere else in the frequency range all till he broke down and built a set of custom drivers to eliminate it and used a phase plug. Now his TD6

Sorry coming from someone who doesn't know why but it just works. I do agree there are some phenomenal sounding speakers that use a dust cap but could they sound better with a phase plug designed right?
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Old 3rd August 2011, 07:21 PM   #53
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Originally Posted by speaker dave View Post
Hi Simon,

Your figure 1 looks like a very useful improvement in the 2 to 5kHz range. Interesting that there was minimal effect above.

I'd spent many hours in years past playing with doping compunds, cutting slots, etc. The only problem is trying not to do permanent damage on a path that doesn't pan out.
While looking through old measurements I've found some more damping experiments measurements on two different drivers that may be of interest. (Apologies to those only interested in phase plug discussion for derailing the thread again, although I have some on-topic phase plug measurements I can post later as well )

What's interesting about these measurements is that they're from two different yet similar models of driver - to the eye they look like a very similar if not identical design (8" paper dual cone) yet they measure very differently indeed, showing how the largely invisible to the eye materials properties and composition is just as important if not more important than the actual physical geometry of the design.

For reference the driver I posted response graphs of previously was a Coral Flat 8 II. (The driver in my signature pic)

The ones I'll post in this message are the Coral Flat 8A. Visually they look like the same driver, but there are some differences - in the particular pair I have, they have very stiff surrounds, probably suffering from stiff surround syndrome with age. (They're close to 40 years old now)

Supposedly a fabric surround but it's so stiff that it's either heavily doped with resin or varnish that has gone hard over time or it's some other material, maybe even phenolic. The Fs is 105Hz because they're so stiff.

On the other hand the Flat 8 II surrounds are extremely soft and supple fabric, (probably porous too, as they don't seem to be doped with anything) so soft that for the same cone mass the Fs is around 38Hz.

An additional difference is that I (very stupidly, in hindsight) coated both cones with a couple of thin layers of a water based rubber and carbon compound. (Otherwise known as the stuff that you coat tyres with....) This had surprisingly little effect on the main cone, but a rather unfortunate effect on the whizzer cone. Unfortunately I don't have a directly comparable before and after measurement for this change, so the baseline measurement I'm providing is after this coating but before adding foam strips.

Given all that, the first response shows the "unmodified" (before foam strips but after ill-advised coating) response of the driver in red - which looks almost nothing like the zig zag response of the previous driver. There is however a dip around 3.2Khz and a large peak at 3.8Khz on the order of 6dB, which is a classic cone breakup resonance, albeit quite a lot better controlled than the other driver.

From what I can remember, the coating I applied made very little difference to this part of the response, so I can only assume that either the paper internal composition is a bit different, or the rather hard, stiff surround is better terminating the edge of the cone at high frequencies.

In any case I wanted to try to smooth out that spike at 3.8Khz, so I experimented with foam strips. I have 3 different combinations which I kept measurements for and I think they're interesting in showing just how much the response can be tailored by the number and position in strips, and the iterative approach I took.

The yellow response in image 1 was my first attempt - consisting of 4 strips at 90 degrees right near the edge of the cone and another 4 in between them spaced in about 20mm from the edge. The big spike is clearly tamed down and the dip at 3.2Khz improved slightly, but I thought I could do better than this.

Configuration 2 in dark blue in image two was achieved by doubling the number of strips but keeping the same basic layout - so now there are 8 equally spaced strips at the edge, and 8 equally spaced strips in between these and offset in by about 20mm. This pulls the response down even further, with the response at 4Khz where we want it but now we have a bit of a hole around 3.2Khz.

4Khz corresponds to the inner strips while 3.2Khz corresponds to the outer strips so I tried removing every alternate outer strip - leaving 8 inner strips and 4 outer strips. The result is the light blue response in image 3. This is flat within +/- 2dB from 2.7Khz to 6Khz, quite an achievement for an 8" whizzer cone driver IMHO, especially compared to the original red response. The droop from 7-8Khz and the peak from 9-12Khz are both due to the aluminium dust cap resonance and its interaction with the whizzer cone output.

What about the large dip at 2.5Khz ? None of the foam strip mods altered this in any way, and this notch is real, not some measurement artefact. Sadly, the cause of this notch is the coating I applied to the whizzer cone. Although I don't have accurate and directly comparable measurements, I know for a fact that this large notch was not present before I coated the whizzer cone. BIG mistake.

The fundamental mechanical resonance of the whizzer cone at ~2.2Khz which is normally fairly low Q, and in phase with the response of the main cone (well, at least less than 120 degrees) causing a modest bump in the response that can be corrected quite well with an RLC compensator or PEQ.

Somehow the coating has altered the cone properties to affect the phase shift and Q to the point where the whizzer cone and main cone are 180 degrees out of phase and almost equal in amplitude on one side of the resonance, so a notch appears.

Nothing I've done to try to undo this mistake has ever returned them to their original no-notch response, and there is nothing that can be done electrically to compensate for it either, so sadly those drivers were never used in a completed design, (and have become a bit of a driver tweaking testbed) because the resonance of that notch is actually quite audible and annoying.

So on this particular driver I think the foam damping strips were highly successful, but the overall response of the driver was ruined by the tampering with the whizzer cone. Pity. If I knew what I knew now I would never have touched the whizzer cone or applied any coatings - I would have gone straight to the foam strips.

In the next post I have another driver where foam strips whilst altering the response, didn't give a satisfactory final result. (Just to show that they can't work miracles on a really bad driver )
Attached Images
File Type: jpg Coral Flat 8A damping strips - 1.jpg (271.5 KB, 356 views)
File Type: jpg Coral Flat 8A damping strips - 2.jpg (266.0 KB, 341 views)
File Type: jpg Coral Flat 8A damping strips - 3.jpg (273.6 KB, 326 views)
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Old 3rd August 2011, 07:47 PM   #54
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This is yet another Coral driver - The Coral 8A-100. My Dad had a whole bunch of these and these were actually the first drivers I experimented with foam block damping on, because there were such obvious resonance problems with them to the point where even just touching the perimeter of the cone with your fingers made a big difference to the sound of the resonances, which is what got me thinking about sticking foam blocks on in the first place, and where to put them.

The one I show the measurements for was the worst one by far - most of the others measured considerably better in the treble, although all of them had the notch at 2.5Khz and peak at 3Khz, and all measured quite differently from each other.

Again physically they look much like the Flat 8, but from what I understand they were a "budget" model manufactured using the same moulds but not necessarily the same materials (or level of quality control...) and sold OEM without the Coral brand. (None of them have Coral written on them, only the model number)

They have a relatively soft fabric surround with a lot of travel like the Flat 8 II, but they're clearly doped with something. They also have a paper dust cap at the base of the whizzer cone rather than the vented aluminium dust cap of the other two models. The cones are jet black, compared to the pale cream colour of the other models, but otherwise appear to be the same geometry, including perimeter stiffening rings. Their sensitivity is about 2dB lower with a higher Qts and the magnet looks a bit cheaper.

The unmodified response is in red in image 1. Yuck! That really is pretty bad, and it's no wonder that particular driver sounded so bad, in fact I would go so far as to say it's defective in some way, as the other 8A-100's whilst not exactly flat, were nowhere near that bad.

The difference between the peaks and dips is close to a staggering 35dB. Yellow in image 1 is more or less the same configuration of strips applied successfully to the Flat 8A in the previous post - you can clearly see a reduction in the peaks at 3Khz and 4Khz, and an improvement in the notch at 3.5Khz, but it's still pretty awful.

No combination that I tried would give significantly better results than this - it just moved the peaks and dips around in frequency. I then started to wonder if a whizzer cone resonance could be the problem, so I tried adding 4 foam strips behind the edge of the whizzer cone, the result is in cyan in image 2. There's a clear improvement from 4-7Khz, but there is still a big notch at 2.5Khz and 3.5Khz.

From this its pretty clear that there are serious cone breakup problems within the whizzer cone on this driver. (But not the others) From memory in this particular driver the whizzer cone was sticking out at least 5mm, maybe 10mm further from the main cone than all the other drivers, so it's possible that this shifts the resonances and relative phase shift of the two cones to the point where destructive interference becomes a problem.

Or it may be a difference in the materials composition - maybe the coating applied to make the cone black (it looks like its been spray painted matte black) has enough of an effect on the critical damping and stiffness to weight ratio of the whizzer to cause problems. (The cone is not nearly as "crisp" as on the better drivers - it feels a bit soft and soggy) Whatever it is, both cones have serious breakup problems that cause a rather ugly summed response which is full of destructive interference and resonances and makes for an essentially unusable driver.

Image 3 is comparing the best damped response I could get from the Flat 8A (dark blue) with the best damped response I could get from the 8A-100 (cyan) - even with all the tweaking its nowhere near as good, despite being largely the same design of driver. (The 650Hz and 1.6Khz notches are measurement environment reflection artefacts, but all the other notches really are in the drivers responses)
Attached Images
File Type: jpg Coral 8A-100 damping strips - 1.jpg (271.0 KB, 318 views)
File Type: jpg Coral 8A-100 damping strips - 2.jpg (276.3 KB, 307 views)
File Type: jpg Coral 8A-100 damping strips - 3.jpg (276.2 KB, 56 views)
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Last edited by DBMandrake; 3rd August 2011 at 08:14 PM.
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Old 3rd August 2011, 08:36 PM   #55
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A really interesting article I read years ago was on the development of the FST kevlar midrange unit in the B&W Nautilus series, where rather than trying to eliminate the standing waves on the cone they harnessed them. The particular fabric weave they used meant that although the cone was circular, the propagation speed in different directions was different, such that acoustically the cone is square.

Because of that standing waves don't form symmetrically right around the radius of the cone, and there are no spot frequency resonances where the effective path length from centre to edge is equal all the way around. Better still, at any given frequency above breakup adjacent sectors of the outer section of the cone are out of phase with each other cancelling their radiation leaving only the centre of the cone active, thus improving dispersion by reducing effective cone area.

(I hope I'm getting this right, it's years since I read the white paper)

Around the same time I thought about adding discrete damping strips near but not at the edge of a conventional cone, and staggering the distance of alternative strips (from the centre) so that as well as providing some resistive damping to the standing waves, would by their being offset in from the edge of the cone alter the pattern of standing waves in adjacent sectors of the cone, and effectively make the cone non-circular for standing waves, much like the B&W driver. Different angular sectors of the cone have different standing wave patterns occurring at different frequencies instead of them all occurring at the same frequencies, which usually happens in a circular homogeneous cone.
I've tracked down a link to the B&W Nautilus 801 design white paper that I was referring to which I first read about 7 years ago:

http://www.hifiportal.co.uk/Articles...ilus%20801.pdf

Excellent reading material for anyone interested in the modal/resonant behaviour of cones and the interesting approach that B&W's engineers took to cleverly turn the modal behaviour of the cone into an advantage instead of trying to eliminate it.

Although it's very different to what I'm doing with my damping strips, they both (as far as I can see) have the effect of preventing the radial standing wave pattern from being identical around the entire rotation of the cone, thus producing different standing wave patterns in different "sectors" of the cone.

There is also a nice little video summary from an interview with Dr Peter Fryer which includes an animation of the cone breakup of their driver, and a simplified explanation of how it works:

Bowers & Wilkins - Kevlar
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Old 3rd August 2011, 11:27 PM   #56
GeneZ is offline GeneZ  United States
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Definitive Technology takes a unique approach with their phase plug. They attach the phase plug to the inner section of the cone via a rubber surround, just like its done on the outer diameter of the driver.


Click the image to open in full size.


The drivers feature Definitive’s patent-pending Balanced Dual Surround System (BDSS) technology that supports the speaker cone at both the inner and outer edges for longer, more linear cone excursion, thereby making lower midrange response more robust than one would expect from a driver of this size. BDSS also imparts greater clarity and finely textured inner detail. The center projecting object is a Wave Guide that prevents sound waves from any given side of the cone from reaching and interfering with sound waves from the opposite side of the cone. This serves to flatten frequency response and improve off-axis dispersion. Every listener in the room hears clear full-range sound with three- dimensional imaging.

Definitive Technology - Technical White Papers
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Old 3rd August 2011, 11:43 PM   #57
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Definitive Technology takes a unique approach with their phase plug. They attach the phase plug to the inner section of the cone via a rubber surround, just like its done on the outer diameter of the driver.
I believe the phase plug is solidly attached to the magnet structure behind and the inner half roll suspension is attached to it. It should work well. As to it being a phase plug and blocking sound from one side of the cone to the other, pardon my scepticism.

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Old 4th August 2011, 12:55 AM   #58
Ron E is offline Ron E  United States
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As to it being a phase plug and blocking sound from one side of the cone to the other, pardon my scepticism.
Looks like a good solution to keep stuff out of the gap...
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Old 4th August 2011, 01:03 AM   #59
GeneZ is offline GeneZ  United States
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As to it being a phase plug and blocking sound from one side of the cone to the other, pardon my scepticism.
I auditioned the smallest monitor they make. ProMonitor 800. It was able to produce bass like no other small speaker I ever heard. They actually vibrated my computer workstation's surface at times. It made me feel a oddly uncomfortable feeling where I rest my arms vibrating like that. They had a richness of sound in the midrange that I really liked. Definitive Technology is definitely onto something. Selling them through Best Buy I think misrepresents their reputation.
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Old 4th August 2011, 06:52 AM   #60
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Looks like a good solution to keep stuff out of the gap...
And introduce a second rubber roll surround of a different size to introduce a second surround dip resonance in the midrange

This second rubber roll surround at the middle is purely there for bass reasons - to eliminate any air chuffing that the gap at a "phase plug" might cause, and potentially act as an alternative to a conventional spider - it's not clear whether the driver has a spider as well. (It's likely it does though, as a rubber roll surround by itself doesn't have much z axis centring control, with most of the centring along the z axis usually provided by a spider, and a rubber surround only providing traverse x-y centring)

However for midrange this extra rubber roll is an unnecessary 5th wheel, and can only be a bad thing. At the edge of a cone a surround is a necessary evil - rubber surrounds are better for bass than fabric or phenolic, but decidedly worse for midrange. (One reason the majority of good full range drivers use doped fabric surrounds, not rubber rolls)

B&W in their FST midrange driver went to great lengths to eliminate the surround at the edge entirely, (replacing it with a compressive glue/silicon bead of some sort behind the edge of the cone) to get rid of surround dip resonance.

On this driver you have two of the things...at some frequency there will be a surround dip resonance from that middle "surround", possibly a different frequency than that of the outer surround. Because it's a forward facing roll there will also be some diffraction of high frequencies from the centre of the cone off the "bump" which wouldn't normally be there, even though it's somewhat rounded.

As for the phase plug itself - as Dave, I'm highly dubious of its benefit or effect - I've tried phase plugs of that shape and length (as well as many others) and one that short in length with a squared off end will introduce severe anomalies in the response above about 3-4Khz due to diffraction from the squared off end.

Apart from plugging the cavity that would normally be present in a dust cap design this phase plug won't be doing much of anything good. If this was a full range driver trying to genuinely produce treble this phase plug would be a very bad design. At the very least it would need to be longer and gradually tapered to a point to get anything approaching a flat treble response.

As it is, I suspect it's a mid-bass driver that's only used up to about 2Khz, so the bad effects of the squared off phase plug will be happening above the crossover filter cut-off frequency - but that also means that any genuine phase plug effects if the phase plug were designed well would also be outside the passband of the driver.

I fail to see the point of the design when the driver likely isn't being used high enough in frequency for a phase plug to have any effect, but they've then added a second surround to eliminate the air chuffing, in the process adding another unwanted surround resonance in the midrange, as well as significant complexity.

I suspect it's more about looks and marketing potential than any genuine improvement in quality. The "white paper" reads more like marketing copy, compared to the B&W white paper which is hard science and research.
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