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Old 10th July 2011, 12:07 PM   #11
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A lot of generalities there about dustcaps having and causing strong resonances. I have more frequently found the opposite, that an appropriate chosen dustcap will reduce driver top end resonances. Of course many dustcaps are simply chosen for good looks.

I designed a 10" sub for Snell with a heatsink on the front of the core pole. This, of course, meant that no dustcap could be used. Cooling was improved but the air rush noises from the leakage path were unfortunate.

The variable air chamber resonance problem sounds like a "straw man" arguement. Really?

David S,
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Old 10th July 2011, 06:51 PM   #12
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Default interesting

Ive often considered removing the dustcap on a pair of visaton AL130, with the aim of improving things a little.

What has stopped me is the possibility of making things worse. Doesnt removing the cap, result in air leaks between the vc former and pole pieces? surely this creates a whistling resonance? I always thought there was another purpose for the cap besides dust protection. wouldnt this rule out a sealed box design?
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Old 10th July 2011, 09:14 PM   #13
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Hi guys, just to throw this in:

A phase plug of sufficient size would, I expect, give better off-axis performance as it stops interference from different parts of the cone, by acting as a reflector. Certainly seems to help my Fostexes.

From Planet10hifi.com
Quote:
The phase plugs work primarily 2 fronts. 1st they fill the hole left when the dust cap removed. The air trapped in the cylinder inside the voice coil and above the pole piece causes what is called an "oil-can resonance". This invariably causes some midrange distress. Removal of the dustcap and filling the hole pretty much eliminates this source of coloration. The 2nd thing the phase plug does is to effectively halve the diameter of the cone. Any side-to-side standing waves, reflections, etc are pushed up about an octave. Related to this is an improvement in high frequency dispersion, widening the sweet-spot, and reducing the laser-like hot spot some full-range drivers suffer from. The smaller effective cone diameter means that the point at which beaming starts is pushed up and due to the shaped nature & the hardness of the plugs some HF energy is redirected.
Chris
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Old 10th July 2011, 09:54 PM   #14
GeneZ is offline GeneZ  United States
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Quote:
Originally Posted by mondogenerator View Post
I always thought there was another purpose for the cap besides dust protection. wouldnt this rule out a sealed box design?
Mine is a sealed box. Mordaunt Short. Small mid/woofer driver, small box...1st order, and phase plug. Very interesting in a nearfield set up. Just breaking them in. The phase plug I believe is solid aluminum.
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Old 10th July 2011, 10:04 PM   #15
18Hurts is offline 18Hurts  United States
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Interesting read,

So for woofers, it is a cooling device (makes sense) For mids it pushes beaming up an octave. I have a set of Infinity midranges from the late 90's, the dust cap is very tall with the peak of it at the level of the edge of the cone. I'm assuming it attempts to perform the same function although it does XO at 3 KHz.

The only speaker I have with an actual phase plug is those Tang Band 3" full ranges, it is aluminum so I'm assuming it both cools the voice coil, limits beaming and for strictly marketing reasons--looks cool.
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Old 10th July 2011, 10:24 PM   #16
GeneZ is offline GeneZ  United States
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This is what I am now breaking in...
Click the image to open in full size.
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Old 10th July 2011, 11:06 PM   #17
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Quote:
Originally Posted by speaker dave View Post
I have more frequently found the opposite, that an appropriate chosen dustcap will reduce driver top end resonances. Of course many dustcaps are simply chosen for good looks.
If the cone has a resonance, the cause of the resonance should be looked at first and corrected. Adding a dustcap may help to some extent, but it's not a solution. A broad damping of the entire cone or selection of proper material for the bandwidth is going to be much more effective than gluing on a dustcap. In a cabinet you can add a brace in a long wall to shorten the distance on the panel. This creates separate resonances that are pushed uphigher in frequency, but on a woofer it doesn't work so well. You are now creating 2 separate resonances in the cone, one the corresponds to the distance from the coil to the dustcap and another that corresponds to the distance from the dustcap to the edge of the cone. In addition you're adding a third resonance from the dustcap itself. Depending on the material properties you may have multiple resonances that are all higher in magnitude than the original you started with.

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I designed a 10" sub for Snell with a heatsink on the front of the core pole. This, of course, meant that no dustcap could be used. Cooling was improved but the air rush noises from the leakage path were unfortunate.
I've never had a single issue with air noises from the leakage path. Keep in mind that air does now flow in one direction but is pushed back and forth multiple times per second. If the path is long and narrow enough the box pressure will never be high enough to create an issue. As with any fluid flow situation the resistance is based upon the ratio of surface area to the amount of free flowing area. With enough resistance there is essentially now air flow to be concerned with.

The TD15's for example have a VC former that is about 3.5" long. The gap inside is .017" wide. There is a huge amount of surface area on the ID of the former and OD of the pole and very little area for air to flow between. In order to have leakage not only does the air need to go through that tiny long resistive chamber, but it first has to go through the venting under the spider, down through the .020" thick x .75" tall gap plate, into the cavity inside the magnets, and then take a 180degree turn up through the VC former. The tighter the gaps both inside and outside the coil, the more resistive to air flow and the more "sealed" the enclosure is.

I have seen issues with other phase plug drivers and it is typically poor design. If a spider is not porous at all and there is no venting under the spider in the frame, you can have much higher pressure differences with excursion. This can force much more air through the gaps than just the internal box pressure alone. I've also seen very wide gaps in drivers with phase plugs and very short VC's with thin top plates at the same time. There is little resistance to any air flow and as a result there can be substantial noise with higher excursions.

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The variable air chamber resonance problem sounds like a "straw man" arguement. Really?
Yes really. Any time you have a chamber of air in a woofer you will have a resonance that you can see on an impedance curve. Unvented chamber under a spider is the most common example if the spider is not porous enough to allow air to pass through easily. The dustcap is the most significant. Take a typical 3" coil woofer. As an example figure in in a 200mL chamber of air and add in a 1" x 3" long vent. You can calculate what the resonance of that chamber will be and you'll see a blip on the impedance curve the directly corresponds to it. As the cone moves in and out, the pole is essentially becoming a piston in a compression chamber. Vary that volume from 100mL to 300mL depending on excursion of the driver while keeping that 1" x 3" long vent constant. See what happens to the tuning of that chamber.

John
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Old 11th July 2011, 06:31 AM   #18
AllenB is offline AllenB  Australia
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Last month I cut out a dustcap. Unfortunately I broke the rules and altered the crossover (slightly) at the same time. The difference was subtle but very nice, like not changing the tonal balance but cleaning things up.

Here is an on axis plot (sans crossover) of : Green = before, Orange = no dustcap, and Red = a simple cylindrical plug. CSD is before and after.
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Old 11th July 2011, 11:50 AM   #19
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Quote:
Originally Posted by John_E_Janowitz View Post
If the cone has a resonance, the cause of the resonance should be looked at first and corrected.
If a cone has a resonance? All cones have many resonances. Even with todays design tools, the acoustical output of the many mechanical modes of any wooofer are still randomly addressed. Dustcaps are one good variable for controlling top end frequency response.

Quote:

I've never had a single issue with air noises from the leakage path. Keep in mind that air does not flow in one direction but is pushed back and forth multiple times per second. If the path is long and narrow enough the box pressure will never be high enough to create an issue.
It is the narrow passage that leads to turbulence and hence air noise.

Quote:
Yes really. Any time you have a chamber of air in a woofer you will have a resonance that you can see on an impedance curve. Unvented chamber under a spider is the most common example if the spider is not porous enough to allow air to pass through easily. The dustcap is the most significant. Take a typical 3" coil woofer. As an example figure in in a 200mL chamber of air and add in a 1" x 3" long vent. You can calculate what the resonance of that chamber will be and you'll see a blip on the impedance curve the directly corresponds to it. As the cone moves in and out, the pole is essentially becoming a piston in a compression chamber. Vary that volume from 100mL to 300mL depending on excursion of the driver while keeping that 1" x 3" long vent constant. See what happens to the tuning of that chamber.

John
A 3 to 1 variation in an under dustcap area? Thats a pretty good excursion.

The blips I've seen in impedance curves are more typically from the surround resonance, one of the lowest frequency resonances. I can't recall anything related to a back chamber Helmholtz resonance. If such a resonance exists I wouldn't worry about it varying with frequency but would just treat it. Many drivers have either venting through the pole or damping under the dustcap to deal with cavity resonances.

Lots of myths flying here about "phase plugs". Some facts: these plugs do not act as phase plugs in the true sense: they do not improve response by equalizing path lengths or propagation times. They are generally not large enough to act as "reflectors" or even occluders of sound.

They may have a minor impact on frequency response. Simply leaving the dustcap off has the greater effect and removes a major variable that driver designers use to improve top end frequency response.

Removal of the dustcap always creates an air leakage path through the woofer. This may or may not create serious air noise at high excursion, depending on factors of coil and gap design, spider material, etc. You may get lucky or you may not.

David S.
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Old 11th July 2011, 02:53 PM   #20
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Quote:
Originally Posted by AllenB View Post
Last month I cut out a dustcap. Unfortunately I broke the rules and altered the crossover (slightly) at the same time. The difference was subtle but very nice, like not changing the tonal balance but cleaning things up.

Here is an on axis plot (sans crossover) of : Green = before, Orange = no dustcap, and Red = a simple cylindrical plug. CSD is before and after.
I did a similar experiment with one of my older spare Coral Flat 8's which had a damaged dust cap, (which are aluminum domes crimped on the voice coil former with a gauze covered vent hole in the middle) and to be honest I don't think removing the dust cap is all that people crack it up to be...

My measurements mirror those of yours pretty much - simply removing the dust cap actually makes the cavity resonances between 1-2Khz measure much worse because you still have a cavity and now that it's exposed it radiates directly instead of being largely obscured by the dust cap with only a small amount of the resonance "leaking out" through the hole and by re-radiation from the dome.

With the dust cap simply removed the result was unsatisfactory, you now have to fill the hole with something to eliminate that cavity resonance - eg a "phase plug". I tried a variety of different lengths, shapes, like cylindrical, bullet etc, and all of them have the same effect of eliminating the cavity resonance between 1-2Khz provided they are a close fit.

Where they all differ is in the 4-8Khz region, where even small changes in the shape of the "phase plug" have a large effect on the response. Although I'm sure it's possible with just the right shaped taper to get a relatively flat response, with trial and error I could not get a response anywhere near as flat from 4-8Khz as the original dust cap being in place.

My conclusion was that for that driver, a phase plug is a mistake. Whilst filling the cavity did improve the resonance between 1-2Khz, it was relatively minor to begin with when the dust cap was in place, and without going to a lot of trouble to develop just the right shaped plug the response from 4-8Khz was all over the place. It's not just a matter of sticking something in that looks like a bullet and hoping for the best

Treble response from 10Khz up whilst slightly flatter was a lot lower in level, and with far worse off-axis response than the original dust cap. On these drivers the dust cap is acting as a horn loaded dome radiator. (With the curvilinear whizzer cone acting as a horn for the dome) Removing the dome means all the treble radiation is coming from the whizzer cone only, so response drops a lot >10Khz, and treble becomes a lot more directional, even with the phase plug in place.

What could have been done instead, is adding soft damping (cotton wool ?) behind the dust cap to eliminate the 1.5Khz cavity resonance and just leave the dust cap in place. It really is an important and active part of the driver, responsible for a lot of the treble response and dispersion, not just a shiny embellishment...

I've found the ~10Khz "oilcan" resonance of the aluminum dust cap can also be largely eliminated without removing it simply by putting a small dent in it near the edge with a small flat blade screwdriver - a mod that I've done to my other units as well. You can't see it unless you look closely, the treble response is not significantly reduced (unlike removing the dust cap) and the response is significantly flatter, with a much better CSD ~10Khz.

Although I can see certain specific cases where a "phase plug" might be useful, I definitely don't think they're a "must do" modification that many people think they are, and unless done just right and only on the right driver, can actually make the performance much worse...
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Last edited by DBMandrake; 11th July 2011 at 03:05 PM.
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