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Old 11th July 2011, 03:37 PM   #21
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Quote:
Originally Posted by speaker dave View Post
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.
I think it's hard to appreciate just how many resonances there are in a speaker. Anything mechanical has a resonance, and resonances can't all be eliminated.

For some resonances the most you can hope for is to control them or push them outside the operating frequency range of the driver...

One might even say that as much as we don't want it to be the case, a completed speaker system (drivers, cabinets, crossovers etc) is a huge messy pile of carefully controlled, balanced and damped resonances

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Dustcaps are one good variable for controlling top end frequency response.
This is especially noticeable on a full range driver, as I was describing in my previous post. Whilst in a woofer a dust cap might not do much except contribute cone area, block air leakage paths, and keep foreign objects out of the gap, in any driver operating above >2Khz or so the dust cap is really important, and for a full range driver it's crucial.

Removing the dust cap on the Flat 8's kills the response above about 8Khz - it falls by many dB. A large portion if not the majority of the treble is radiating from the dust cap, so frivolously cutting it out completely changes the treble response.
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It is the narrow passage that leads to turbulence and hence air noise.
I've wondered about the hole in the dust cap on the Flat 8's - it's about 8mm diameter in a 30mm dome, (with a tiny gauze behind it) and when they're producing bass you can really feel the air squirting in and out of the hole if you hold your hand close to it, but I can't say I've noticed a turbulence noise.

I'm not sure whether the hole is there to minimize cone breakup that might otherwise occur in the middle of the dome in the treble, or whether its there to vent the pressure - the aluminum is extremely thin, dents rather easily, and I guess could be buckled or dented by the air pressure during bass if not vented.
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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.
Again relating to the Flat 8's - they don't have any venting or damping behind the dust cap, and this does appear to cause modulation of the treble with bass excursion due to the changing cavity size. The cavity is 30mm wide and 17mm deep, with an Xmax of +/- 3mm, so the cavity height can change from 14mm to 20mm with bass excursion peaks.

On the frequency response this shows up as a whole series of small but significant "ripples" which look like comb filtering which gradually start to appear from about 8Khz up which "slide" up and down in frequency with the cone position - you can see it clearly change with a real time FR analyzer when deflecting the cone with a DC bias. (It doesn't show up on the impedance curve though)

I assume that they only show up in the response at higher frequencies even though the fundamental cavity resonance is lower because only at the treble frequencies is the dome the majority contributor of the radiation of the driver. At lower treble / upper midrange frequencies although the resonance modulation of the cavity behind the dust cap is still occurring the dust cap is a rather small proportion of the total radiation area and is thus swamped by the response from the rest of the cone...

In hindsight I think this is a significant source of audible IM distortion between bass and treble - even if the motor and cone are fairly linear the bass excursion will amplitude modulate treble frequencies due to the "sliding comb filter". I'm pretty sure I can hear this at higher volume levels when using them full range, but when crossed over with a tweeter I can't hear any IM at the same playback levels.

A "phase plug" would eliminate the problem of course (at the expense of a large loss of treble) but I don't see why proper damping behind the dome wouldn't also solve the problem without killing the treble performance.
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Old 11th July 2011, 04:13 PM   #22
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Originally Posted by AllenB View Post
CSD is before and after.
Didn't notice this at first, but something is very screwy with your CSD graph - 79ms on the time axis ? Normally you would measure driver decay over 2 to 4ms or so. Apart from being impossible to do a reflection free 79ms gated measurement in a house, no driver should take that long to decay

If you really did use a 79ms gate time you'll be collecting a ton of room reflections, so most of that decay will be room reverberation decay not the drivers own decay response, which should typically be at least ~25dB down in under 2ms.

Your CSD graphs also look very different to mine graphically even though I use ARTA as well. Old version of ARTA perhaps ?
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Old 11th July 2011, 06:19 PM   #23
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Quote:
Originally Posted by speaker dave View Post
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.
Of course the cone will have a resonance. However I was referring to a very problematic resonance that needs to be corrected for. As an example, a TD15M is +/- 2.5dB up to over 4KHz on axis while an AV15 has a very narrow breakup mode at 2KHz that is only about 1/4 octave wide but nearly 10dB in magnitude. Obviously the aluminum cone AV15 wouldn't be a driver suggested to play up to 1KHz. As another example, the initial cone samples I got for the TD6M with a standard hard pro-audio style paper pulp would play smoothly to 8KHz but then had a large breakup at 8.5KHz of nearly 10dB. I took a tip that Nick McKinney got from Roger Russell a long time ago and made a switch to a Kapok pulp for the cone. This is a softer pulp that is much more well damped internally. The driver now doesn't play as high, only to about 5KHz but then rolls off smoothly with almost no breakup. There is no dustcap that could correct this.

As another example, look at the 18sound 6ND410. This driver had many issues, but in this case it was the dustcap that gave it the claimed efficiency. They claim 101dB 1W which it only exceeds around the breakup peak at 4.5KHz. I tried everything to damp the resonance. Added mass, stuck foam on the cone, putty, nothing could fully correct the issues. The following is the impedance curve measured a few ways. Red was the natural impedance curve with no modifications. You can see the issues at 4KHz. I added mass to the dustcap to try to damp it. The resonance is still there but shifted lower in frequency as you can see in the blue curve. You can also see the Fs has shifted lower as the mass added was significant. Finally I just cut the dustcap off as you can see in the yellow curve. Fs raised very slightly but the bump from the dustcap resonance is gone.

Click the image to open in full size.

As I mentioned though, they count on this resonance for efficiency. You can see the effects of this dustcap on the entire bandwidth here. light blue is with the dustcap and yellow is without. Obviously this is an extreme example. It is literally the most extreme I have ever seen.

Click the image to open in full size.


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It is the narrow passage that leads to turbulence and hence air noise.
In order to have turbulence you need to have air flow. If there is sufficient resistance to prevent the airflow as I mentioned, there will be no turbulence or noise. Again, this simply comes down to proper design and taking these effects into account. Remember if you are dealing only with the box pressure inside the cabinet, this is easy to address. If you are however forcing air through the gap based upon the motion of the driver then the pressure can be substantially higher.

As a more common example, hook 100ft of 4" diameter hose up to a dust collector. You'll have noise at the far end as you are drawing a significant amount of air in. Now take 100ft of 1/4" hose and hook it up to the same dust collector. Same amount of pressure, same distance, but now you have much more resistance to the air flow. It will be completely silent as the flow is slowed to almost nothing even though the pressure is the same.

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A 3 to 1 variation in an under dustcap area? Thats a pretty good excursion.
The larger the dustcap, the less this compression ratio will be obviously. However with many drivers using inverted dustcaps and highly extended poles now this chamber can be very small. As an example on the AV15's I have a 2.5" VC with a 1" pole vent. At 30mm inward excursion there is approximately 1/2" from the top of the pole to the bottom of dustcap. This gives about 2.5 cubic inches of volume under the dustcap. At 30mm forward this chamber is expanded to 14.8 cubic inches. In this case it is approximately a 5.9:1 ratio. Drivers that are even higher excursion like many of the XBL drivers out there now, the TC LMS drivers, etc will have an even higher ratio. The 3:1 I stated is quite conservative really.

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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.
Everything that can resonate will. Spiders, cones, surround, dustcaps, air chamber under the spider, air chamber under the dustcap, etc. You can easily isolate which blip equates to which part. Just because there are other causes of resonance doesn't mean we just ignore some of them. It's also not only the issue of the resonance but also the difference in pressure within the chamber and the loading it places on the driver. You can put something inside the chamber, but that doesn't eliminate the issue, it only puts a bandaid over it.


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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.
Agreed that the main benefit isn't in a woofer phase plug acting as a phase plug in the traditional sense. It's getting rid of the negative affects of the dustcap and air chamber under it that become the more important issue. There are some slight benefits in the upper response if a phase plug is long enough to come up to the front of the cone itself but they are very minor. I've never specifically seen a case where a dustcap improved the upper end response though. It has always been the opposite. If you want to damp a cone resonance, edge treatment at the outside of the cone where it attaches to the surround is a much more feasible option.

Quote:
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.
I do agree that you likely won't have great results if you just take any random driver and remove the dustcap. If it hasn't been designed properly to prevent noise from air leakage then you will likely have problems. For something like open baffle use where there is no box pressure with a driver that has proper venting under the spider, the results could be good. If there is no pressure in the intended application there is no leakage. Also some initial testing can tell if the dustcap is the problem. Measure response and impedance. Add some mass to the dustcap. If the upper end response greatly changes and the bump in impedance curve shifts lower in frequency and is more broad, that is a good indication that the dustcap is an issue. If it doesn't change anything, go on to testing other things.

John

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Old 11th July 2011, 07:26 PM   #24
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There is no advantage to using a phase plug or open dust cap...none what so ever....with either you get a lot of out of phase signal eminateing from the driver.....Some people might think they are hearing more detail but they are hearing phase smear and group delay ? an out of phase signal ( time and phase) . . .this offsets any concern for cavity pressure or resonance...ask yourself this ? When have you ever scene a studio monitor, true studio monitor (stuff recording engineers use) or professional speaker with a phase plug or open dust cap...you don't and wont see them...if someone finds one to tell me about it is not one that is bought or used by anyone ! ! !...
You will also find many more monitors have rear facing ports than front; to keep out of phase midrange overwhelmed by direct signal. . . .vented voice coils, spiders and poles with damped dust caps do just fine regarding any resonance in the cavity...I have even scene wool felt on top of the pole to further damp dust cap radiation . . .

have a nice day,
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Old 11th July 2011, 08:08 PM   #25
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Originally Posted by erickalan77 View Post
There is no advantage to using a phase plug or open dust cap...none what so ever....with either you get a lot of out of phase signal eminateing from the driver.....Some people might think they are hearing more detail but they are hearing phase smear and group delay ? an out of phase signal ( time and phase) . . .
Well, changing an existing well designed driver from a dust cap to a phase plug is certainly a crap shoot unless you have good measurement equipment and a lot of patience and time on your hands to develop exactly the right phase plug that will (hopefully) bring you back to a near flat response, but even then I agree you can easily end up worse off than you started, as in the example I gave.

Converting a full range driver from dust cap to phase plug seems to be a popular pastime amongst full range purists, (along with de-whizzering...) but whilst it certainly makes a significant change in the response I'm yet to be convinced that it's unambiguously a net positive result, or that it's even worth attempting.

The two main issues that it addresses (cavity resonance around 1-2Khz and dust cap resonance around 10Khz) can both be addressed in other ways IMHO, without the drawbacks of a "phase plug".

That's not to say that a well designed driver can't be made with a phase plug in the first place though.
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When have you ever scene a studio monitor, true studio monitor (stuff recording engineers use) or professional speaker with a phase plug or open dust cap...you don't and wont see them...if someone finds one to tell me about it is not one that is bought or used by anyone ! ! !...
I think that's being a bit harsh - I can think of at least one - the B&W Nautilus series (801 in particular) were sometimes used in recording studios - not as the near-field monitors obviously, but as full size monitors, and they have phase plugs in their midrange drivers. While they're not a speaker without flaws, I wouldn't point my finger at the phase plug as an issue in that design...
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You will also find many more monitors have rear facing ports than front; to keep out of phase midrange overwhelmed by direct signal. . . .
Agree 100% there. Ports on the front are a terrible terrible idea for midrange quality, especially on a 2 way but even on a 3 way, and something which I will never do again.

Doesn't matter how much stuffing is in the cabinet, (within reason) you'll get enough leakage in the midrange at the port's tube resonance frequency (often between 500-1000Hz) to cause easily measurable and audible colouration in the midrange. (More midrange output from the port at the resonance than from the driver is not uncommon even with a well lined cabinet)

It's not just out of phase either, the tube resonance causes a phase rotation that when summed with the drivers direct response results in both a peak and a notch in the FR, however as well as that the spurious output is also time delayed on the order of 2ms or more depending on cabinet/port size, smearing the signal badly in the time domain.

The exact same port on the rear of the cabinet eliminates the problem to the point where you can't measure it or hear it, even when the speaker is not far from the front wall, so there's no good reason I can see to put a port on the front baffle and a lot of reasons not to...
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Old 12th July 2011, 12:53 AM   #26
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Originally Posted by John_E_Janowitz View Post

As another example, look at the 18sound 6ND410. This driver had many issues, but in this case it was the dustcap that gave it the claimed efficiency. They claim 101dB 1W which it only exceeds around the breakup peak at 4.5KHz. I tried everything to damp the resonance. Added mass, stuck foam on the cone, putty, nothing could fully correct the issues. The following is the impedance curve measured a few ways. Red was the natural impedance curve with no modifications. You can see the issues at 4KHz. I added mass to the dustcap to try to damp it. The resonance is still there but shifted lower in frequency as you can see in the blue curve. You can also see the Fs has shifted lower as the mass added was significant. Finally I just cut the dustcap off as you can see in the yellow curve. Fs raised very slightly but the bump from the dustcap resonance is gone.
It is not clear that the issue is a dustcap resonance. How do you know that is the case? Your two curves are dramatically different, almost looking like different woofers. One aspect not discussed yet is that dustcap removal loses woofer area and drops sensitivity. Your yellow curve looks like and extreme example of that, or is the difference due to the added mass?
Quote:

As I mentioned though, they count on this resonance for efficiency. You can see the effects of this dustcap on the entire bandwidth here. light blue is with the dustcap and yellow is without. Obviously this is an extreme example. It is literally the most extreme I have ever seen.
This is not a typical with/without dustcap comparison. Again either the added mass is part of the difference or you have lost significant radiating area (I don't know what the dustcap diameter was). For most of the comparisons I've done the top octave peak and rolloff change with different dustcaps and the sensitivity drops a little from lost area. You can't really count on a resonance for efficiency as its bandwidth is too narrow.

Quote:
As a more common example, hook 100ft of 4" diameter hose up to a dust collector. You'll have noise at the far end as you are drawing a significant amount of air in. Now take 100ft of 1/4" hose and hook it up to the same dust collector. Same amount of pressure, same distance, but now you have much more resistance to the air flow. It will be completely silent as the flow is slowed to almost nothing even though the pressure is the same.
Not sure that is a fair analogy. The volume of air pumped would be the same between a small gap and a large gap. Clearly, the velocity goes up with a smaller gap.

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The larger the dustcap, the less this compression ratio will be obviously. However with many drivers using inverted dustcaps and highly extended poles now this chamber can be very small. As an example on the AV15's I have a 2.5" VC with a 1" pole vent. At 30mm inward excursion there is approximately 1/2" from the top of the pole to the bottom of dustcap. This gives about 2.5 cubic inches of volume under the dustcap. At 30mm forward this chamber is expanded to 14.8 cubic inches. In this case it is approximately a 5.9:1 ratio. Drivers that are even higher excursion like many of the XBL drivers out there now, the TC LMS drivers, etc will have an even higher ratio. The 3:1 I stated is quite conservative really.

John
We're talking about drivers with 60mm of excursion? Impressive woofers indeed. When people are talking about phase plugs I assume they are talking about modest output 6 or 8" units. If your woofers give you 60mm peak to peak then you should consider them as LF air pumps and assure that no midrange frequencies get to them. I can'timagine midrange sounding clean on top of 60 mm excursion for the usual IM (both FM and AM distortion) effects. I certainly wouldn't remove the dustcaps from such a woofer as there would highly likely be air noise problems at those excursions.

IM can be caused by several mechanisms at extreme excursions. Any mechanism that changes frequency response with excursion would create incidental IM for frequencies that varied in level. I have heard woofers with such high excursion that the coils were leaving the gap, the inductance was dropping dramatically (for the peaks of excursion) and HF energy was coming and going with the inductance variation. But I still haven't heard the effects of a varying under-dustcap Helmholtz resonance. (And if I did I would find a way to damp the resonance before considering removing the dustcap.)

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Old 12th July 2011, 12:57 AM   #27
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Originally Posted by DBMandrake View Post
Well, changing an existing well designed driver from a dust cap to a phase plug is certainly a crap shoot unless you have good measurement equipment and a lot of patience and time on your hands to develop exactly the right phase plug that will (hopefully) bring you back to a near flat response, but even then I agree you can easily end up worse off than you started, as in the example I gave.

Converting a full range driver from dust cap to phase plug seems to be a popular pastime amongst full range purists, (along with de-whizzering...) but whilst it certainly makes a significant change in the response I'm yet to be convinced that it's unambiguously a net positive result, or that it's even worth attempting.
Agreed.

Quote:
It's not just out of phase either, the tube resonance causes a phase rotation that when summed with the drivers direct response results in both a peak and a notch in the FR, however as well as that the spurious output is also time delayed on the order of 2ms or more depending on cabinet/port size, smearing the signal badly in the time domain.
Just don't tell the TL people any of this.

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Old 12th July 2011, 01:33 AM   #28
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Originally Posted by DBMandrake View Post
Didn't notice this at first, but something is very screwy with your CSD graph - 79ms on the time axis ?
Thanks DBMandrake. I was attempting to extract off axis information from the on axis measurements, but I didn't mean to post them that way. Allow me to try again

First and second are before picking up the razor and after adding the plug. The third is without the dustcap.
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Old 12th July 2011, 02:49 AM   #29
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Quote:
Originally Posted by speaker dave View Post
It is not clear that the issue is a dustcap resonance. How do you know that is the case? Your two curves are dramatically different, almost looking like different woofers. One aspect not discussed yet is that dustcap removal loses woofer area and drops sensitivity. Your yellow curve looks like and extreme example of that, or is the difference due to the added mass?
Please re-read what I posted. As explained, adding mass to the dustcap broadened and lowered the resonance. You can tell mass was added as the bottom impedance peak is lower in frequency in the blue curve. When I cut off the dustcap any mass added to the dustcap was also gone. The Fs is clearly higher than the original driver so there is now less mass. This was very specifically a dustcap issue. Not only did I add mass to it at one point, i damped it with the coating we use on cloth surrounds which shifted the breakup around. Cutting it off eliminated it completely. Again to be fair this was the most extreme case I have seen and not the norm. I also tested 6-7" drivers by B&C, PHL, Beyma, etc. Only the Beyma with the phase plug had a smooth upper end with little breakup.

Quote:
This is not a typical with/without dustcap comparison. Again either the added mass is part of the difference or you have lost significant radiating area (I don't know what the dustcap diameter was). For most of the comparisons I've done the top octave peak and rolloff change with different dustcaps and the sensitivity drops a little from lost area. You can't really count on a resonance for efficiency as its bandwidth is too narrow.
See above. The diameter of the dustcap is 2.25" on a 7" driver. The VC however is 1.75" so that would be the amount of area lost. It can account for the loss of sensitivity somewhat but you can see that the entire shape of the response curve is now altered. The massive peak is gone and it is in general much more flat.

Quote:
Not sure that is a fair analogy. The volume of air pumped would be the same between a small gap and a large gap. Clearly, the velocity goes up with a smaller gap.
IF the flow was allowed to fully pass through, and the passage could handle the pressure then yes this would be the case. However if the resistance is high enough, there is very little if any flow. You need enough pressure to overcome the resistance for any flow to take place. As another example take a garden hose. The water pressure connected to the hose is always a constant. You can calculate the flow rate based on the pressure and the hose length taking into account the resistance of the internal surface area of the hose. Now take a smaller hose again say 1/4" diameter the same length. While the length is the same and water pressure is the same, there is much more resistance to the flow. The velocity of the water coming out won't increase. It actually decreases due to added resistance and there is also much less flow coming out. The idea is to get the resistance up enough to eliminate any issues with velocity. If the internal pressure isn't significant enough to overcome the resistance through the small gap, it will never escape.

Quote:
We're talking about drivers with 60mm of excursion? Impressive woofers indeed. When people are talking about phase plugs I assume they are talking about modest output 6 or 8" units. If your woofers give you 60mm peak to peak then you should consider them as LF air pumps and assure that no midrange frequencies get to them. I can'timagine midrange sounding clean on top of 60 mm excursion for the usual IM (both FM and AM distortion) effects. I certainly wouldn't remove the dustcaps from such a woofer as there would highly likely be air noise problems at those excursions.
I've been building and selling the TD woofers for about 6 years now. Previous to that Lambda Acoustics was selling them since 2001. Not all of them are that high excursion. That was actually an AV15 without phase plug to prove a point about the chamber volume. The TD15M's however are a 6mm Xmax driver with about 12mm one way suspension travel and clean response on axis to 4KHz. The phase plug instead of the dustcap is a huge part of that extended response as are the cloth surround and the cone profile and material. While the TD15M wasn't measured by augerpro, the LO15 was. This uses the same cone, surround, spider, but a shorter underhung coil. The physical soft parts are all the same so the breakups are primarily the same. Of most interest is the nearfield response, +/-2.5dB to over 3KHz:

(*edit - apparently you need to copy the link and past as it won't load here)
Click the image to open in full size.

You can find the rest of the data here:

AE Speakers LO15 4ohm - drivervault

A TD15X or H though has 14mm Xmax and about 22mm one way suspension travel. Again the phase plug is instrumental in getting the clean upper end response. You can see the measurements augerpro did of the TD15X here:

Frequency Response - drivervault

There are no issues with air noise. Both the TD15M's and X's and H's are used for wide band use. I have TD15H's which are similar, but with a copper coil vs aluminum, in Studio 4 at Chicago Recording Company. The guy who claims that "nobody" uses drivers with phase plugs may want to check with them. I can give multiple other examples of those using them in studios but won't waste the time now.

Quote:
IM can be caused by several mechanisms at extreme excursions. Any mechanism that changes frequency response with excursion would create incidental IM for frequencies that varied in level. I have heard woofers with such high excursion that the coils were leaving the gap, the inductance was dropping dramatically (for the peaks of excursion) and HF energy was coming and going with the inductance variation. But I still haven't heard the effects of a varying under-dustcap Helmholtz resonance. (And if I did I would find a way to damp the resonance before considering removing the dustcap.)
Yes there are many issues leading to IMD. The thing is that most often drivers are designed with intent to address one or two of the causes but in a way to ignore others. I often link people back to this poster as a good reference for the causes of various distortions.

Click the image to open in full size.

As you mention, inductance variation is a huge issue for this. The typical S shaped inductance curve can cause some serious issues, especially when doing passive crossovers. If inductance is varying with excursion, so is the upper end of the response curve. As the impedance curve is also changing, values for passive crossover components are no longer valid to keep a good phase relationship at the crossover point either. Removing that variation is key to any wide band driver or any case when a driver has to be used in a region where the impedance curve has begun to rise, which is almost always. The full copper sleeve of the TD woofers linearizes inductance nearly perfectly. In fact all of the TD woofers measured by augerpro were excellent in Le vs X measurements. He stated when measuring the LO15:

"whippersnapper02@yahoo.com - Jun 26, 2009 9:30 AM

I know, why only the 5mm plot? Well this was the last TD driver I tested Le(x) for after seeing all the others what's the point? Put a well designed full copper sleeve on the polepiece and Le(x) will be excellent. In the case of ALL the TD drivers it's flawless.
"


As you an see in the TD15X at +/- 13mm there is no change in the impedance curve. (*edit - apparently you need to copy the link and past as it won't load here)

Click the image to open in full size.

Not only that but the full copper sleeve also nearly eliminates flux modulation as well which is another HUGE factor in distortion, both IMD and harmonic, specifically odd orders. It's a reason that out of all the drivers tested the TD woofers also had the lowest distortion by a wide margin. Sorry if I got a little off track from the phase plug discussion here.

One final cause of IMD is due to varied acoustic loading on the cone. This leads to the properly sized phase plug argument. One of the things that were found to lead to IMD issues in the original TD drivers was the varied loading on the cone as the VC former came up around the curved portion of the phase plug. This was found by playing a 1KHz tone on top of a 20hz tone to high excursions. The solution in this case was a longer phase plug with a longer straight section before the curve. Once the former never came past the curved section, the loading was constant at all excursions and it eliminated this issue. In reality the shape of the plug at the point is nearly meaningless in the case of the woofer. It's the length that is the critical factor.


John

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Old 12th July 2011, 11:11 AM   #30
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Not to be obtuse but I'm still not following your woofer comparison. They appear to be very different woofers measured under different circumstances.

There is an across the board 3 to 4 dB sensitivity drop. If the mass you previously added was removed (as shown in your impedance curves) and the area loss doesn't account for it then what is the difference? You can't claim that the dust cap resonances caused an across the board sensitivity difference, it just doesn't work that way. I also note that the measuring windows are different (fine fuzz on the cyan curve indicates a long measuring window). Plus a periodic wiggle around 1 k for the yellow curve indicates a strong reflection in the measurement not present in the other case. There are similarities in the dual peaks at 4.3 and 5 kHz, suggesting the same woofers but the other differences are too great for the changes you suggest.

A point on woofer peaks. Although most people assume that HF woofer peaks must come from the woofer center, most modal analysis shows that strong upper modes come from activity around the woofer cone's perimeter. Removing (or adding) a dustcap can be enough of a change to modify the output, but this does not indicate that the effect was purely happening on the dustcap (as evidenced by the 4.3 and 5k peaks still being there for both curves). At the same time your premise was that a strong resonance existed in the volume under the dustcap, the primary impetus for removing it. I'm still not seeing any evidence of that.

I'll stand by my initial assertions: removing a dustcap and adding a phase plug will take away a significant variable for adjusting the woofer frequency response, reduce the woofer's effective area and provide an air leak path that will frequently cause air noise problems. You can find the odd unit that overcomes all of these difficulties, but in most cases they are there. Certainly, pulling off a dustcap and adding a phase plug in hopes of improving a given woofer is misguided.

David S.
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