Hi Dave.
I don't have the data available. I did it back in the days at KEF on the 2pi baffle.
I might be able to find some other 4pi measurements of recent drivers where I have been trying to investigate response peaks by pulling off the dustcaps. As you have been pointing out, this is often fruitless because the issues are mainly from the cone, not the dustcap!
I concluded that given this, it is often a good idea to use a small diameter rigid dustcap bonded directly to the voice coil former to help stiffen the former.
My latest project, 4" aluminum phase plugs for BMS 18" woofers:
The phase plug goes partly in the shorting ring that sits on top ot the pole piece and they form electrically and thermally one piece. Each plug weights ~2.8kg, they dissipate heat and act as a heat buffer.
BMS woofers have triple shorting ring motors, so the plug did not reduce inductance at the VC's resting point but I think it linearizes inductance (Le(X)) as the higher order harmonics (>H3, also H3 reduced here and there) reduced nicely in the middle and upper bass area.
On the right is the phase plugified woofer (look for ex. at the pink, blue and orange lines between 50hz and 200Hz). Measurement distance ~65cm, woofer was in 36cm deep U-frame baffle. The woofer is BMS 18N850, neodium motor (modified cone and damped basket).
Couple of sine tone comparisons, the phase plugified is on the right again:
Above pictures in numbers:
100Hz:
Without phase plug / With phase plug / Difference
H3: 0,153% (-56,3dB) / 0,100% (-60dB) / -3,7dB
H4: 0,0201% (-53,4dB) / 0,288% (-50,8dB) / + 2,6dB
H5: 0,166% (-55,6dB) / 0,0771% (-62,3dB) / -6,7dB
H6: 0,0155% (-76,2dB) / 0,0123% (-78,2dB) / -2dB
H7: 0,0702% (-63,1dB / 0,0207% (-73,7dB) / -10,6dB
H8: 0,0098% (-80,2dB) / 0,0096% (-80,4dB) / -0,2dB
H9: 0,0332% (-69,6dB) / 0,0123% (-78,2dB) / -8,6dB
130Hz:
Without phase plug / With phase plug / Difference
H3: 0,262% (-51.6dB) / 0,0970% (-60,3dB) / -8,7dB
H4: 0,0287% (-70.8dB) / 0,0111% (-79,1dB) / -8,3dB
H5: 0,284% (-50,9dB) / 0,125% (-58,1dB) / -7,2dB
H6: 0,0345% (-69,2dB) / 0,0191% (-74,4dB) / -5,2dB
H7: 0,0936% (-60,6dB) / 0,0360% (-68,9dB) / -8,3dB
H8: 0,0276% (-71,2dB) / 0,0067% (-83,5dB) / -12,3dB
H9: 0,0107% (-79,4dB) / 0,0037% (-88,6dB) / -9,2dB
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
The phase plug goes partly in the shorting ring that sits on top ot the pole piece and they form electrically and thermally one piece. Each plug weights ~2.8kg, they dissipate heat and act as a heat buffer.
BMS woofers have triple shorting ring motors, so the plug did not reduce inductance at the VC's resting point but I think it linearizes inductance (Le(X)) as the higher order harmonics (>H3, also H3 reduced here and there) reduced nicely in the middle and upper bass area.
On the right is the phase plugified woofer (look for ex. at the pink, blue and orange lines between 50hz and 200Hz). Measurement distance ~65cm, woofer was in 36cm deep U-frame baffle. The woofer is BMS 18N850, neodium motor (modified cone and damped basket).
An externally hosted image should be here but it was not working when we last tested it.
Couple of sine tone comparisons, the phase plugified is on the right again:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Above pictures in numbers:
100Hz:
Without phase plug / With phase plug / Difference
H3: 0,153% (-56,3dB) / 0,100% (-60dB) / -3,7dB
H4: 0,0201% (-53,4dB) / 0,288% (-50,8dB) / + 2,6dB
H5: 0,166% (-55,6dB) / 0,0771% (-62,3dB) / -6,7dB
H6: 0,0155% (-76,2dB) / 0,0123% (-78,2dB) / -2dB
H7: 0,0702% (-63,1dB / 0,0207% (-73,7dB) / -10,6dB
H8: 0,0098% (-80,2dB) / 0,0096% (-80,4dB) / -0,2dB
H9: 0,0332% (-69,6dB) / 0,0123% (-78,2dB) / -8,6dB
130Hz:
Without phase plug / With phase plug / Difference
H3: 0,262% (-51.6dB) / 0,0970% (-60,3dB) / -8,7dB
H4: 0,0287% (-70.8dB) / 0,0111% (-79,1dB) / -8,3dB
H5: 0,284% (-50,9dB) / 0,125% (-58,1dB) / -7,2dB
H6: 0,0345% (-69,2dB) / 0,0191% (-74,4dB) / -5,2dB
H7: 0,0936% (-60,6dB) / 0,0360% (-68,9dB) / -8,3dB
H8: 0,0276% (-71,2dB) / 0,0067% (-83,5dB) / -12,3dB
H9: 0,0107% (-79,4dB) / 0,0037% (-88,6dB) / -9,2dB
Heh Cal, thanks I quess!
Have not had time to measure the effects of the aluminum phase plug in the BMS 18S430v2 (ferrite motor, 8mm calculated xmax, real-life linear excursion 10-11mm) and BMS 18N860 (neodium motor, 19mm calculated xmax). I suspect that the phasing plug might effect them differently than 18N850. Ferrite motors are more suspectible to flux modulation than neodium, and the longer VC of 18N860 is more in the phasing plug's area in the resting point, so it might result in inductance reduction. This effect of phase plug (phase plug acting as a shorting ring) is not often discussed. The 18N850 has only ~0,65mH inductance at the resting point (due to the triple shorting ring motor) and Le(X) is top-notch linear according to Klippel measurements, yet it still benefitted quite greatly from the phase plug's inductrance linearization/flux modulation reduction (or whatever is that caused the higher order harmonics to go down especially in the middle/upper bass).
Have not had time to measure the effects of the aluminum phase plug in the BMS 18S430v2 (ferrite motor, 8mm calculated xmax, real-life linear excursion 10-11mm) and BMS 18N860 (neodium motor, 19mm calculated xmax). I suspect that the phasing plug might effect them differently than 18N850. Ferrite motors are more suspectible to flux modulation than neodium, and the longer VC of 18N860 is more in the phasing plug's area in the resting point, so it might result in inductance reduction. This effect of phase plug (phase plug acting as a shorting ring) is not often discussed. The 18N850 has only ~0,65mH inductance at the resting point (due to the triple shorting ring motor) and Le(X) is top-notch linear according to Klippel measurements, yet it still benefitted quite greatly from the phase plug's inductrance linearization/flux modulation reduction (or whatever is that caused the higher order harmonics to go down especially in the middle/upper bass).
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Excellent work - thanks. It does not surprise me in the least, but quite contrary to the loudspeaker dogma.
Perhaps I have missed an entry,so excuse me for re-calling that the origin for fitting phalli to speaker cavities appears to originate with F.H.Brittain and the GEC BCS 1851 metal cone speaker.( Wireless World,Nov.1952 ).The purpose of the device, which was called a "bung",was to remove a severe dip in the response at 2.5 kHz caused by destructive interference from opposite sections of the cone. D.A. Barlow called this phenomenon,Interference at Cone Apex,and his cure to remove a dip at 1100 Hz resulted in fitting a large spherically curved dome to the cone.(AES Loudspeaker Anthology,Vol. 1-25 ).
I polished them after they came from the machining, the surface was quite clean (like in the first picture). I polished them attached to a hand drill, it was quite quick operation. Thanks for the tip, I will try that on one plug to see what it does. I have some 000 steel wool somewhere. Wax is good for finishing and protection against oxidation.
To my understanding the phase plug does not alter t&s parametres besides possibly the inductance. In this case it did not affect the resting point inductance, but the motor got more linear, maybe Le(X), flux modulation or something else got better. I have not tried this on any midwoofers, they came with the phase plug. For fullrangers I made aluminum phase plugs, and noticed slight reduction in inductance and higher order harmonics reduction but the plug made the cone resonances somewhat worse in CSD. Good decay and time domain behaviour is more important in fullrangers imo. I get nice higher order harmonic reduction also from current driving the fullranger anyway.
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If you do this, make sure to follow up with another polishing step, otherwise you may see rouge (iron contamination).
Apropos of phase plug, here's the abstract of an engineering brief for a paper to be delivered at the AES Dublin Convention:
EB05-2 New Engineering Method for Design and Optimization of Phasing Plug and Dome-Shaped Compression Chamber of Horn Drivers—Alexander Voishvillo, JBL/Harman Professional Solutions - Northridge, CA, USA
In this work an accurate analytical solution is found for the sound field in a dome-shaped compression chamber. This simplifies the design and optimization of the compression chamber’s annular exits to suppress high-frequency air resonances. In earlier works by other authors, the solution is also found in spherical coordinates. For low-curvature chambers, an approximation in the form of Bessel function summation was used. For high-curvature compression chambers an analytical approximation did not work and FEA had to be used. The new proposed method is based on Mehler-Dirichlet analytical integral presentation of Legendre functions. This approach handles high-curvature dome chambers and does not require using numerical methods. An evaluation of this new method’s applicability to chambers with various different curvatures was implemented.
Engineering Brief 517
EB05-2 New Engineering Method for Design and Optimization of Phasing Plug and Dome-Shaped Compression Chamber of Horn Drivers—Alexander Voishvillo, JBL/Harman Professional Solutions - Northridge, CA, USA
In this work an accurate analytical solution is found for the sound field in a dome-shaped compression chamber. This simplifies the design and optimization of the compression chamber’s annular exits to suppress high-frequency air resonances. In earlier works by other authors, the solution is also found in spherical coordinates. For low-curvature chambers, an approximation in the form of Bessel function summation was used. For high-curvature compression chambers an analytical approximation did not work and FEA had to be used. The new proposed method is based on Mehler-Dirichlet analytical integral presentation of Legendre functions. This approach handles high-curvature dome chambers and does not require using numerical methods. An evaluation of this new method’s applicability to chambers with various different curvatures was implemented.
Engineering Brief 517
https://www.eton-gmbh.com/fileadmin...Datenblaetter/SYMPHONY_II_8-512_C8_32_HEX.pdf
https://www.eton-gmbh.com/fileadmin...Datenblaetter/SYMPHONY_II_8-412_C8_32_HEX.pdf
Same drivers but one with phase plug and another without.
Now as per the overall discussions and as per the original driver designers there are anyway two points taken into consideration.
1. Using phase plug will anyway give better off axis response as much of the dustcap is contributing to it.
2. Also what makes the point interesting is the cavity resonance behind the dustcap is a major concern especially between 1Khz to 2Khz so phaseplug would be better I guess
3. Now only concern what I see is the thin air gap if that doesnt chuff the sound then absolutely phase plug will be a better option.
So why many don`t use one reason is cost second is some dust might enter into the gap and get the coil damaged ( which is rare case I guess )
If using a phase plug results in better CSD / time domain performance I would rather do something else for the other problem but use the phaseplug.
This is not confirmation bias but would like to build some drivers now its time to choose phaseplug or not. Im building a 8 inch, 10 and 15 inch.
I`m ONLY concerned about the thin gap chuffing can anyone give me final conclusion on this.
The cones will have +/-12mm of excursion at max for 10 inch and 8 inch so using phaseplug gives chuffing noise for such excursions? Most of the cases the drivers will be in ported config.
https://www.eton-gmbh.com/fileadmin...Datenblaetter/SYMPHONY_II_8-412_C8_32_HEX.pdf
Same drivers but one with phase plug and another without.
Now as per the overall discussions and as per the original driver designers there are anyway two points taken into consideration.
1. Using phase plug will anyway give better off axis response as much of the dustcap is contributing to it.
2. Also what makes the point interesting is the cavity resonance behind the dustcap is a major concern especially between 1Khz to 2Khz so phaseplug would be better I guess
3. Now only concern what I see is the thin air gap if that doesnt chuff the sound then absolutely phase plug will be a better option.
So why many don`t use one reason is cost second is some dust might enter into the gap and get the coil damaged ( which is rare case I guess )
If using a phase plug results in better CSD / time domain performance I would rather do something else for the other problem but use the phaseplug.
This is not confirmation bias but would like to build some drivers now its time to choose phaseplug or not. Im building a 8 inch, 10 and 15 inch.
I`m ONLY concerned about the thin gap chuffing can anyone give me final conclusion on this.
The cones will have +/-12mm of excursion at max for 10 inch and 8 inch so using phaseplug gives chuffing noise for such excursions? Most of the cases the drivers will be in ported config.
This is often taken to be more than it is, it's not something to worry about here.
http://www.seas.no/index.php?option...rfxp&catid=44:utv-prestige-woofers&Itemid=461
http://www.seas.no/index.php?option...8s-w26fx002&catid=49:excel-woofers&Itemid=359
http://www.seas.no/index.php?option=com_content&view=article&id=345&Itemid=497
There are many seas drivers with phaseplugs I guess there wouldnt be a chuffing problem if so seas wouldnt have introduced it.
Coming to the dustcap breakup well its a point to concern. Looking at 18 sound drivers their response is only onaxis and not off axis shown if shown one would understand where the breakup is starting.
Third
The off axis response
aesthetics
pole piece cooling and reduced power compression
Cavity resonance behind the dust cap and turbulence resonances
cons is slight drop in sensitivity can be sacrificed because of the above pros.
cost can be reduced by using wood but it will not have enough heat transfer.
http://www.seas.no/index.php?option...8s-w26fx002&catid=49:excel-woofers&Itemid=359
http://www.seas.no/index.php?option=com_content&view=article&id=345&Itemid=497
There are many seas drivers with phaseplugs I guess there wouldnt be a chuffing problem if so seas wouldnt have introduced it.
Coming to the dustcap breakup well its a point to concern. Looking at 18 sound drivers their response is only onaxis and not off axis shown if shown one would understand where the breakup is starting.
Third
The off axis response
aesthetics
pole piece cooling and reduced power compression
Cavity resonance behind the dust cap and turbulence resonances
cons is slight drop in sensitivity can be sacrificed because of the above pros.
cost can be reduced by using wood but it will not have enough heat transfer.
About 10 years or so back I cut the dustcap off 15" drivers which I intended using up to 1200Hz.
I cut away the dustcaps and used a spice container which was made from a cardboard tube, and had a steel bottom plate so it attached to the magnet. Here's another shot...
There was no problem with chuffing. In fact, there wasn't much gained from the phase plug at all. I did get results from that ring of foam weatherstripping you can see about a third of the way out.
I cut away the dustcaps and used a spice container which was made from a cardboard tube, and had a steel bottom plate so it attached to the magnet. Here's another shot...
There was no problem with chuffing. In fact, there wasn't much gained from the phase plug at all. I did get results from that ring of foam weatherstripping you can see about a third of the way out.
What this boils down to is overall responses, because it kind of doesn't matter what the off-axis does, as long as the on-axis does it too. It's the differences that matter. Looking at the two similar eton drivers you showed first, I'm having trouble seeing quickly which one looks better. If I were buying them and had to make a choice, I would need to study them.
