Moving vs fixed phase plug for sealed-cabinet mix cubes?

[Scoox]

I want to build a pair of mix cubes (like Auratones but on a budget). I bought a pair of Visaton B 100 drivers. I just received them and I've noticed the phase plug is actually fixed, that is, the cone slides over it which means there is a small but definite gap between the two. Now, the whole point of a sealed cabinet is to prevent air flowing into and out of the cabinet... if there's a gap around the phase plug, can it still be considered a sealed cabinet? Are drivers with a fixed phase plug, or without one, designed to be used with ported cabinets? Will I get any distortion or noise if I try to install these speakers in a sealed cabinet as the air whizzes in and out through the narrow gap?

I'm also curious, are there any advantages to using a fixed phase plug vs a plug that's attached to the cone and moves with it? I assume the main benefit of the fixed plug is that the cone would be lighter, which gets the driver closer to the theoretical ideal "infinitely stiff and zero mass" cone.

 

[alex-z]

Phase plugs are fixed, to avoid adding moving mass. They have two functions, aiding high frequency directivity, and reducing resonance that would build up behind a dust cap due to air pressure.

The air leakage will impact your sealed cabinet response. This is unlikely to cause audible problems, just a slight efficiency loss. Most modeling software will label this as "Ql", with a higher number corresponding to reduced air leakage. The other downside is that the phase plug slightly reduces the surface area of the woofer, and central cone stiffness.

Overall, the quality of the motor structure and surround matters more than the dust cap vs phase plug debate.

 

[Scoox]

Phase plugs are fixed, to avoid adding moving mass. They have two functions, aiding high frequency directivity, and reducing resonance that would build up behind a dust cap due to air pressure.

The air leakage will impact your sealed cabinet response. This is unlikely to cause audible problems, just a slight efficiency loss. Most modeling software will label this as "Ql", with a higher number corresponding to reduced air leakage. The other downside is that the phase plug slightly reduces the surface area of the woofer, and central cone stiffness.

Overall, the quality of the motor structure and surround matters more than the dust cap vs phase plug debate.

What does QI stand for? I've googled it but nothing relevant came up. After reading your reply I think I might end up returning the drivers. At 77 €/ea. I don't want to be making compromises. It's a bit annoying the specs don't mention this detail.

 

[alex-z]

Ql just describes the loss factor of air leakage in the box. It is not directly linked to a physical interaction, but is used as a static element in modeling software.

Qa is for losses from damping material within the box.

Qp is for port airflow loss.

Speaker design is 100% compromise, regardless of price point. The spec sheet doesn't mention it because that is an inherent property of phase plug drivers. If you move to a driver with a dust cap, you will gain maybe 5% more efficiency, and increase your "Ql" but high frequency directivity and dust cap resonance will become slightly worse.

If you want "mix cubes" then the compromise list is significant. Any "full-range" driver is going to have significant narrowing at high frequencies. The only way to make such a design work is a high quality coaxial, like you might find in a KEF or Genelec speaker.

 

[airborne]

phase plugs like this are used on audiophool speakers to make them look more expensive

sort of like exposed vacuum tubes on audiophool amps

you messed up because you should have gotten a driver with a copper phase plug - what is the point of having a phase plug that looks plastic ?

it saves them money because they don't need to vent the motor and they charge you more for the cool look

win-win for them

in theory removing the dust cap could allow them to do things like use extended pole piece for a more symmetrical flux in the gap but in reality if that mattered you would see designs like this outside the audiophool realm ...

as for box leakage i assume it works out somehow - but when i had a gap in in a sealed subwoofer box using TC Sounds LMS driver it made farting sounds until i sealed it with silicone ...

but i never heard about that problem with drivers like yours ...

 

[Scoox]

phase plugs like this are used on audiophool speakers to make them look more expensive

sort of like exposed vacuum tubes on audiophool amps

you messed up because you should have gotten a driver with a copper phase plug - what is the point of having a phase plug that looks plastic ?

it saves them money because they don't need to vent the motor and they charge you more for the cool look

win-win for them

in theory removing the dust cap could allow them to do things like use extended pole piece for a more symmetrical flux in the gap but in reality if that mattered you would see designs like this outside the audiophool realm ...

as for box leakage i assume it works out somehow - but when i had a gap in in a sealed subwoofer box using TC Sounds LMS driver it made farting sounds until i sealed it with silicone ...

but i never heard about that problem with drivers like yours ...

Actually the ones I got have metal phase plugs. I don't know what metal but it looks like anodized aluminium (I can make out the CNC milling pattern on the surface).

 

[airborne]

Actually the ones I got have metal phase plugs. I don't know what metal but it looks like anodized aluminium (I can make out the CNC milling pattern on the surface).

there you go !

expensive !

the goal of hi-fi is to make things heavy and expensive.

this is called engineering ! ( and not marketing as evil trolls would have you believe ! )

 

[Vladimir Filevski]

phase plugs like this are used on audiophool speakers to make them look more expensive
sort of like exposed vacuum tubes on audiophool amps
you messed up because you should have gotten a driver with a copper phase plug - what is the point of having a phase plug that looks plastic ?

Nothing worse than ignorance person with huge ego.
Phase plugs in midwoofers have their use, as alex-z correctly pointed out:

Phase plugs are fixed, to avoid adding moving mass. They have two functions, aiding high frequency directivity, and reducing resonance that would build up behind a dust cap due to air pressure.