Dust caps vs Phase plugs (off axis measurements) 

@RAW-CAt A "Dust Cap" is typically defined as any type of cap that conceals and seals off the voice coil and coil gap in the motor from the front of the cone and the air mass in front of it. The "Dust Cap" can be shaped as a "Phase Plug" as we see here in the non-STATUS Alpine midwoofer, but it is still considered a "Dust Cap" and not a "Phase Plug" because it seals the voice coil and coil gap from the front. Solid "Phase PLUGS" are physically attached to the pole piece or motor system and are fixed in place and do not move +/- with the motion of the cone. These Phase PLUGS are primarily used to dissipate heat and reduce power compression. However, using a Phase Plug also reduces the Sd (piston or cone area), so it is a tradeoff between efficiency and power compression. With the "Phase Cap", you don't lose the Sd, but you also aren't taking advantage of a true Phase Plug's thermal dissipation properties. As magnets heat up, they start to lose some of their "BL" (motor force) or magnetic strength. Especially if the driver does not have a central rear pole vent in the motor system, a Phase Plug will often be implemented to help do the job of cooling the coil and magnet/motor. For example, take a look at the previous generation Alpine Type-X SPX-13PRO and SPX-17PRO midwoofers. In midwoofers and midrange drivers, the intersection of the dust cap and the main body of the cone nearly always creates a resonance that results in a small breakup and "blip" in both the impedance plot and FR, as shown in some of your measurements. The physical size or diameter of the Dust Cap or "Phase Cap" in relation to the main cone determines the frequency at which this resonance & breakup will occur. And also due to their physical size, a Phase Plug or Phase Cap will only affect the highest frequencies in the driver's intended passband. They are most often used to minimize a Single Large Resonance and breakup and instead reduce it's effects by "spreading out" that single resonance into multiple smaller resonances across the passband, and/or pushing the resonance further up in frequency where they will be out of the intended passband (above the XO point) where they will be less audible and less detrimental. A single large resonance and breakup will be easily audible as distortion, whereas multiple small resonances and breakups that are spread out will most likely be below our threshold for detecting distortion.

Hum, it appears that the phase plug doesn't have much of an impact, but in all fairness it seems like 6.5 inch drivers suck in the higher frequency range anyway which is why most of us run three way front stages to begin with. I think you're right, it would be really interesting to see if there is any impact on the midrange drivers with or without the phase plug. Those are the drivers we're using for their upper frequencies where i would expect to see the phase plug make a difference. We'll be waiting to see that video for sure! Keep up the great work on the video content 👍.

I was worried about taking mine off..and replacing with flat dust cap..now I won't mind

Thanks for this hard work. It's very illustrative.

Very Very cool tests. Super interesting... And fun to watch. Good stuff

The gem I got from this: higher frequencies are focused beam-like energies. Makes sense, can almost visualize it👍

Maybe they use phase plugs to appeal to home audiophiles where a single driver playing full range is more common?

Ok so my cdts have the removable type phase plugs that are pointy and screw into the motor and one of my kicks 5.25” midwoofers had backed out so much over time and gotten loose to the point it fell off and I didn’t realize it for a few months. The speaker sounded muffled and like it lost its direction this was happening to left front kick panel woofer that I experniced this issue and it was not playing as well and handling the bass at higher volumes like the right side kick was handling.

@RAW-CAt You should try to get ahold of Jason Kemmerer and have him on a livestream interview as a guest. 😉 Jason worked at Alpine Electronics during its heyday and was responsible for the design and technology used in many of the Alpine Type-X and Type-R components and subwoofers. He holds multiple Patents for loudspeaker and electronics design...do a search for "Jason Kemerer Justia Patents". He was most recently working for Scosche and is now the head of Wvtech (WaveTech) which make car audio OEM Integration Interfaces with DSP.

Why are we doing this outside where there are so many other noises can interfere with the results, instead of inside to simulate a closed auto cabin?

Wouldn't the measurements be more accurate in an open baffle? So the cabinet reflections dont cause distortion on the 2nd and 3rd harmonics?

Whats the distance between the speaker and microphone?

How Faital sounds compared to SB?

What about a dual cone loudspeakers?