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Why are phase plugs on the higher end drivers and just dust caps on the lower end drivers?

What purpose does the phase plug provide?

Main benefit is that it aids off axis response. Higher frequencies radiate from the driver near the center of the cone, and if the shorter waves radiate from one side to another, they will interfere with one another-causing cancellation and reinforcement at given frequencies. The phase plug is shaped to prevent this from occuring. This mainly aids in upper midrange for larger midbass drivers and into the treble range for midranges, basically where you'd typically cross a driver over at.

The other benefits including removing heat from the motor (assuming the phase plug is capable of dissipating heat, such as aluminum or copper). And another is that it eliminates a potential place of resonance where a dustcap would be.

I have heard of a few reasons. Not sure which one is the most correct, but here they are anyways.

A phase plug essentially eliminates the need for a dust cap, which in turn, reduces cone mass, permitting extended frequency response.

It allows more airflow through the motor structure, which improves cooling and thus increases power handling.

A Phase plug also helps to reduce on-axis beaming by dispersing higher frequencies that the cone is producing.

The phase plug also helps to break up standing waves that form on the cone when it starts creating frequencies smaller than the cones diameter. This allows the speaker to play higher without sacrificing SQ.

Well crap, you beat me to it.

I was just thinking about this earlier today...I figured it wasn't cause they look cooler.

Yes, I forgot to mention the marketability of the phase plug. It's hard for a potential customer to resist the temptation to touch the phase plug nipple I know that's the first thing I did when I got the Excels in :happy055:

As mentioned - essentially to extend the high-frequency response of the driver in a useable way - without the "beaming" that happens when you play frequencies on a speaker whose wavelengths are smaller than the cone diameter (aka. high frequencies on a large cone - the off-axis response starts to suffer cancellations, from sound waves eminating from opposite sides of the cone, beginning to cancel with one another... as you move off-axis, those pathlengths start to develop differences in length that approach 1/2 of that higher-frequency wavelength - and start to cancel).

But it's no magic wand - it helps, but it doesn't cure... and it's only effective really to extend the response up just a little farther up the musical spectrum... it isn't going to turn a 6.5" mid into something that can play up to 10kHz and sound good as you walk around the room.

The other half of your question:
Why do high-end speakers sometimes have them, and low-end speakers never do?
Answer: You can design the crossover around it.

With a cheap component set, you can run the crossover point lower... or perhaps "in your market segment" you just don't care if you start flirting with a little beaming as you get near your crossover frequency.
With a higher end set, you might want to run that Xover point as high as you can - relegating that tweeter as much to "ambient" duty as you can get away with. In terms of setting up a more realistic soundstage, that's something a higher-end clientele might appreciate, where a cheap-speaker customer isn't likely have a clue.

Plus - as mentioned - it looks impressive, significant - so damnit, then it just must be. Perception IS reality, for sure.

well then here is another question for you...If they can customize the crossover from these, why does my system have virtually no midbass? Recognizing that the midbass driver doesn't have the phase plug, could it be cause i am running the passive x-overs rather than going all active?

Also, while I am on the subject, would it be better to rewire the amps to include the mid/tweet comps on one 2.150, and the midbass on the other 2.150, or continue with the 2.150 per side (tweet, mid, midbass)?

I just can't seem to get a decent amount of Midbass from these K3P's

Hell, I have a line on an SRX2 for the tweets, then I can run a dedicated Audison amp to each set of speakers. I am thinking on keeping the mounting of the crossovers the same cause of the looks, but running them active off the 8455..then the sub x-over'd on the 1.400

Mo powa!!!! You'd be better off going active and throwing more on those midbass drivers, especially considering that you have the midranges and tweets in the dash (primary energy from the drivers + reflected energy from the windshield) that are already more efficient than the dedicated midbass driver. You'll probably get much better results imaging-wise from the flexibility of going active as well.

K2Ps themselves generally have plenty of midbass output, but not the best low end due to the driver design. Both of the midbasses in that set are a high Qts, high Fs driver designed specifically for more of a pronounced output down to around 80hz, and a more rapid rolloff below that. Subjectively, this can lead to "tighter" sounding bass. Sounds like some form of installation issue, not necessarily because of a poor install (which we ALL know isn't the case), but perhaps due to the size of the doors you've got them in amongst other factors. But are you wanting midbass or subbass from your mids? A car door performs more like a leaky sealed box than IB, though, and saying that the K3P midbass already has an Fs of around 68hz IB, that can raise a bit if you have a smaller car door, and even more if you've really done a lot of work sealing that door up via deadening and baffles.