Tweeter Question...

[jpb6061]

Sorry if this has been discussed, but I couldn't search out an answer in the archives.

What effect does the phase plug have on a tweeter such as the Focal TC 120 TD5 (or the TC 120 TDX 2, of which I have a pair)?

I note that Wilson takes them off, Thorsten took them off in his Watt/Puppy Klone project as well.

I'm doing my own version of the same and was wondering!

If they ought to come off.... How do they?

JB

[JonMarsh]

Strictly speaking, Wilson buys them without the phase plug. I take them off, too. Hard phase plugs "improve" the off axis dispersion, but they also create some resonance and energy storage. I prefer TC120dx2 without. Side cutters work fine...

~Jon

[dlr]

Phase plug resonances? I don't see it.

Hard phase plugs "improve" the off axis dispersion, but they also create some resonance and energy storage.
~Jon

How so? Phase plugs improve the on- and off-axis from what I've seen. They also eliminate the air cavity beneath the cone, normally vented through the pole piece, that is itself a resonant space. I don't see how the phase plug can in any way create resonances. To what do you attribute resonances?

Also, if you cut off a phase plug, you've taken away some of the heat dissipation of the driver.

dlr

[Gerald Jansen]

Different Thang

Dear dlr,

Please consider the carefully crafted design of a TC120xxxxx, before barging in. This "phase plug" is a diffuser type of construction *in front* of a concave dome, so your concerns are way out of line.

Cheers,
Gerald

[dlr]

oops

Dear dlr,

Please consider the carefully crafted design of a TC120xxxxx, before barging in. This "phase plug" is a diffuser type of construction *in front* of a concave dome, so your concerns are way out of line.

Cheers,
Gerald

I wasn't thinking of phase "shield" as I've always referred to it. You're right, it doesn't apply to the tweeter variety.

dlr

[Gerald Jansen]

I wasn't thinking of phase "shield" as I've always referred to it.
dlr

Good to see you back. I did not intend to scare you away. I'm curious why you use the term "phase shield" for a phase plug (and vented pole piece)? Is it a translation from another language?

Hard phase plugs "improve" the off axis dispersion, but they also create some resonance and energy storage.

Do you have an idea weather the resonance and energy storage are caused by it's acoustic design, or by the non-rigid fragile construction of the diffuser (leading to secondary radiation)?

[ThomasW]

Good to see you back. I did not intend to scare you away. I'm curious why you use the term "phase shield" for a phase plug (and vented pole piece)? Is it a translation from another language?

One would imagine that Dave's use of the term, and the subsequent confusion, was a function of the previous posts in the thread referring to a "phase plug" instead of a "diffuser".

And I'm reasonably sure that Dave's been around long enough that one discourteous comment won't scare him away.... :wink:

[cjd]

Which reminds me, I really want to try Dave's tweak of the RS28. But I really don't want to take things apart. :P

C

[JonMarsh]

I don't like taking tweeters apart unless I've got some testing means to verify the distortion and "tune" the centering when putting it back together.

DLR"s "phase sheild" is a "soft" one, not a "hard" one (careful about typos there), and is probablyl the best solution if one wants some of the dispersion improvement without the drawbacks of a hard diffuser.

JUST my 0.02.

BTW, phase shields vary considerably in design and construction, and the ones I really get worked up about are those used by Vifa and in the past by MB Quart- they have a really nasty signature, IMO, and are what got me worked up about them originally.

~Jon

[Gerald Jansen]

I'm curious why you use the term "phase shield" for a phase plug (and vented pole piece)?

Ok, I found the answer at your website (www.speakerdesign.net) in article on the Dayton tweeter modification. Very Interesting.

[dlr]

My brain had momentarily stopped working

BTW, phase shields vary considerably in design and construction, and the ones I really get worked up about are those used by Vifa and in the past by MB Quart- they have a really nasty signature, IMO, and are what got me worked up about them originally.

~Jon

I haven't heard many hard-dome tweeter systems, so I've not built up a preference for or against as some have. Many folks have voiced the same sentiment as you, though, as I'm sure you know.

When I read your post I saw "plug" andI immediately forgot that the subject was tweeters. My brain slipped a gear and went into woofer mode.

But I do still find your comment interesting. In my limited work with a phase shielded tweeter (the RS28), I found that the phase shield improved the on-axis response and the off-axis a bit. I think, though, that the influence of the shield is very dependent on the dome and shield geometry. A lot of manufacturer graphs of their hard-dome tweeters do show really bad off-axis results even with a shield.

The RS28 has some of the best off-axis response of any tweeter I've tested, insofar as smoothness is concerned. Take off that shield and the on-axis response above 10K suffers badly due to phase.

dlr

[dlr]

Terms

Good to see you back. I did not intend to scare you away. I'm curious why you use the term "phase shield" for a phase plug (and vented pole piece)? Is it a translation from another language?


Do you have an idea weather the resonance and energy storage are caused by it's acoustic design, or by the non-rigid fragile construction of the diffuser (leading to secondary radiation)?

I've always seen the description as phase shield. It's drawn from the fact that it blocks, or "shields", the listener from the output at the center of the driver. This is done because a hard-dome tweeter does not have any inherent internal damping in the dome and the speed of sound is much faster in a hard material. The result is that the signal is emitted from all points on the dome at nearly the same time. At high frequencies the wavelength is short, to very small time delays cause interference between the output at the former attachment point on the rim and the output from the tip of the dome, hence the droop one sees if there is not shield.

The resonance is simply due to the fact that there is an air cavity between the shield and the dome. The blocked signal is reflected back to the dome which itself then reflects the reflection. This sets up a resonance determined by the gap distance.

My tweek of the RS28 was an attempt to dissipate this energy rather than reflect it.

I suppose that there could also be some resonance in the structure itself, I hadn't considered that possiblity. But since most are fairly secure, I suspect it's minimal. Certanily one such as the RS28 won't suffer from this, the shield being attached to the rear of the mesh cover.

dlr

[dlr]

It's one of the easier ones

Which reminds me, I really want to try Dave's tweak of the RS28. But I really don't want to take things apart. :P

C

The nice thing about this tweek is the ease and reversibility. The RS 28 has a cover over the diaphragm assembly that includes the mesh grill. The sensitive part (diaphragm assembly) does not need to be moved. As long as you're careful in separating the cover (it tends to stick), it should leave the rest unscathed.

As tweaks go, though, this one is on the minor side, IMO.

dlr