Cone construction question

[Tyson]

Why do some woofers have a fully concave inner surface, ala the Peerless Nomex woofers that I have:



While others have a convex inner surface, a "bubble" in the center, ala the Scan Speak Revelator midwoofer?

[DcibeL]

Technical term...dustcap.

Sometimes it is only cosmetic, sometimes it helps with some frequency response, off-axis response, power compression...really depends on the driver, engineers likely try a driver with both and see what measures best.

IMO, I think most dustcaps are not optimal. They are attached to the cone, when their best location would be to attach directly to the voice coil, such as many Fostex drivers, MarkAudio, and Eminence beta8 are done.

I suppose the size of the dust cap will also affect the driver performance, speaker design is made of compromises, what you see is usually a combination of what the marketing department thinks looks pretty, and what the engineering department thinks performs well.

Maybe I am just talking out of my A...who knows.

[Moondog55]

A quick re-read of the "Cook-Book" tells me that the HF is radiated from the centre of the driver and that the dust cover can have a definite effect on the quality of the higher frequencies.

Vifa used to use a soft dust cover on small woofers and mid-bass drivers and a rough textured coating on their dedicated mid-ranges, M-11 comes to mind

[Tyson]

So dust caps are cosmetic and tend to be an engineering compromise? It would seem that the Scan Speak would not use one, if that were the case, since it performs so well in tests. Also, if you're gonna plop something into the center of the driver, wouldn't a phase plug be a much better aesthetic solution, which also has better engineering payoff too?

[Andrew Eckhardt]

The inverted dustcap thing is pretty rarely an improvement for wideband drivers. In that case I think it's usually done more for the "Ooo neato!" effect rather than better response. Inverted can reinforce the cone over a wider gluing interface, maybe why it's seen a lot on higher power woofers.

Phase plug forfeits the cone reinforcement and the coil cooling pump effect of the dustcap, again not a real good tradeoff for very high output.

[gedlee]

Originally Posted by Tyson

Why do some woofers have a fully concave inner surface, ala the Peerless Nomex woofers that I have:

While others have a convex inner surface, a "bubble" in the center, ala the Scan Speak Revelator midwoofer?

Speakers of this genre are often purchased based on looks, so looking different is a selling point.

TAD drivers are the best looking speakers that I have ever seen, but they perform the same as all the rest. They just look a lot better. The looks net an additional $2000 per driver in some cases. Worth doing I suppose.

[DcibeL]

Earl, looks are a more important factor in this "genre" when compared to high sensitivity pro-audio drivers, but not the sole reason to purchase, that's for sure.

Tyson, I think you misunderstood me. The dust cap itself is not an engineering compromise nor purely cosmetic, but the entire speaker is a combination of compromises. The choice of the dust cap is one of which to balance the compromises to create a good performing loudspeaker. Because speakers are made by companies, part of the design choice is undoubtedly a combination of cosmetic appearance as well as acoustic performance.

Phase plugs are along the same lines, however a phase plug attached directly to the pole piece will affect the motor, so part of the choice to use or not to use a phase plug may come from the motor design, among other things.

Peerless exclusive speakers have a phase plug, but they did not completely remove the dust cap, just put a hole in it. There are a couple possible reasons for this:

-removing the dust cap affected frequency response in an unwanted manner
-cosmetic consistency between models of the same product line
-added mass necessary to obtain desired T/S parameters

Who knows, the voice coil former comes up to the dust cap on these speakers, if the dust cap were not there, the voice coil former could be made shorter, reducing weight further. Maybe something to do with the moving mass, or power handling...Lots of possibilities for the choices made that resulted in the final product.

[tvrgeek]

I would think the concave cap would require a longer VC former down to the motor. Extra mass and move the acoustic center back further. Can't see an advantage. I do happen to like both drivers shown.

[CLS]

I think some of the larger-than-VC dust caps have some degree of structure enhancement to the cones. Such a cap tends to be solid materials and shaped flatter. On where it joint the cone (somewhere between VC and surround) makes another support to fight against the distortion of the cone shape. I guess the overall stiffness contributed by this would probably larger than thickening the cone itself.

This should be another effective factor to 'tune' the breakup behavior.

Ref: Eminence Lab12, 18sound 6ND430 ....

[woody]

The inverted dust cap may limit high frequency responce. The reason I say that is I rember looking at the frequency responce of a 7" Dynaudio driver with an inverted dust cap ( realy the cone and dust cap were one piece) and a Focal tweeter with
an inverted dome. The Dynaudio was fine to about 2.8k then started a verry steep
roaloff the Focal tweeter was fine to about 18k then also started a verry steep roaloff. In both cases the roaloff looked steeper than 24db/oct.