Driver dustcaps.

[Chrisbee]

Hi

I've seen every possible variation of dustcaps and cones on speakers and sub drivers over the years.

From the standard convex dustcap, through flat to inverted bowls. Little convex dustcaps seem to have given way to ever larger domes. The fad now is for smooth bowl shapes.

The standard speaker cone is hollow on one side and a solid cone on the other. So it is a highly assymetrical arrangement for pushing air around anyway. I know the cone is made that shape for stiffness but the back of the cone is rarely seen by the listener.

Does a bowl move more air than a flat plate? The back of the cone will never change much however one tries to do to modify the dustcap shape.

Does the shape of the dustcap and its effect on the cone's visible form really have any effect on a driver's sound or performance? Can it be heard or measured? Or is it just a sales gimmick to make the humble dustcap look more sexy?

[drf]

is that a can of worms I hear being opened? I have heard many arguments for and angainst most styles of dust cap. To be honest I haven't seen measurements that prove one is better than another but I will accept it if they are out there. Some of the theories I have heard are:

1) convex dust caps cause turbulent distorton as the convex DC and the concave cone shift air differently.

2) dust caps should be vented to prevent pressure and excessive damping between the inside of the VC and the pole piece.

3) dust caps should be a straight cork plug from the pole piece out to prevent harmonic distortion.

4) dust caps should be concave inline symetry with the cone to prevent intermodulation distortion.

5) dust caps should never be aluminium as this causes early breakup.

personally I think its a visual thing and they all sound about the same.

[russ.will]

I've always felt that there is something right about no dust cap. No real reason, but it seems inherently right to do away with moving mass that can be surplanted by a well designed grill covering the entire driver.

Lets face it, any number of drivers with dust caps have vented pole pieces so dust is getting in one way or another, so as far as I can tell, its mostly about cosmetics.

The only time I can see it making sense is when you have a truly continuous profile convex cone like that used in the PB-13. The rigidity of the diaphragm would be compromised by the discontinuity of cutting it. Or is it just an oversized inverted dustcap for the cone behind?

I'm still not sure what the point of that ruddy great thing is in the middle of B&W drivers though?

Russell

[jwhite8086]

What kind of dustcaps are these
http://www.cerwinvegamobile.com/strokerpro15.php

[russ.will]

Oo! Err? Front and rear spiders AND a dustcap?

I'm not sure about much with that one, but in the UK, we might describe it's looks as "a tarts handbag".

Russell

[phaseshift]

The shape, mass, material and method of adhesion of dust caps all have an impact on the overall output and response characteristics of a driver.

Is there a best? Probably not because your best may be another mans's worst; depends on intended application and desired result.

One of the bigger issues in dealing with dust caps is the overall cone breakup modes. This ends up having an effect on your response as there will be parts of the cone assembly moving out of phase with other parts of the cone assembly. You can model this fairly well with a couple of FEA platforms and a few of the speaker specific modeling programs. You can also measure it directly using laser scanning of the cone, but that is tricky and only a few people are making systems which are accurate; Wolfgang Klipel's system being at the top of that game for now.

The adhesive and attachment method also have an effect on the performance as does the stiffness of the cap. Some designers like a very stiff cap, others a very compliant one- soft rubber etc… Again, it is design preference and intended use that dictact these- on top of that, you have designer preference as there may be more than one way to accomplish the goal.

How big of an effect does the dust cap have? Anywhere from minor to significant; again, depending n the design. Next time I get a chance and think of it, I will build and measure the same speaker with the same dust cap- one measurement with the dust cap on in typical convex style (dome = outwards) and then invert the thing to a concave attachment (dome = inwards). Obviously, that would be only one situation, but it may be better than trying to explain the how, where and so on.....

Speaker with no dust cap- There are tons of them. Coaxial speakers have no dust cap and many work quite well. However, if you take the coaxially mounted tweeter off the speaker, you can measure significant differences that are not within the tweeter bandwidth (of course, no tweeter / no signal at those frequencies, but you can also see different issues; typically changes in the modes at or near the wavelength equal to the VC diameter. Some of this is strictly acoustics- the tweeter post does effect the response and having a big old hole where that was does impact it as well. Some of it is thought to be related to the near filed response and phase changes from refraction. Of course there is some air leakage as well, but this is probably not as significant in this case as the other points.

[Hi Ho]

What about phase plugs? The Dayton RS drivers I used to build my speakers have phase plugs and I have always admired their sound. Why aren't phase plugs used on Subs?

[phaseshift]

Hi Ho wrote:

What about phase plugs? The Dayton RS drivers I used to build my speakers have phase plugs and I have always admired their sound. Why aren't phase plugs used on Subs?

Many reasons, but wavelength being the primary one....

[Hi Ho]

So are you saying phase plugs don't live up to the hype in any speaker or they simply aren't practical in a subwoofer application.

[drf]

Although some would argue there is too much hype surrounding phase plugs, I would say that for subs and any low freq (with respect to cone diametre) a phase plug will do nothing. Phase plugs seem to only be of benefit at the point when a cone would naturally start to breakup.

Like all designs features of a driver, it is compromise between one attribute and another.