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speaker dust cap: size, shape and the effect thereof

OldogNewTrick

OldogNewTrick

Dec 28, 2004
1,816
1,237
Germany, EU
#1
I have noticed a lot of different designs, sizes, shapes of dust caps on speakers.
From a teeny, weeny, flat lil paper covering the voice coil former innards on some vintage speakers to the more modern type "round" cone (some small, some large to cover most of the actual cone) to flat ones for sub woofers. Different materials from paper to aluminum.


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A lot of the mid range projection / dispersion / range is attributed to the cone design.

We have discussed the influence of magnet material on driver performance, but never cone design.


Please enlighten me :cool:
 
#2
A dustcap can serve as a separate midrange/high frequency radiator. Smaller lighter caps extend response higher, but also will result in a response hole between the low frequency output of the cone and the high frequency output of the dome. Medium size/mass caps won't go as high but give smoother broadband response. Large caps as shown in the EA are seen on pure woofers and do not extend response higher. With very large/heavy caps as shown in the lowermost picture the high cap mass lowers the driver resonance, usually for subwoofer use.
 
#3
billfitzmaurice said:
A dustcap can serve as a separate midrange/high frequency radiator. Smaller lighter caps extend response higher, but also will result in a response hole between the low frequency output of the cone and the high frequency output of the dome. Medium size/mass caps won't go as high but give smoother broadband response. Large caps as shown in the EA are seen on pure woofers and do not extend response higher. With very large/heavy caps as shown in the lowermost picture the high cap mass lowers the driver resonance, usually for subwoofer use.
Click to expand...


word ! :cool:

uhh, ahhh... end of discussion :smug:
 
#4
Almost.....

There are large caps that are "hard" and paradoxically don't do much extension.... they are there mostly to make it look like the voice coil is huge.

There are also large caps which are somewhat transparent to sound, and those allow the lower portion of the cone to have significant effect, while still having the desired marketing effect.

A cap which is the same size as the coil former, combined with a shallow cone, will have the most useful response extension effect. Example would be the older Peavey speakers with (IIRC) one-piece former and cap. I think those were only on the replaceable basket units.... you'd never be able to aline the coil correctly unless it was done from behind with a fixture. the magnet and one-piece cap would be in the way otherwise.
 
#6
Fuzzbass said:
I used to love the sound of metal-capped fifteens, because they seemed brighter and edgier than paper-capped ones. Was that true or was I listening with my eyes?
Click to expand...
Bill was just talking about the old Altecs with metal caps being much brighter than anything else so apparently there's something to it. I once asked him about if that made a difference, and he said that it was more related to the stiffness of the material than anything.
 
#7
Yes those and the JBL 120/130/140 are brighter but as Bill also said they only have this directly on axis in a very narrow beam. Works well for a mic in single speaker recording though.
Maybe we could extend this thread to discuss the effect of any cone striations, the embossed radial lines that allow the flexibility for the center of the cone slightly more freedom to move independently.
Then add in discussion of the effects of wizzers those little sub cones that you see on some drivers.
Altec also made a speaker called a duplex which had a single striation in
the cone providing a radial stiff suspension line at just under half cone diameter. Not much power handling but might be interesting for lower level recording.:bassist:
 
#8
I believe a key part of how the dustcap behaves is how it is fixed to the voicecoil and then how the cone is fixed to both. If you have the dustcap firmly on the voicecoil but then a looser join between cone and voicecoil and cone and dustcap you end up with a mechanical crossover (which suffers similar challenges to electrical crossovers regarding phase) so the voicecoil perceives a smaller moving mass at higer frequencies, which then radiate from the dustcap. Whizzer cones make significant use of this. However it should be noted that unless you keep the voicecoil inductance low enough the increased impedance at higher frequencies will reduce the treble output. There's also the whole self-damping and bell modes thing, hence aluminium dustcaps adding quite a lot of spikey treble.

Alex
 
#10
Bassmec said:
Yes those and the JBL 120/130/140 are brighter
Click to expand...
JBLs with aluminum domes had higher extension than those without that used the same motor, as in the case of E120 versus 2206, E140 versus 2226 etc., though the dome wasn't the only part of the equation. That said, JBL HF extension was meager compared to aluminum domed Altecs, and also paled compared to paper dome EVs. OTOH cheaper Utahs and the like with aluminum domes had nowhere near the HF extension of JBLs. So while the dome size and material does have an effect, response is determined by a number of factors, not just one.
 
#11
I have a JBL E-155 18" with a chrome dome. It measured out surprisingly high, up in the KHz, on-axis.

I bought a dome replacement kit to replace the dented one. On the E155, the dome is just outside the voice coil diameter. This is common on all the E-series drivers. My E110 drivers have a smaller coil, but their dome is also coil sized.
 
#12
Just for grins, Ampeg had a custom Eminence cast-frame 15 at one time (now surely discontinued)....

That 15 had a fairly wide dispersion and decently in-phase response up to 5 kHz, most of which, IIRC, was due to the cap and the fact that the cap and coil were pretty much matched for size. The above is based on TEF data, which we had because I also used that same 15" on some 2-way PA speakers, such as the Audio-Centron ACE-1 and others.

That was pretty darn good response from a 15, especially considering that on serious PA speakers, such as the Audio-Centron CE-34, we limited 10" speakers to under 1kHz due to beaming.
 
#13
There's also the effect of acousticly open materials like linen and felts. These tend to be both brighter and smoother in the midrange.

It's the combination of many variables and tradeoffs, including size, geometry, stiffness, mass, acoustical transparency, relationship to VC bobbin attachment, etc.
 

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