What Happens If I Remove The Dustcap? Is My Speaker D-E-A-D?

[Patrick Bateman]

If I remove the dustcap from a speaker, is the speaker ruined?

The reason that I ask is that I am thinking about trying to DIY some coincident coaxial speakers.

I recently listened to a set of the KEF coincident speakers at Speakerlab in Seattle, and the KEFs do a *lot* of the things that I like so much about the Unity horns which I adore. The KEFs have that incredible ability to articulate sounds in the midrange. I've also noticed with Unity horns that there's an ability to hear details in recordings which are 'smeared' by everything else. It's not an abundance of detail, but a *coherence* which stems from the very very very tight alignment of midrange and tweeter. Basically, once you get the midrange and the tweeter in sync in the time domain, it fixes a whole ton of problems.

I nearly bought them, but the KEF wasn't perfect; it doesn't 'disappear' the way that my Summas do, but the midrange was world-class. (I think the KEF could use a better cabinet.) There's a place in Florida that sells the woofers but they don't have them in stock and I'm impatient. I looked at the Seas coincident coaxial, but at $300 a pair, it's a bit pricey and the efficiency is poor. I'd buy a set of the Insignia coaxials if there were still for sale. (Just discovered them on Zaph's site.)

Anyways, back to the question - if I cut the dustcap off a woofer, is it curtains? I concede that this will change the Thiele Small parameters, and it will alter the frequency response. Both of those things can be dealt with. But will cutting a hole in the cone allow air from the FRONT of the cone to enter the box via the throat of the woofer? I'm thinking the answer is "no" if the voice coil is not vented.

But I've never done this, so I am only guessing.


Picture this, but with me adding a tweeter to the voice coil. (not my pic)

[a_tewinkel]

I have no experience with this, but here's a DIY coax made from a 5" Monacor woofer with nice pics: click (Dutch, by the way). As you can see this guy's made a cap to mount the tweeter.

Maybe you could close it off if the pole piece is vented, like with the plastic cap on the pictures? On the other hand a vented pole piece might be suitable for pulling through some wires for the tweeter.

[weltersys]

In the bad old days I saw and heard many JBL D-120, D-130, D140 that had the aluminum dustcaps "blown off", they sounded OK without that pinging bit.

Never listened close enough to hear if they were chuffing though.

I did cut the dustcap off a damaged EVX-150 X, which has a hole clear through to the back plate, could not hear chuffing over the sound of the Kapton blisters dragging...

A_tewinkel suggestion is good, if chuffing was a problem the vent hole could be closed.

Art

[Altec Best]

If you close off the vent you can overheat the voice coils. Not a good idea IMHO.Woofers need to be vented either front or back.

[John Sheerin]

Air can potentially enter the box by going past the coil ID and back up past the coil OD and through the spider (or exit in reverse). However that's probably not that big a deal - the clearances are small, so it would probably just change the Q of the box a bit by creating more leakage. As mentioned, you will be changing the thermal design of the motor. The dust cap won't pump air past the coil anymore. If you keep power dissipation low enough, that should not be an issue. If the motor was not well designed to begin with, you could potentially improve things...

[Radugazon]

I did this surgery on many occasions, the results are variable.

On big mid bass, you can eventually see the disappearance of a nasty peak in the end of the range, obviously a cavity resonance. This improvement can be heard when running a sweep, and only becomes interesting if the driver is used close to its upper limits. No other consequences. It's sometimes a positive trick.

On small FR, you can have a more annoying disappearance : the Hf are gone. Negative.

With vented poles pieces, I have never seen an influence on the low range, but this on OB where there is already no bass and no pressure for generating aerodynamic noises. I guess it would be very problematic with sealed or ported baffles.

What kind of tweeter are you planning to use ? In front, like the dutch link, or behind the magnet with a kind of horn ? If running passive, this last can help to compensate a phase shift. Last word, if you use the proper solvents for getting rid of the glue (normally cyano acrylate are not used here), it's possible to make something clean and put back the dustcap later, even with paper cones.

[simon7000]

You will need to add a skirt from the tweeter edge to the woofer cone to keep out dust. There is a very big magnet back there and any magnetic dust will find it a nice place to jam up your voice coil.

Yes you will change parameters a bit, but you can always replace the dust cap if you don't like the results. They are a fairly standard part as is the speaker glue.

[AllenB]

I ran some plots of this process but I can't seem to find them right now. My experiences were similar to Radugazon. I replaced the dustcap with some phase plugs and I have no complaints.

[Patrick Bateman]

Originally Posted by Radugazon

I did this surgery on many occasions, the results are variable.

On big mid bass, you can eventually see the disappearance of a nasty peak in the end of the range, obviously a cavity resonance. This improvement can be heard when running a sweep, and only becomes interesting if the driver is used close to its upper limits. No other consequences. It's sometimes a positive trick.

On small FR, you can have a more annoying disappearance : the Hf are gone. Negative.

With vented poles pieces, I have never seen an influence on the low range, but this on OB where there is already no bass and no pressure for generating aerodynamic noises. I guess it would be very problematic with sealed or ported baffles.

What kind of tweeter are you planning to use ? In front, like the dutch link, or behind the magnet with a kind of horn ? If running passive, this last can help to compensate a phase shift. Last word, if you use the proper solvents for getting rid of the glue (normally cyano acrylate are not used here), it's possible to make something clean and put back the dustcap later, even with paper cones.

Basically something like the Seas H1260 which costs $280 a pair:



Using something like the SB Acoustics SB15NRXC30-4 which costs $112 a pair:


And something like Seas 27TAFNC/G which costs $66 a pair:


The more I look at the tolerances, the more I wonder if I should just find a used set of Kef's with the UNI-Q driver on Ebay though. Getting that tweeter into the voice coil will be tight, and saving $160 might not be worth the risk of trashing the woofers to do the surgery. Hmmm.

[planet10]

I've done quite a few dustcapectomies...



Mostly on full ranges with whizzer cones. On the (unfortunately discontinued) Pioneer B20, a small tweeter mountsd on a cylondrical (1 3/8" dowel) was a successful experiemnt.

As per Radugazon's comment on small FRs i agree, but can be a fix for a pushed in dustcap.

Pwesonally i have moved to what i call co-incedent drivers -- moving the XO low enuff that all the driver are within a 1/4 wl at the XO,

dave

dave