Dust Caps: “The Deal on Dust Caps” by Anthony Lucas
Anthony Lucas is the head of Technical Support at Eminence speakers. I sent him over some questions about the dust cap you see in the center of a typical guitar amp speaker. I had an idea that they helped shape the sound to a certain degree but did not realize how much of affect they had. Anthony dishes all the dirt in this interview I entitled “The Deal on Dust Caps”.
300guitars: Hi Anthony. First let’s start off with you explaining what the dust cap on a guitar amp speaker is.
Anthony Lucas: Hi Billy. Thanks for having me again. I’m glad you picked this topic. I’m not sure people realize just how important this design feature is. The dust cap is the round, often dome shaped, component in the center of the cone that covers the voice coil and magnetic gap. A lot of people incorrectly assume that the diameter of the dust cap represents the voice coil diameter, but that is rarely the case.
300guitars: What mechanical purpose does it serve?
Anthony Lucas: The dust cap’s main purpose is to keep dust and debris out of the magnetic gap. Even the smallest particle getting into the magnetic gap can create problems.
300guitars: How many different materials are used for the dust cap?
Anthony Lucas: Eminence uses cloth, paper, felt, and a few aluminum dust caps for guitar speakers. Aluminum was more commonly used on older/vintage designs. It’s tough to work with because it dents very easily!
300guitars: Explain how each different material affects the sound.
Anthony Lucas: What people might not know about dust caps, in general, is that they color/shape the upper-mids and highs. Each material has a different effect because they vary in stiffness and porosity. Designers can use this to their advantage to help voice an associated frequency range.
When we use cloth, it is generally just a screen type of dust cap…what we call a Zurette. It is relatively neutral, meaning it colors the sound very little, if any. You hear more of the signature response of the cone with this type of dust cap.
Paper is harder and stiffer and can add peaks and extension to the highs.
Felt is used to tame a bright, peaky cone response. The result is warmer, smoother, more mellow upper-mids and highs. I think it is the most coloring material.
Aluminum is even stiffer than paper caps and adds significant top end extension. The disadvantage is that it’s also heavier. The added mass lowers SPL. Aluminum lends a unique quality to the highs that you will either love the detail it adds or hate because it’s too harsh. I like to refer to it as a “jangly” top end. While I have seen/heard instances where it’s worked out well with the right amp and cab, it is generally not a good material for hi-gain applications. It just gets strange and messy…..sounds like your tone just crashed into something that didn’t give.
300guitars: Does size matter? How does the size affect the sound?
Anthony Lucas: I hope we’re still talking about dust caps! Yes, the size of the dust cap matters. The size determines where the dust cap will be attached to the cone. Different frequencies are generated from different areas of the cone….fundamental frequencies and harmonics. When you attach something to a specific spot on the cone, you are adding rigidity and mass to that area. Most guitar cones are very thin and lightweight, so a little change can have a big impact. It can affect the way the cone responds to the signal (transient response, linear motion) and to some degree, the break up characteristics. If you watch a guitar speaker under a strobe light, you will see that the cone is excited in all different directions, especially the more you push it. It’s not just a linear, up and down movement like you might expect.
Some speaker designs use a dust cap that is installed on the voice coil former rather than attached to the cone. We call this a “pop-on” dust cap and we will use it to smooth out the response in the 2kHz-5kHz frequency range. Yeah, everybody’s thinking….”Smooth out the top end response!?!?! Where you get those pop-on dust caps????” I can see all the questions on how to change a dust cap coming in now, and of course, the “how do I repair my speaker that I damaged trying to modify the dust cap” ones. Well, it’s not usually that desirable to do this type of “smoothing” on a guitar speaker because the peaks and dips often create more detail and interest sonically.
The shape of the cap is another factor. It affects the way the speaker projects the frequencies from the center of the cone. You’ll notice that some dust caps are more bulbous or more conical than others. The smaller, more conical ones seem to add more “sizzle” to the top end.
300guitars: How is the dust cap held onto the cone?
Anthony Lucas: Most dust caps have a lip, which is used to glue it to the cone. Ones without lips are centered and a bead of adhesive is applied around the perimeter of the dust cap, adhering it to the cone.
300guitars: Do you ever recommend “beam blocking” types of diffusers for the dust cap?
Anthony Lucas: I have personally never tried one…..I have the resources to easily change the dust cap if I don’t like the highs…haha. I do hear good things about them and that they’re effective at curing harshness on the highs.
300guitars: Thanks again for taking the time for another insightful interview about speakers Anthony..!!!
Anthony Lucas: Thank you, Billy….always a pleasure!