cone breakup

[crissty]

What is cone breakup, and what type of cones have what breakups.
Can somebody help.

[Mr Evil]

At low frequencies a cone moves as a whole. This is the 'pistonic' area of operation. At higher frequencies the cone starts to flex, leading to resonances. This is what is referred to as 'breakup'.

These resonances are at fixed frequencies and are thus not harmonically related to the input signal. Non-harmonic distortions like this sound bad, and so should be avoided like the plague.

Breakup behaviour depends on cone material and geometry. Paper has very well damped breakup modes, which is one reason why it's such a commonly used material. More rigid materials, like metal, have very bad breakup modes, but fortunately they tend to be higher in frequency and so can be avoided with care.

There are ways to control breakup modes by treating the cone with various coatings.

[crissty]

Ok..
How do we get the breakup modes of a particular speaker, is there any particular software, testing method for it.

[crissty]

OH.... And more stiffer the cone, the better, as it will push the breakup mode further.

[rjb]

Its rather more complex than that. All cones breakup, in fact manufactures like Jorden build this in to their cones to get wider frequency response. So there is "bad" break-up and "good" breakup.
"Bad" breakup is where strong resonance is triggered which produces an output is not necessarily related to the sound desired.
As above, paper has good internal damping so it is easier to control resonances, and acheive a smooth sound over a reasonably wide frequency range. For these crossovers can be simpler. In general rigid materials produce resonances at higher frequencies than softer materials, but much worse in magnitude, such as the Seas metal cones. For these more complex crossovers are needed, to limit their ouput to the pistonic range.

However breakup can be controlled using rigid materials as well. as Jorden has shown. He manages to make his cones pistonic at low frequencies, but flex in a smoothly controlled manner to effectively reduce cone area at higher frequencies, producing a wide frequency range. (This is very much simplified, so please don"t jump on me- go read his book).

Opinions on cone material are just that, much depends on the designer and the intended purpose. So you cannot simply say sofy bad, hard good.

I am not aware of any software that measures breakup. An experienced person can see indications in the freqence response and impedence curves that will cause him to suspect breakup. There are a number of good speaker designers on the net that discuss this in relation to their enclosure and crossover designs

KEF did a lot of work using visual interference patterns and lasers that if you are interested is worth reading. B&W have also done more since. Not really for the DIYer.

[John Sheerin]

Originally posted by crissty
Ok..
How do we get the breakup modes of a particular speaker, is there any particular software, testing method for it.

You can model the breakup of a cone with FEA, but this will only be as accurate as your material properties are, among other things. This kind of tool isn't available to most people anyway.

If the speaker has undamped breakup modes (ie, with large amplitude), you can run the speaker under a stroboscope and watch the cone surface move.

Or you can whip out your laser vibrometer and measure the velocity of the cone surface at a bunch of different points to generate a map of the cone. There's an AES preprint showing this on the JBL Pro website. It's 'High frequency components for high output articulated line arrays' by Doug Button if you're interested. See p.27.

[rjb]

I forgot to mention that waterfall curves are useful in identifying major resonances.
For a DIYer these are probably the easiest and simplest tool to identify the frequency, but not the location of the resonance. With experience you can make an intelligent guess, and hence trial a possible solution.
MarkMcK has posted a lot of information in this forum about improving small TB drivers, either with glue rings or physical distortions which is worth reading.
VAF is an Australian manufacturer that damps some cone resonances with felt pads, (which also add mass as well).

[Salas]

In a plain vanila frequency response chart, damped breakups usually manifest themselfs as a series of horizontal S type smallish ripples. In a metal cone you get a hard peak or a twin peaks instead. Its always there where you see long tails on the related waterfall.