What really happens to speaker cones...?

more than a miniscule amount of flex would make a cone fall apart. remember most are paper glued to the dust cap and surround.

ideally and hypothetically - there is no flex.

somebody else probably knows more on how much they flex.

 

After Duke's more thorough post I thought I should clarify that what I meant here was seeing the actual cone-in-motion stuff, and not seeing the graphs that he's kindly linked.

 

Cones do indeed flex at higher frequencies, with the outer edge of the cone moving out-of-phase with the central region. This is called "breakup", and it results in the upper frequencies extending higher and dispersing wider than they otherwise would have. The downside is that the response tends to be quite rough in the breakup region, and usually you have at least one large response peak. Depending on the application, that response peak can be a good thing or a bad thing.

You can often see evidence of cone breakup in a frequency response curve. Here's an example:

http://www.eminence.com/pdf/legend-bp102.pdf

The BP102 starts rolling off at about 300 Hz, but then starting about 600 Hz the cone goes into breakup mode which extends the top end by another octave and a half.

Here's a spec sheet on a 12" Selenium woofer that includes polar plots on the second page. Rigid piston theory would predict that the pattern would get narrower and narrower as we go up in frequency, but here we see the pattern hardly narrows at all between 800 Hz and 2 kHz:

http://www.parts-express.com/pdf/264-334.pdf

In contrast, here you can see the radiation pattern of a 12" woofer with a more rigid cone. The on-axis response is quite a bit smoother, but note in particular how much narrower the pattern is at 2 kHz:

http://www.parts-express.com/pdf/264-370.pdf

Much of the woofer designer's art goes into juggling cone breakup and other tradeoffs to get the best compromise for a particular application.

 

Wow. Blew off my original post there ; }

 

In the world of hi-fi, some companies have equipment that uses lasers to measure the movement of the cone, at all the different parts of it. There are a few images out there that have been used in advertising.

 

Serious pro designers also use laser accelerometers to analyze cone movement.

 

Billoetjen, cone breakup in and of itself is never good, but all cone breakup is not created equal.

If it's not causing severe peaks and dips, chances are breakup is not of great audible significance. In the world of high-end home audio I deal with fullrange electrostatic loudspeakers, which are legendary for their retrieval of low-level detail and nuance, and whose flimsy saran-wrap diaphragms are always in low-level breakup.

In an ideal situation, yes we would eliminate breakup by crossing over well below the point where cone flexture sets in. In practice, it's more realistic to aim for crossing over below the point where breakup is a serious problem. Intuitively even a little cone breakup seems bad because it means we're not replicating the input waveform, but replicating waveforms isn't really critical to the human hearing mechanism.

Now it just so happens that one of the people who posted in this thread has designed bass cabs which cross over from woofer to midrange at a low enough frequency to avoid or at least minimize the woofer's contribution in its breakup region. I'm speaking of course of greenboy's magnificent fEarful cabs. In my opinion their primary advantage is their much improved ratiation pattern, but the midrange takes over well before woofer breakup becomes severe and so neatly and elegantly sidesteps that issue.

 

A surprising amount of the signature tone of a bass cab is from the break-up nodes. A really blatant example is that of the Hartke aluminium cone speakers which have very loud peaks due to the lack of self-damping. Another interesting example is the very undamped cone of the coax whizzer cone on a Wizzy 10 or 12. When you get into the world of hi-fi you spend your time avoiding them as much as possible and it's very important to design a crossover that minimises their colourations but in my bass cabs I wouldn't be without their beneficial effects.

Alex

 

You really can't avoid breakup modes, you can only pick for the ones that serve you best, and make use of them in the best fashion for your purpose.

Very simple example: some single-cone enclosures have breakup that actually aids in off-axis response as compared to ones using drivers that are very shrill and don't spread that response but an iota.

 

Thanks for the polite corrections, Alex and greenboy! I went too far in saying that breakup "in and of itself is never a good thing"; it definitely has significant benefits especially in a musical instrument speaker application. And if breakup benefits bass cabs, then where in the world would guitar cabs be without cone breakup??

And by the way now that Alex has joined us here, we have two participants who have designed bass cabs which neatly sidestep nasty woofer breakups by going to a high quality midrange driver.

 

Terrific thread, guys! Very informative.

 

Yes, in home hi-fi the ideal is usually to operate the drivers in their pistonic ranges as much as possible. That's less practical in pro-sound or instruments, because it means you have to use a smaller driver.

You can tell to some extent by looking at the graph of the driver's impedance curve. A driver in its pistonic range will have a nice, smooth curve. Ripples in the curve indicate a resonance of some type, although I don't know if it's always a breakup mode from the cone. Stereophile magazine prints the impedance curve when they review a hi-fi speaker, because a ripple in it is a hint of some resonance, whether it's cone breakup or often a resonance of the speaker box.

 

Like this 'cept different.

 

Yep, like this. The FR and the impedance plot kinda show a relationship as the first ripples show up where serious breakup is just getting started. Em sure smooths their FR plots but that makes it all the more apparent how big the major peaks and chasms are.

 

Nice visualization demo, and no wonder we are mystical about "organized sonics".

 

Here's another example. Ignore the underlay of the heavily smoothed Em 3012LF (the lower, thin line starting at the left side of the chart). Instead, look at the thick line (and the thin line that diverges from it at around 750 Hz) which represents a 6.5" 18 Sound 6ND410 midrange driver' FR. The 18 Sound plot has not been smoothed (locally averaged) nearly as much.

The thin diverging line starting at 750 Hz is the FR at 45° off-axis. Notice that it follows pretty nicely the main plot until at around 2.2 to 2.5 K Hz it drops to a level about 6 dB down from the on-axis response, until 3K Hz where it begins to plummet more rapidly, with a characteristic jag or two along the way. Then look above the 2.5 to 3K plateau after the first drop, and the rapid plummet, and you see a couple of spikes in the main response. Pretty indicative of breakup.

The 18 Sound mid cone really is designed well. It has very flat passband for such a powerful driver, and excellent off-axis response right into the end of pistonic beaming limit and beyond, and it does a marvelous job on axis, of controlling the breakup without real big peak/chasm issues.