Speaker fart mode

Mitchell_Cote

I understand distortion from a clipping amp but all I know is that divers can "fall apart" when the volume is turned up.  Especially cheap drivers with simple motors.  Can somebody explain what exactly is happening when a driver goes into fart mode.

Thanks

jr@mac

Extremely high odd-order distortion as the voice coil is leaving the gap, and sometimes as the voice coil hits the back plate, and sometimes as turbulence from too-little voice coil venting, sometimes thermal compression. All are generally related to excursion demands. In many cases, vented enclosure tuning is a culprit. A lot of drivers model well, but due to various limitations the excursion demands exceed their capabilities to perform well. I have found nothing to absolutely correlate that against, unfortunately, as drivers across all price platforms exhibit seemingly unpredictable behavior in a vented alignment. Extremely expensive Morel drivers are unable to handle even moderate SPL in a vented alignment, and a dirt-cheap MCM 5" can be cranked six ways from Tuesday, sooo... who knows.

Any particular driver giving you problems?

Mitchell_Cote

Well explained JR, thanks.  

I did a quick and dirty woofer swap in my 2000 GMC 1500 so there are may problems with the install right of the bat. As well as getting them clean power.  I may clean it up our just wait and do it correctly with good divers and subs I have on hand.  This summer if time permits as the truck is being used by a friend that is 4 hours away. 
This is the woofer I used.
https://www.parts-express.com/tc-6024-6-1-2-treated-paper-cone-woofer-with-foam-surround-4-ohm--299-2196

jr@mac

Yeah - that driver may not be high Q enough for a very large, lossy enclosure like an automotive install offers. May be able to clean it up considerably with a 300uF capacitor in series with it, though. 

Mzisserson

I have observed driver flattus to be the result of softer cone materials like paper and poly flexing as it is being tugged on by the surround anxiously trying to keep the coil in the gap. 

Johnny touched on a couple of good points, too. Inadiquate venting and porous dust capa cam add up to quite a bit of audible fart sound when a driver is pushed. 

I have also found harder materials such as Al cones to have less far, but when they are pushed beyond their limits its not a gentile push, rather BAM! You found the limit! Or Accutons let you know by simply shattering! 

It is strange. Cost has zero correlation to how well a speaker does when pushed. Scans and Seas tend to behave best, I have found. Goldwood and MCM also lead the pack in terms of going gently into the night. 

Some of the worstI have encountered: SB, Morel, Dayton (eapically the RS and DS line)...


Bare in mind we are talking extremes. Many drivers work just dandy when pushed a little. Not that many can keep their composure and seemingly "self protect" when pushed beyond 90%

Mitchell_Cote

Thanks for the input gents.  I was thinking of an LR high pass at 60 Hz but I like your idea of a 300uf cap better JR. I'll go with that.