If Xmax shouldn't be exceeded, why have a Xlim?

I'm going to throw in my baseless opinion here.

As you drive air harder, it's elasticity and thermal compliance and all the physics stuff becomes relevant. An absolutely perfect pistonal speaker is still going to distort at high volume because it's going to get mounted in a real space, not a theoretical anechoic one, and cos that's the nature of air and ultimately, eardrums.

Yes it's distortion, but it's not hard high order clipping, it bears more similarity to power compression. I'd much rather hear a dynamic system that exceeds xmax on the transients than one that used outboard processing to bring everything below xmax at the same rms level.

EDIT - like above poster, i mean this applies to subs. Sounds rough on top where linearity properly matters.


Edited by greeef - 25 January 2011 at 9:38pm

From what I understand from all the above, driving a sub past xmax has more of a compression than distortion effect - like an analog tape deck. Obviously within reasonable power levels!
I've produced/written dance music for a good few years now and the best method I've found for making basslines sound Phat (with the capital P!) Is by adding a little soft tube compression. In planning for and designing subs I have always worked within xmax figures but now I think my mind has changed, maybe an extra couple of mm will add some warmth without necessarily destroying equipment???
^^^^all for sub and nowt else though!

This all comes down to how much you want to hear distortion and how much equipment you want to cook.  Good judgment comes from experience, and experience comes from bad judgment.  You can exceed your xmax at the expense of some driver distortion during the musical transients.  Since they're transients, you can musically get away with considerable driver distortion, so long as the amplifiers don't clip.  That's another kettle of fish.  Eventually, exceeding xmax far enough, for long enough, will rip apart the surround or burn the voice coil, and after a recone job, you will have a new feeling for how much you can exceed those limits.  Since I go entirely with vented bass cabinets, the mechanical limits in that 1/3rd octave above Fb get to me first.  I try to stay in the 6-10dB-down range for running power at the amplifiers.  It seems to be a good trade-off between ruined voice coils and torn surrounds.  As a side note, I have never had a spider fail.  In the mids/tops, I see thermal limits first.  Got another lesson on that recently.  The photo's I see of burned voice coils mostly show that some part of the coil was out of the gap.  Some of them suggest poor reconing techniques, meaning the voice coil wasn't centered magnetically during the recone job.

The nice thing about a large xlim is having the ability to survive some monstrous transient, like when a mike gets dropped or a signal processor gets accidentally turned off.  If the driver can take that transient physical abuse, great.  You made it through another day.

+1 snowflake and rotorbar!

Edited by S DeXter - 15 February 2011 at 7:13pm

cooky1257 wrote: There's two halves to a waveform, if the coil is out of the gap on the upstroke then isn't the downstroke dependent on the surround and spider returning the coil to a driven position in time?.

Do you mean there will be an assymetry of motion? The surround and spider are always responsible for driver position, not just out of xmax...?

cooky1257 wrote: The pistonic behaviour is only guaranteed within xmax limits as I understand it. Start approaching xmech and you'll bottom your drivers, quite possibly wrecking them.

Pistonic behaviour relates to cone breakup modes (cone flexing and resonating) rather than xmax effects . Loudspeaker modellers assume the driver to act like a perfect 'piston' (unless you are using FEA!).

Alot of newer drivers have quite large Xmechs....

there is also the possibility with horn loaded subs of using the front chamber to roll-off the HF at right frequency to drastically reduce the amount of third harmonic. if the third harmonic is kept to about a third of the level of the second harmonic the distortion will sound a lot more musical. so if the horn has a low cut-off of 40 Hz it will have high excursion around 60Hz. Second harmonics of this will be 120Hz and third will be 180Hz. so if the front chamber can be sized to begin HF roll-off after 120Hz and to be at least 6dB down (in addition to the mass roll-off of the driver) at 180Hz there could be some improvement to the sound. this is all in theory BTW but will try it in my next prototype