Excursion- do you design around it for woofers?

seedlings

Member
2007-04-20 9:19 pm
How closely do you watch the woofer excursion graphs when designing a cabinet?

It’s possible to adjust parameters so that all the excursion is used at lower power than rated, or to design so that full rated power corresponds to maximum excursion. I didn’t know if there was any common observed practice. Xmax isn’t usually one of the more trusted specs.

I’m preparing to build and designing for xmax limit at full power, based on crossover freq.
 
I tend to optimize for a worst-case scenario, ie- 100dB. This means really loud, capable in larger rooms, and keeping chuffing at a lower threshold. If it will work at this level, it will be fine in all but the most stressful uses. Some smaller designs can't breach 95dB as a worst-case, then the larger and louder scenarios are not as favorable for the design. Try to keep the Xmax at spec+15% down to tuning for vented designs, and the overshoot below there is sometimes exaggerated in the models due to lack of suspension compliance being modeled accurately. For sealed designs, keep the Xmax at the same spec down to F3 if possible.

Later,
Wolf
 

seedlings

Member
2007-04-20 9:19 pm
Very reasonable. Thank you. From what I’ve read, xmax isn’t a mechanical limit where the former clacks against the magnet, rather the point where the magnet begins losing linear control. So I have modified my simulation from the 5mm spec xmax parameter, up 20% to 6mm at max power. That actually improved the amplitude response graph. I don’t expect to reach full power, but nice to have available.
 
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How closely do you watch the woofer excursion graphs when designing a cabinet?

I pay it some attention. I build low power systems that are Xmax limited.

I normally assume cheap drivers are lying on their Xmax figures. I normally guesstimate Xmax = 3mm on vintage woofers, or woofers that have no info.

I sometimes use simulations (e.g. Hornresp or this Piston Excursion calculator) to tell me which part of a system will probably be the weaker link, for the crossover points I'm considering. I don't care if the simulations are imperfect, I just use them for a rough idea. They are pretty good (predictive) for this.

Example: my current toy, foreground in the pic, has a vintage 18" OB woofer supporting a 2" wide band driver. Both parts of the system will hit Xmax relatively easily. If the Xover is too low, the wide band driver will be the the weaker link. The Xmax simulations got me pretty close the the Xover point I ended up going with.

In theory, I thought the 18" would also hit Xmax easily, and not go low and loud enough. I was planning to make a 4-way, roughly:

2 x 15 (the box in the background of the pic, <80Hz)
1 x 18 (midbass)
Synergy style (mids and treble) - the 2" is in the apex of a simple conical, synergy style horn, and is based on a prototype version which had mids.

...bit in practice, I got hooked on the half-finished system, and have been using it as a simple 2-way for months. Because lazy. The main flaw is that I'm running the woofer too high.

For me, the limited Xmax (relatively low max SPL) is not an issue.
 

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...bit in practice, I got hooked on the half-finished system, and have been using it as a simple 2-way for months. Because lazy.

That sounds familiar.

How closely do you watch the woofer excursion graphs when designing a cabinet?

It’s possible to adjust parameters so that all the excursion is used at lower power than rated, or to design so that full rated power corresponds to maximum excursion. I didn’t know if there was any common observed practice. Xmax isn’t usually one of the more trusted specs.

I’m preparing to build and designing for xmax limit at full power, based on crossover freq.

Same for me, more or less.

I try to match the amp to worst recommended use scenarios, but the speaker size dictates driver size, and I very rarely buy drivers with more than 6mm xmax. So in my case I let membrane area and desired SPL dictate bass response.
 

YSDR

Member
2013-11-24 8:13 pm
From what I’ve read, xmax isn’t a mechanical limit where the former clacks against the magnet, rather the point where the magnet begins losing linear control.

You are right, the physical limit of the movement is called Xmech or Xlimit.
In reality, some non-linearity are always present in a driver and it's possible to reaching audible levels of non-linearities within Xmax and this can be a magnet-coil interaction too. And there is drivers that are linear (their values within the Klippel limits) beyond Xmax.
Xmax is just the theoretical/mathematical linear excursion limit.