What exactly is xmas (millimeters) & sensitivity/efficiency used for?

Besides that xmas is "the measure of a speakers cone maximum excursion in one direction while maintaining a linear behavior" what does it have to do with subs? Why would you want a higher number?

I also wanted to ask about sensitivity, its defined by crutchfield as:
"An efficiency or sensitivity rating tells you how efficiently speakers converts power into sound. The higher the number, the louder the speakers will play."

but what sonicelectronix says is that: "Sensitivity/efficiency is not a good indicator of a sub's output capability and shouldn't be used to compare subs to determine which one is 'louder'"

Are they possibly talking about for example a lowly lets say 300w sub with a very high sensitivity isn't as good as for example a 900w sub with a slightly lower sensitivity/efficiency?

 

The xmas means how far the cone is moving. The further it moves, the more air it displaces and the more "thump" you get.

 

So basically, more Xmas = more air displaced which ='s a harder hitting bass?
This doesn't mean the kid you can hear 2 blocks down the street with a really crappy sounding system?

 

Technically,
If one speaker is rated 300wrms at 90DB sensitivity it would be louder than a 900wrms at 88db.
Actually, one could potentially generalize and say this but unless Physically testing both units in a real world setup you won't know. Factors like BL and other TS spec's come into play but at the end of the day is a complex relationship between EVERYTHING. You cannot simply look at one vs another and say this is louder because abc. You can guess and postulate but thats why the SPL game is so Huge and One person does 168 db with 1 sub and another can break 140 with the same thing.


There are very complex relationships between the TS parameters and what a speaker does / sound like. Honestly, There is way to much to even explain or get into, and unless your planning on competing or even then the "specs" don't mean all that much. Specs do have some importance when selecting a driver ( getting complicated) as some of the specs tells one that this given sub is better suited for a SEALED Box, or would be good in a 4th order BP vs ported etc.

Example of this - Sundown has their X series and ZV4 series with the mega roll surround With HUGE xmax, i mean massive!! but yet their " spl " surround is much smaller, Less xmax but is louder...

Im just saying, this is a very complex questions and honestly could take weeks of understanding and explanations from An Expert, And Expert i mean the people that design em lol.

 

Shoot. And this is why I want to go into mechanical engineering because it like blows my mind but I love it at the same time.
Thanks for explaining that btw.

 

xmax is one thing to look at when determining the potential maximum output of a speaker. the other important part of this is cone area. cone area and maximum linear excursion form a direct relationship to how much air the speaker is physically capable of moving which is a reflection of potential volume.

efficiency / sensitivity is basically the volume that speaker produces at 1 watt of power from 1 meter away. So it can be used, very loosely, to compare speakers if they are within the same power range, but it's more for reference when matching drivers in designing a speaker system. It doesn't have very much use in car audio applications.

But like manethon said, these are just specs and until you have a very good understanding of all specs and other factors that come into play in real world applications, don't think too much about them.

And good choice on mechanical engineering. stick with it!