how big of a factor does the Fs play in a sub ?

[ShadeTreeMechanic]

From the formulas that I am familiar with, the Fs is used to calculate F3 which is the 3db down point on the response curve. It is possible to have port tuning below this 3db down point which is what you are seeing. Picking a driver with a lower Fs will move the 3db down point lower as you have found out.

[Bayuk89]

Yea i will do some more research, i want my truck to be a wind tunnel lol the current build will play down to 28hz but its not as loud or powerful as 35hz +

Im prolly asking too much out of these subs, bryan chamberlain told me im better off running an X series to play down to the lows i want.

[Triticum Agricolam]

following this intrested what's the follow up on this... i see triticum looking at this

Lol, you caught me :-)

I've been thinking about if/what to post on this thread. It's a complicated subject because you can't view just one T/S parameter by itself, they are all interrelated and have to be interpreted together as a whole.

Fs is a function of moving mass (measured as MMS) and suspension compliance (stiffness, measured as CMS or Vas). If you increase moving mass or decrease Vas/CMS that will lower the Fs, and if you decrease moving mass or increase Vas/CMS it will raise the Fs. To make things more complicated, in addition to changing the Fs of the sub, changing moving mass and/or suspension compliance also will change the QES/QTS of the driver, so in an attempt to just change the Fs you really end up with a completely different sub.

When it comes to the frequency response, some interesting things happen. For example increasing moving mass or increasing Vas/CMS have opposite effects on the FS of the sub, but they have similar effect on the frequency response. They will both increase output at tuning and reduce it everywhere else. This is because increasing MMS and increasing Vas/CMS both make it harder to move the cone. Since at tuning there is very little cone movement not much changes, but above and below that were there is more cone movement that movement is restricted thus the loss in output above and below tuning.

So for Bayuk, if you want to do an interesting experiment, you can increase the MMS of your sub by adding weight to the cones. This will lower the Fs and raise the QES/QTS, then you can hear the difference for yourself. As easy and temporarily way to add weight to the cones is using magnets. You have to pull your subs out to do it but you put magnets on the top and bottom side of the cone so the cone is getting clamped between them. Its going to take a few to make a difference. The MMS of your SA-18s is about a pound and you want to increase that by 50% or so. If you try this be sure to evenly space the magnet around the cone and don't play the sub too loudly, you don't want to shake any of them loose. If you decide you want to permanently increase the moving mass, this can be done by adding heavy dustcaps.

[Triticum Agricolam]

Shadetree

http://stereointegrity.com/product/ds4-18-subwoofer/#tab-specifications

I was looking at this sub as a replacement for SA-18's i can tune wayyy low but prolly wont play well above 45 hz.... its on sale for about $20 less than MSRP due to being discontinued and bringing back the old HT18 sub in their line up.

That sub will pay just fine above 45 hz, if the box is right :-)

[Bayuk89]

Thanks tritcum !! ? i dont think i would do the temporary magnets myself. I could add a bigger dust cap like an 8" over my existing cap ? One thing i found thats annoying is my SA-18 uses an 18" cone but 2.5 coil.... so the support of options isnt as good as an 18" cone with 3" or 4" coil i would do a custom recone with heavier cone along with spider and surround options but IDK if its worth it on these subs...

Your opinion tritcum, would it be worth a custom recone on my SA-18's or would it be better to buy the DS4 subs ? The DS4 has 27mm of xmax vs 19mm on mine, it has a 4" coil vs 2.5" on mine. Fs is 17hz vs 33hz on mine.... its $229 each shipped.

[Bayuk89]

http://stereointegrity.com/product/ds4-18-subwoofer/

http://sundownaudio.com/index.php/products/discontinued/item/sa-18-series

Here is the links to the DS4 and SA-18 with the subs T/S specs... i want to play mid 20's in the bandwidth. If i can get that out of my SA's i will keep them. If not i will put them on the shelf and get the DS4. If you have the time to look over the subs. Id love to hear your valuable opinion.

[Triticum Agricolam]

Those DS-4 subs are an amazing deal. I love Stereo Integrity and you would be getting a TON of sub for the money, but........its probably not worth it to switch to those subs unless you have room for a WAY bigger box.

The DS-4 subs are heavily geared towards home theater use where people want raw output down to below 20 Hz, and those subs will do that very well. They probably do it better than anything else out there for the money. However they need a very low tuned, huge box to perform to their best. If you to just drop those DS-4 subs into your current box they really would not be any improvement at all. To really be better than what you have now they would need to be in at least a 16 cu ft box tuned to around 23 hz.

[Bayuk89]

I could wall my truck. Im 10.5 net below my window i could fit another 6 cubes i bet.

[Bayuk89]

Im gonna do some thinking on the DS4. Only bad thing for me is they only offer 2ohm option... so if i got them i either wire .5 ohm or 2 ohm