# How do Fs, Xmax and Qtc affect Free Air Subs?



## Argento (Apr 9, 2011)

I’ve been reading numerous threads about this and I’m still uninformed. I’m a novice, so don’t hesitate to correct me or call me out on something wrong, b/c I’d like to know. From what I’ve read a good free air sub application has a Qts >.6 or .7, a high Fs (>40?) and a high value of Xmax. Are these characteristics sufficient to base a sub selection for IB scenario?

==========
I’m asking b/c I want to upgrade my Stealthbox 8IB4 Sub with one that fits in the Stealthbox.

The 8IB4 comes with the Stealthbox and has less than kind reviews, but it was designed for free air so I used it’s specs as my baseline: Qts = .684 and Xmax = 8.6mm and Fs = 37.5Hz. 

There is a great thread here on free air subs and I found that the DLS AO8 has dimensions that may allow it to fit in the Stealthbox surround. Qts = 0.34, Xmax = 9mm, Fs = 32Hz. Reviews are very good for this sub and I found them for $126/ea w/ free shipping. Single VC.

One of the usual replacement suspects is the ID8: Qts = 0.332, Xmax = 15mm, Fs = 24.7Hz. Reviews are also very good and these are available for $119.99/ea w/free shipping. DVC.

As you can see, the options I found have low Qts compared to the baseline. How important is it to run a high Qts? I considered using 1VC on the ID8 and short the other VC to increase Qts. Is this worth the effort over just using the AO8 or the ID8 as-is? I know I won’t get the perfect solution, but I’d like to understand the shortcomings if possible.


Thanks for any help.


----------



## ryan s (Dec 19, 2006)

Glancing at the Qts is only a good "quick check"...subs with a .2 Q may not be usable, for example. Some are, but most drivers built today are meant to go in a tiny box. Small Vas + low Qts. On the other hand, a .6 might have a bump in Q when installed.

What's important is what the total Q becomes once the sub is in its airspace. For example, a pair of .4 Qts subs in 14 cu ft models to .7 total Q...which is considered "optimal." This is extremely simplified theory, and the environment of the car plays a huge role in what happens when you go from simulating a sub to actually installing it.

How much space is your trunk? Stealthbox for IB subs...never know JL made those (I'm assuming it's JL, at least).

You want the lowest Fs and highest Xmax, also. Cone area is essential, as well. The better question is: If you can fit an 8", could you fit a 10 or 12 instead?  Or a pair of some size?


----------



## St. Dark (Mar 19, 2008)

Which StealthBox do you have? I had the same thoughts as Ryan S...I don't think that the 8IB4 was what came in there. You should be able to look up your SB on JL's site and it'll tell you which sub is supposed to be in there. Which will in turn help you find a suitable replacement if that model is no longer available.


----------



## Argento (Apr 9, 2011)

ryan s said:


> Glancing at the Qts is only a good "quick check"...subs with a .2 Q may not be usable, for example. Some are, but most drivers built today are meant to go in a tiny box. Small Vas + low Qts. On the other hand, a .6 might have a bump in Q when installed.
> 
> What's important is what the total Q becomes once the sub is in its airspace. For example, a pair of .4 Qts subs in 14 cu ft models to .7 total Q...which is considered "optimal." This is extremely simplified theory, and the environment of the car plays a huge role in what happens when you go from simulating a sub to actually installing it.
> 
> ...





Thanks for the response. I'd guess I'm closer to 13 cu ft. I'm trying to upgrade oem sound and keep my trunk, hence the stealthboxes off the rear deck. 

So if I'm going with Qty.2 subs using the same trunk space, theoretically I'd double Qts...which gets me above .6. However, my wiring options become limited to the following:

Qty.2 ID8 1/4; Wired Independently 1 CH per 2ohm VC; Helix Precision B4, 90W/150W per Ch at 2 Ohms. 

Should I worry about 1CH per VC? 

Will double check JL sub tonight.


----------



## Argento (Apr 9, 2011)

St. Dark said:


> Which StealthBox do you have? I had the same thoughts as Ryan S...I don't think that the 8IB4 was what came in there. You should be able to look up your SB on JL's site and it'll tell you which sub is supposed to be in there. Which will in turn help you find a suitable replacement if that model is no longer available.



JL Audio Stealthbox for 1998 BMW E36 M3.


----------



## St. Dark (Mar 19, 2008)

Well, hard to type with my foot in my mouth...but that box does indeed come with the IB8". Dunno about the bad reviews, that sub was mourned by many when it was discoed. I'd try it with them if they aren't bad/surrounds rotted/etc.


----------



## sqshoestring (Jun 19, 2007)

The Qts of a sub is your Q when mounted IB. When you use a box then that changes everything, model it, I model my IB subs too.

Very roughly Fs is how low that sub might play, Qts of .7 should give flat response IB down to near Fs (just like a box can at Q .7), and the Vas and qts can show you how damped the speaker is. IB subs have little dampening that is why a box affects them (with bass rolloff) much more than a low qts sub where you need a tiny box to push the Q back up....IB subs a huge box will start pushing Q up. All else said more xmax will make more bass, even IB if you EQ it. IB subs will work in a box provided you don't need low bass from it (usually seen by high qts). Some of the pioneers are like this, some have some strange specs.


----------



## Moon Track (Mar 10, 2011)

Actually, these parameter in combination with BL can be used to recognize a free air sub from modeled by software xmax at rated power but they don’t speak about quality of the sound. Any speaker can be used as a free air, just only with less power than it was determined by manufacturer for enclosed design because of xmax limits. The difference between them that the free air sub can allow a thinner diaphragm and smaller coil, in other words a more light diaphragm , that have an effect on the transient response of the speaker . Only reviews can help here.


----------



## ryan s (Dec 19, 2006)

Argento said:


> Thanks for the response. I'd guess I'm closer to 13 cu ft. I'm trying to upgrade oem sound and keep my trunk, hence the stealthboxes off the rear deck.
> 
> So if I'm going with Qty.2 subs using the same trunk space, theoretically I'd double Qts...which gets me above .6. However, my wiring options become limited to the following:
> 
> ...


I'd be more concerned about finding the actual Q (and response curve, and SPL, and excursion) in that 13 cubes that worrying about the Qts outside the car  My .41 Qts sub models at .58 in my trunk (and I'm ok with that for now). Filling my trunk halfway makes it so much better. Adding another sub would do the same.

The ID8s...do you mean they're dual 2 ohm? Sure, you could run one voice coil per channel. For ease of wiring, I'd wire the coils to 4 ohms on each sub, then bridge the amp for 2 channels at 4 ohms per channel. 90-150w per sub will be more than enough.


----------



## Argento (Apr 9, 2011)

Thanks for all the responses and great information. 

I guess reviews are pretty subjective...however the reviews were on a bmw forum, not an audio forum. I plan to listen to both subs.

Thanks for the wiring suggestion. I thought 90/150 for a sub rated 180/300might be too low, but it's probably more reliable sound than an independent VC wiring setup. 


I may have more questions after install...


Thanks again!


----------



## sqshoestring (Jun 19, 2007)

In general when a sub is used IB you cut the boxed rating in half, sometimes you don't even need that much and some subs can handle over half. If a sub is sold as an IB sub, then that would not apply. It even said that in the manual for the Infinity 1252W I used IB they were 300rms and it said 150rms when IB.


----------



## ryan s (Dec 19, 2006)

For non-IB-specific subs, 1/2 rated power (Pe or RMS) is the "rule of thumb" just because their suspension may be a bit weaker due to being able to be shoved into a box. IB-specifics subs can't go in anything but a huge box so they're optimized for that environment.

The IDs will work...just be careful at low frequencies and high volume as they can go past their excursion limits


----------



## sqshoestring (Jun 19, 2007)

Or you can go nuts and get something like a JBL GTi that has shorting rings, you can't really xmax it. But they are not optimal tune (but will work with EQ) and are heavy and expensive. Would be impressive however. I don't know what all subs have rings, they often don't say. RF used to call them braking rings.


----------

