20 replies to this topic

[wazzab4]

I want to know what makes a IB sub a IB sub

I have been told you need a good Xmax,
Need a Q not sure which one Qes Qms Qts around .60,
And it should be crossed over above its FS.

Is there anything else i need to know and are the comments above right.
I am looking for a 8in SUB for my rear parcelshelf but the main thing i want to know is how to find one for my self.

Thanks in advance Wazza

[Gonadman2]

I think that Qes (Electrical Q - voice coil winding, magnetic flux) and Qms (Mechanical Q - Spider, Cone, Roll surround etc) have something to do with Qts (Total Q), being those 2 values added together to give Qts.

I have found that my Focal 27V2's with a Qts of 0.51 are superb in IB.

I know Fi in the us make a range of dedicated IB subs and they are quite cheap. syd_monster is using a Mach 5 IXL sub as well which are quite inexpensive also. I'm sure the 21V2 would work alright if you have to have an 8" but I think it would be too small for decent SQ levels of output.

With IB, power is lower as you will find the mechanical limits of the woofer much sooner. You need to compensate for this with multiple woofers and large sizes so that output isn't compromised. 8" would not go very loud in an IB configuration before you found the mechanical limits of the woofer.

You need to ensure that the front wave of the woofer is separated from the back wave of the woofer or cancellation will occur. This means the boot area needs to be sealed off completely, otherwise it won't work. I have experienced this first hand, if you want to have a look at my build thread there is more detail there.

[Pulse-R]

if you want to give it lots of power, it needs a big XMAX.

other than that, the rest is a trade-off.
If it's going in the parcel shelf/behind the seat/etc. then it's not truly 'IB' as that would mean you have to vent the boot substantially into the air outside.

Q has little effect on how good it sounds, try modeling a box with the size of your boot as the volume... 300 litre or so.

[Fury♫]

Different lines of thought over what "Q" is ideal.
I've seen for/against for high Q, low Q, ideal Q... etc...

As the ideal Q for flattest reponse is 0.707, i chose a sub with a very close Q (.66 iirc).
The boot will have a slight impact on the Q, which should accomodate this nicely.

Xmax is very important. The more linear excursion, the better. IB needs excursion for low en response.

[syd-monster]

x2 basic simple figures waz, Qts of .7 as per Fury and a light cone (Mms). But realistically any sub "could" work I.B.

[wazzab4]

OK i would of though 300 liters be enough to consider it IB.
I did know about front wave and rear wave.
Do i need to consider its Fs as in to how low thne sub might play.
EG i have found a fs40hz and one at 28hz does it mean the 28hz one will play lower at the end of the day, or would a Q of .707 and a big XMAX be better and forget the Fs. I guess what i am asking is what would play flat with a roll off the best.

even easyer can you mould a sub for IB in a program to give you a idea of how it might play.

Edited by wazzab4, 09 December 2009 - 09:15 PM.

[~thematt~]

You wont be able to achieve true IB in a car, because of the restriction in boot volume. True IB is considered to be the point where the change in volume of the enclosure doesnt impact the Q of the driver at all. Nominally, its between 2 to 10 times VAS. Obviously, if you have a 300litre boot, you'll need a driver with a VAS of lower then 30Litres to be considered optimal (unlikely).

Unfortunately, this isnt possible, and therefore you have a 'hack' situation known as Trunk Baffle. This is as close as you'll get to IB in a car. I've gone through the Qts arrangements on here before, but just note that volume changes in the enclosure only ever cause it to rise, never to fall.

So basically, you start with a value of Qts, and when put into an enclosure, it rises to become Qtc. The best 'trade-off' figure for Qtc is 0.707 (inverse of square root 2), but anything between 0.5 (critically damped) to 0.707 is fantastic. Just note that because your boot will influence, your Qtc will rise. So if you get a 0.707 Qts woofer, your final response will be higher.

No replacement for displacement. If you haven't got cone area, you'll need LOTS of linear displacement.

Fs isnt as bad as you might think, nor as critical. Its just the point where the motor loses control of the woofer. You can easily play the woofer below this point, as well as above it. In fact, you'll be surprised how effective getting a high Fs woofer and running it low can actually be!!

[wazzab4]

linear displacement is Xmax is that right.

[br85]

Displacement (vd) is basically the amount of air the driver can move whilst remaining linear.

To calculate vd, multiply cone area (sd) by xmax (one way). You want to do your unit conversion so you end up with cc's or litres to compare drivers.

500 cc's one-way or 0.5L should be more than sufficient. Good luck getting there will a single 8 though. I'm sure it can be done, but it's not likely to be cheap. Any reason you can't use 2 X 8"s?

If you follow ~thematt~'s advice, the obvious thing to do to minimise boot loading is to find something with a relatively low vas.

[shiny_car]

don't confuse 'total Q' with Qts (not that i'm good at explaining them!). for IB, the sub typically should have a high Qts, so over 0.50 is ideal.

[Cyberpunky]

theory Vs real world hmmm
I went from a dedicated IB (QTS 0.7)[1 x 12] to using 3 subs that had QTS of 0.41. Bottom line was subs sounded awesome but power handling was reduced(in theory) but as i only fed 3 x 12s 200 rms each,when rating was 125wrms each, result awesome sq and SPL(relative)

As the SPL guys say nothing beats cone area. 3 x 12s rocked. sure QTS wasnt ideal in theory but in real world it made a few ppl slink down into their seats, and sounded damn fine. it is a 2nd order(maybe first order..damn been away too long) config though so no losers...tight as 2 coats of paint

peace
Cyberpunky

Edited by Cyberpunky, 10 December 2009 - 02:14 AM.

[Luke352]

Cyberpunky, on Dec 10 2009, 02:10 AM, said:

theory Vs real world hmmm
I went from a dedicated IB (QTS 0.7)[1 x 12] to using 3 subs that had QTS of 0.41. Bottom line was subs sounded awesome but power handling was reduced(in theory) but as i only fed 3 x 12s 200 rms each,when rating was 125wrms each, result awesome sq and SPL(relative)

As the SPL guys say nothing beats cone area. 3 x 12s rocked. sure QTS wasnt ideal in theory but in real world it made a few ppl slink down into their seats, and sounded damn fine. it is a 2nd order(maybe first order..damn been away too long) config though so no losers...tight as 2 coats of paint

peace
Cyberpunky

Actually the theory when you apply the knowledge of the environment the drivers will be placed into says the 3 x 12's were always going to work better.

As has been pointed out a driver that has been designed for true IB (Qts 0.707) when placed into a non true IB setup will end up with a total Q well above 0.707, so it won't end up sounding that good.

Whereas the 3 x 12's with there Qts of 0.41 each when all placed into the boot would've had there total Q raised up to an acceptable point around 0.6 probably.

But yep SQ or SPL there is no replacement for displacement.

[~thematt~]

shiny_car, on Dec 9 2009, 10:49 PM, said:

don't confuse 'total Q' with Qts (not that i'm good at explaining them!). for IB, the sub typically should have a high Qts, so over 0.50 is ideal.

Qtc is combined 'Total Q'. The Total Q of the entire system. Driver, environment, enclosure, amplifier, AP mats etc. etc. This can be changed by changing one of the system variables.

Qts is the 'Total Q' of the driver alone. Purely Mechanical (Qms) and Electrical (Qes). This is fixed. Qts= Qms*Qes/(Qms+Qes)

Here is an explanation I made a while back on various figures of Q
http://www.mobileelectronics.com.au/forums...t&p=1002746

In a car, you'd want a Qtc of between 0.6 and 0.707. People 'suggest' various figures for Qts to get you as close to these as possible, without really knowing how the trunk enclosure impacts the system. This, and VAS, are obviously critical to getting as close to perfect as possible.

[Matt VIP]

is there any way of measuring Qtc in car?

[Matt VIP]

here's a good discussion on IB 8's wazza

http://www.diymobileaudio.com/forum/diy-mo...e-air-subs.html