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View Full Version : What does F3 actually mean?

96civ
12-05-2005, 10:35 PM
I've built a box that is approximately 1.62 cubic feet and I've calculated the F3 variable for my Kicker 12" L5 sub to what I think is correct.

Sealed
Vb = 1.62
Qtc = .8922
Fb = 50.758
F3 = 49.534

What exactly does the 49.534 mean as far as tweaking the sub and amplifier, if that even has anything to do with it. I have a A8000T Audiobahn amp rated at 800W x 1 @ 2 ohms with a crossover.

Any help would be appreciated, thanks all.

snb778
12-05-2005, 10:48 PM
I believe its the frequency that your sub will hit the hardest???? no????

Johnny Drama
12-05-2005, 11:15 PM
I've built a box that is approximately 1.62 cubic feet and I've calculated the F3 variable for my Kicker 12" L5 sub to what I think is correct.

Sealed
Vb = 1.62
Qtc = .8922
Fb = 50.758
F3 = 49.534

What exactly does the 49.534 mean as far as tweaking the sub and amplifier, if that even has anything to do with it. I have a A8000T Audiobahn amp rated at 800W x 1 @ 2 ohms with a crossover.

Any help would be appreciated, thanks all.

That is the note at which the sub's sound level will decrese by -3db and also the point at which you will start to notice the sound level to drop or fade away.

LuNaTiC
12-06-2005, 12:09 AM
fs.....

96civ
12-06-2005, 09:34 AM
So If i set the crossover to something lower say around 35Hz... that would help?

squeak9798
12-06-2005, 12:34 PM
WTF guys :confused:

Don't listen to the other two.....Jntar is correct.

It's the point at which the sub's frequency response has decreased by 3db, and will begin it's low-end rolloff of ~12db/oct (in a sealed enclosure.....ported has a rolloff more around 24db/oct).

Notice that this is the roll-off of the low end of the response, which means the frequencies below the F3 (-3db point) will begin to decrease in amplitude by approximately 12db/oct. Think of it like a mechanical highpass on your subwoofer with a 12db/oct slope and the F3 being the crossover point. Below the F3, the output of the subwoofer will begin being decreased by 12db/oct.

Luckily for us, in an automotive enviornment....we get about 12db/oct of "boost" (cabin gain/transfer function) starting at around 60-70hz due to the small space we are dealing with. Which means you can have a relatively high F3 point for the enclosure, yet still have in-car response flat to 10hz. Because the "boost" of the enviornment is counter-acting this drop in output from the sub/enclosure.

Clear as mud??

Tirefryr
12-06-2005, 12:41 PM
fs.....

Box built to Vas, tuned to Fs, should have an F3 of Fs, but this is the only time you'll see this.

And as Squeak has said, transfer function is a lovely thing.

96civ
12-06-2005, 02:24 PM
OK so the F3 doesn't really have to do with anything as far as setting the subsonic filter or crossover. Or does it mean that I should set the crossover at about 50Hz? I understand that that's the point at which the octaves start to decrease at 12db, but what does having a high F3 or a low F3 indicate? Sorry if I sound retarded, but I'm a newb to the whole box building variables and equations lol.

OldManTod
12-06-2005, 02:55 PM
you know for a bunch of fuqin retards, you guys always amaze me at how you pull this stuff (as exact as a science it is) out of your a\$\$... Good Job!!

- T

squeak9798
12-06-2005, 02:58 PM
OK so the F3 doesn't really have to do with anything as far as setting the subsonic filter or crossover.

Nothing to do with either of them.

I understand that that's the point at which the octaves start to decrease at 12db, but what does having a high F3 or a low F3 indicate? Sorry if I sound retarded, but I'm a newb to the whole box building variables and equations lol.

Common sense should prevail here :D ;)

What do you think happens when the system starts it roll-off at a lower point in the frequency response? What happens to the output at those lower frequencies? Just think about it for a second........

Also realize that having too low of an F3 for an enclosure going in a vehicle could result in an overemphasis of the lower octaves, causing it to sound somewhat "boomy" because of that transfer function I discussed earlier.

Tirefryr
12-06-2005, 03:00 PM
If the enclosure is sealed there is no need for you to use the infrasonic filter.

96civ
12-06-2005, 06:38 PM
Nothing to do with either of them.

Common sense should prevail here :D ;)

What do you think happens when the system starts it roll-off at a lower point in the frequency response? What happens to the output at those lower frequencies? Just think about it for a second........

Also realize that having too low of an F3 for an enclosure going in a vehicle could result in an overemphasis of the lower octaves, causing it to sound somewhat "boomy" because of that transfer function I discussed earlier.

Aha, so the roll off rate, F3, is the point where some low-pass filter start to filter out frequencies greater than F3 at a derivative of -12db/octave. Now I see why that's a retarded question, thanks squeak.

squeak9798
12-06-2005, 06:42 PM
Aha, so the roll off rate, F3, is the point where some low-pass filter start to filter out frequencies greater than F3 at a derivative of -12db/octave. Now I see why that's a retarded question, thanks squeak.

:uhoh:

F3 would be like a highpass point, not a lowpass point. Below F3 the output begins to decrease, not above it. Which would mean an enclosure with a lower F3 would have more output at lower frequencies compared to a higher F3.

And, FYI, a Qtc .707 alignment has the lowest F3

96civ
12-06-2005, 06:52 PM
I think I meant to say it differently than what I stated it previously. Let me explain this scenario and tell me whether or not my reasoning is correct.

According to the graph, frequencies under F3 (let F3 = 50Hz) which might be 35Hz do not experience roll-off because the angular frequency is smaller, but frequencies higher than F3 -- 75Hz for instance experience the drop-off? Or is the previous statement backwards as to what really happens?

flakko
12-06-2005, 06:54 PM
then why in SQ cars do people prefer sealed boxes? it has the flattest response yes, but with the F3 being so high, wont the sub NOT reproduce the music in its true form? (even though you have that in car gain/transfer thingie)

wont a ported box tuned lower give you a flat response as well?

squeak9798
12-06-2005, 07:00 PM
I think I meant to say it differently than what I stated it previously. Let me explain this scenario and tell me whether or not my reasoning is correct.

According to the graph, frequencies under F3 (let F3 = 50Hz) which might be 35Hz do not experience roll-off because the angular frequency is smaller, but frequencies higher than F3 -- 75Hz for instance experience the drop-off? Or is the previous statement backwards as to what really happens?

WTF? That looks more like a graph for a crossover or inductance related rolloff....which is NOT what you are referring to.

Basically.....you are looking at the wrong ******* graph.

THIS is the type of graph you should be looking at:

http://www.carstereo.com/Images/SealResp.gif

96civ
12-06-2005, 07:04 PM
****. My bad dude. The graph I was looking at was ****ing me up. That clears everything up. Sorry about that.

squeak9798
12-06-2005, 07:06 PM
then why in SQ cars do people prefer sealed boxes? it has the flattest response yes, but with the F3 being so high, wont the sub NOT reproduce the music in its true form? (even though you have that in car gain/transfer thingie)

wont a ported box tuned lower give you a flat response as well?

Is it possible to obtain flat response in a car with a ported enclosure? Yes...but it's not easy, and you'd basically be defeating the purpose of using a ported enclosure in the first place. Remember....that port typically gives you a nice hump in the frequency response.....which is NOT what you really want.

And no.....using a sealed enclosure doesn't hamper the "reproduction of music". What you want is an accurate reproduction at the listening position, which INCLUDES cabin gain. If you, for example, created a famn dancy enclosure that had an F3 of 10hz and tossed it in a vehicle, you'd have a RISING response in the low end due to cabin gain, which would cause a "boomy" sound. But if you can get the rolloff of the enclosure and the transfer function of the vehicle to work together, and can get a nice, smooth in-car frequency response with a very low in-car F3....which IS what you want.

squeak9798
12-06-2005, 07:08 PM
****. My bad dude. The graph I was looking at was ****ing me up. That clears everything up. Sorry about that.

Well.....just so you don't feel stupid, a "F3" point for a crossover is essentially the crossover point and can relate to both highpass and lowpass......so you weren't really using the "wrong" terminology or graph in a general sense.....you were just using the wrong graph and description for the particular question you were asking here, in relation to sealed enclosures and their response ;)

It can get kind of confusing, since the same term can be used in a few different places.......

flakko
12-06-2005, 07:16 PM
Is it possible to obtain flat response in a car with a ported enclosure? Yes...but it's not easy, and you'd basically be defeating the purpose of using a ported enclosure in the first place. Remember....that port typically gives you a nice hump in the frequency response.....which is NOT what you really want.

And no.....using a sealed enclosure doesn't hamper the "reproduction of music". What you want is an accurate reproduction at the listening position, which INCLUDES cabin gain. If you, for example, created a famn dancy enclosure that had an F3 of 10hz and tossed it in a vehicle, you'd have a RISING response in the low end due to cabin gain, which would cause a "boomy" sound. But if you can get the rolloff of the enclosure and the transfer function of the vehicle to work together, and can get a nice, smooth in-car frequency response with a very low in-car F3....which IS what you want.

so a low tuned box (around 28-30 hz) will actually be "boomier" than a higher tuned box (33-35), if the transfer function is not taken into consideration.

squeak9798
12-06-2005, 07:26 PM
so a low tuned box (around 28-30 hz) will actually be "boomier" than a higher tuned box (33-35), if the transfer function is not taken into consideration.

Too many factors to consider (such as enclosure size, etc)....but also remember that 1) your car has a resonant frequency, closer you tune to that the more output you'll have, 2) generally the higher you tune, the peakier the enclosure itself will be

Tirefryr
12-06-2005, 07:27 PM
so a low tuned box (around 28-30 hz) will actually be "boomier" than a higher tuned box (33-35), if the transfer function is not taken into consideration.

I doubt you'll notice much a difference between those tunes to ear, in-car.

Take into consideration his earlier comment on the transfer function and boominess with a flat curve before being loaded in-car. The typical peak with the vented enclosure is now non-existent. Load the vehicle in car and the transfer function will act as an EQ effectively creating in a peak in the lower octaves which is where the boominess comes from.

With the tranfer function removed, you are taking away the increase or peak, therefore reverting back to the anechoic measurement.

LuNaTiC
12-06-2005, 09:02 PM
Wow sorry, Didn't realize it really was F3...