Determining F3

[TheNewSon 0]

What exactly is the F3 of a speaker, and what should that number be relative to a ported box's tuning frequency, if at all?

-C

[theabunai 3]

it is the frequency at -3db I believe...

It is used to project a subs rolloff

[TheNewSon 0]

So does that mean that you can change a subwoofer's F3 depending on how you build the box or set the amp?

-C

[vladd 26]

depends on what you mean. If you're talking about an F3 that's listed with T/S parameters then no you can't change that because that's the -3db point of that speaker free air. But when building a box the size and tuning of the box will change your F3 point, and can change it drastically

[TheNewSon 0]

So I still don't really understand what it is. Does that mean that the F3 is sort of the high end of the port tuning range, and anything above the F3 will drop off sharply in volume? What would a typical F3 be for a sub with a tuning frequency of say 30 hz?

-C

[vladd 26]

F3 just means that at that frequency the sub has rolled off 3db. anything below that will be rolled off more and at an exponential rate usually.

[Tirefryr 1,742]

The F3 will be dependent upon the application. Enclosure size, tuning, placement, etc, will all affect the F3. You could have your crossover set to 80Hz and the sub may nto even play that high in your application.

[vladd 26]

there...notice how the tuning of 45hz(red line) has an F3 of ~35hz and the 20hz tuning (green line) has an f3 of about 29hz. f3 is just where the line crosses the -3db mark, but those are just winisd graphs and in no way show in vehicle response

[Tommythecat 0]

F3 just means that at that frequency the sub has rolled off 3db. anything below that will be rolled off more and at an exponential rate usually.

Actually it's quite linear - at least once you get out of the transition area between the upper-end response and the eventual speaker/enclosure rolloff.

[vladd 26]

yea i noticed that on the winISD plot, i always thought it was exponential for some reason though.

[theabunai 3]

I always view the f3 like a Plot or point on the freq graph... it really tells you about the roll-off and at what rate/how drastic....

change the enclosure/port size and the f3 will change.... it can show the system to have a flat or peaky response....

I use it as a guideline to fine tune the enclosure.... I like a flatter response so when i design a box i check the f3 to see just how "slight" the response change is...

[TheNewSon 0]

Ahh, I see now. So I find it interesting that the F3 can be both above and below the tuning frequency. I thought the tuning frequency was usually the loudest point of the system, but apparently not.

-C

[vladd 26]

while winISD doesn't show it, a speaker will roll off above and below its frequency range, upper end is usually from inductance

[theabunai 3]

while winISD doesn't show it, a speaker will roll off above and below its frequency range, upper end is usually from inductance

Yupp... vladd is one of the few people that gives accurate info...

[///M5 2,833]

yea i noticed that on the winISD plot, i always thought it was exponential for some reason though.

It is if we weren't plotting in dB but in sound pressure. By definition a linear response in a dB scale is logarithmic.

[helotaxi 46]

yea i noticed that on the winISD plot, i always thought it was exponential for some reason though.

If you were to plot it as a function of power response, or on a linear scale (rather than logarithmic scales as octave and dB scales both are) it would be an exponential decay.

F3 is not a spec of the driver, Fs is. F3 is a spec of box/sub combo and is the point where the response is 3dB below the normalized level. It is usually referenced as the bottom of the usable freq range (3dB is a 100% difference in power after all) when talking about box rolloff. This will change in-car vs large room as the cabin gain of the car will extend the usable bottom end fairly significantly in a sealed system (not as much in a ported setup because of the phase cancellation from the port below tuning). The other significance of the F3 is the rollof does become constant below that point. There is rolloff above the F3 but is plots as a curved decay rather than the linear (on the normally used scales anyway) decay below that point.