FE207E BSC + BR Questions

[rjbond3rd]

Greetings. I have a pair of Fostex FE207E drivers in old KLH boxes of 25.67 liters, and I used WinISD to calc a port of 3" diameter, 4" length. The drivers have 50 hours on them, and sound great to me (compared to when new).

Using test tones, I can determine that they have lots of energy at 50Hz but absolutely nothing at 40Hz -- not so much as a rumble.

1. BSC Question -- I have them flush against a wall, and the bass is still not yet balanced. Obviously then, the BSC is going to do a lot more than merely compensate for baffle step, true? Wouldn't it be more accurate to say "equalization filter"?

2. Baffle Width -- If the answer to 1 is yes, does the width of the baffle really have anything to do with the circuit (given that they are already flush against a wall)?

3. BR Alignments -- Everything I've read on BR says you go after a precise alignment (QB3, B4, BE4, IB4 etc.) But in practice, it seems as if people don't do this -- instead, they just sim various volumes in WinISD etc. and pick the one they like (assuming the port is calculated correctly).

So if everyone does just fine casually simming various BR volumes/ports, is the whole notion of alignments just superfluous in practice? Doesn't it ultimately come down to arbitrary fine-tuning as opposed to "matching" the textbook alignment?

Thanks in advance!

[planet10]

Originally posted by rjbond3rd
3. BR Alignments -- Everything I've read on BR says you go after a precise alignment (QB3, B4, BE4, IB4 etc.) But in practice, it seems as if people don't do this -- instead, they just sim various volumes in WinISD etc. and pick the one they like (assuming the port is calculated correctly).

Alignments are an artifact of when they were being derived -- a time when math was done by hand & with slide rules.

dave

[vinylkid58]

Originally posted by rjbond3rd
Using test tones, I can determine that they have lots of energy at 50Hz but absolutely nothing at 40Hz -- not so much as a rumble.

You should be at approx. -14db @ 40Hz.

1. BSC Question -- I have them flush against a wall, and the bass is still not yet balanced. Obviously then, the BSC is going to do a lot more than merely compensate for baffle step, true? Wouldn't it be more accurate to say "equalization filter"?

BSC isn't going to help the lack of low bass output.

3. BR Alignments -- Everything I've read on BR says you go after a precise alignment (QB3, B4, BE4, IB4 etc.) But in practice, it seems as if people don't do this -- instead, they just sim various volumes in WinISD etc. and pick the one they like (assuming the port is calculated correctly).

WinISD alpha has a QB3 alignment option.

So if everyone does just fine casually simming various BR volumes/ports, is the whole notion of alignments just superfluous in practice?

I wouldn't think so. Isn't that why we have T/S numbers to work with?

Jeff

[Scottmoose]

A step filter is indeed a form of Eq -it just happens to be targeted at correcting for baffle step loss rather than other issues. It won't really increase the ultimate extension of your speaker, but it will balance things out better.

T/S BR alignments are useful, but not Holy Writ. Typically, for a de rigure BR, it's a good idea to go for some form of EBS alignment, which doesn't necessarily have to match what the T/S derived equations say. You do what will work best for you, not be rail-roaded by a set of useful, but not essential, equations.

[rjbond3rd]

Originally posted by planet10
Alignments are an artifact of when they were being derived -- a time when math was done by hand & with slide rules.

dave

Aha! That makes sense, as I've been reading some rather old texts (1970's and 80's). Thank you!

Originally posted by vinylkid58 [B]
You should be at approx. -14db @ 40Hz.

Hi vinylkid58, you're absolutely right -- I didn't have it cranked up quite high enough, but now I feel the air movement at the port, and it is definitely there. Thank you! (I also swapped in a revised port length of 5.28".) My cubicle is next to the HVAC closet and I think it masked the sound.

Originally posted by Scottmoose
A step filter is indeed a form of Eq -it just happens to be targeted at correcting for baffle step loss rather than other issues. (...)

Hi Scottmoose,

Gotcha -- though if the speaker is already up against a wall, presumably there isn't any (or much) step loss? So I was speculating that what I'm really doing is building a somewhat arbitrary "tone control" or "sound shaper" circuit rather than a step-loss compensator per se. And that would give one the freedom to choose a frequency not dictated by baffle width, I suppose. Whatever improves the sound is fine by me -- flush against the wall, it's still just a tad thin.

Originally posted by Scottmoose
Typically, for a de rigure BR, it's a good idea to go for some form of EBS alignment, which doesn't necessarily have to match what the T/S derived equations say...

May I ask how one gets an EBS? Is it merely a question of playing with the box size and tuning frequency until a nice "bass shelf" appears? Or is this a precise mathematically defined "alignment" like B4, QB3 etc.?

Thank you very much in advance, sir!

[Scottmoose]

Depends on how deep the cabinets are & other factors as to how severe the step-loss will be if placed against a rear wall. There will still be a degree of it. How objectionable it will be only you can tell.

There are a couple of T/S style alignments for EBS, but it doesn't really matter. Adjust to suit your own requirements.

Assuming pure T/S math, then the box will be larger than a max-flat 4th order T/S alignment, the volume of which is Vb = 15Vas*(Qt^2.87). The -3db point of said box will be F3 = Fs * (Vas/Vb)^0.5 'Required' tuning frequency will be Fb = Fs * (Vas/Vb)^0.32
And ripple (in db) from a nominal flat response will be taken from Ripple = 20 log10 ((2.6)(Qt)((Vas/Vb)^0.35)) The theory is that it's best not to exceed ~1db of ripple, which is sensible enough. But you don't really need any of this unless you're after mathematically perfect enclosures; as Dave noted above, the alignments, though very useful guides, come from a time when simulation software was not generally available to most DIYers. Actually, I still like the older, simpler equations for BR cabinets that pre-date T/S parameters, but horses for courses.

[rjbond3rd]

Hi Scottmoose,

Thank you for the equations -- you've just given me hours and hours of fiddling.

I made the classic mistake of listening to "flat" response and concluding that it was "wrong". Really, I guess I have a personal preference for a slightly exaggerated bass.

I'm just a newb, but these crude boxes already accomplish my main goal, which was to get past the intolerable flabby bass I was hearing in the low-end retail speakers. I can hear every note on the five-string electric bass (Steely Dan), it just lacks a little weight. But no flab so mission accomplished. I guess we spend the rest of our lives trying to get that last 10%.

Thank you again for the equations, sir!

[Scottmoose]

You don't have to use them. They're only one way.

[rjbond3rd]

Hi Scottmoose, gotcha -- I think I will scratch out a few BR's on paper with equations in order to learn something. I appreciate your help.

Going back to the original post (my FE207e in a 25.67 BR), I added the BSC and it sounds fantastic. The circuit is merely a 2.5 mH inductor in parallel with an 8-ohm L-pad (Zobel to be added but it sounds too good right now to power down for soldering.)

The value for the inductor was recommended by Bob Brines for an 11.5" baffle here:

http://www.diyaudio.com/forums/showt...25#post1003625

Gmilitano's Javascript BSC calculator was helpful:

http://diyaudioprojects.com/Technica...it-Calculator/

In the end, I think I will keep the L-Pad for small adjustments. It's nice to have a couple knobs to turn... Thank you again!