Tuning port on MLTL : how ?

[sadhill]

Hi,

I am trying to tune the port on a Fostex FE207E MLTL.

What is the best way to achieve it ? I dont get anywhere by ear only, with sinus generator at low frequencies (38Hz Fb for this Fostex).

Furthermore, should it be done before or after adjusting the filters ?

It seems to work equally well for low frequencies with any length or without tube in the port !!! (presently 4" diameter and 4" length)...

Quite surprising...

Thanks for help

[loninappleton]

Yes, I have used a variety of speakers in the same MLTL box and found them both striking.

GM uses Martin's MathCad models to do some. Maybe he will chime in here.

[sadhill]

Let me add that the design is done according to MJK MathCad sheet.

[GM]

Normally you tune the cab before designing any BSC.

You can pick a frequency to tune to and get 'close enough' using MJK's software or measure to find the find the flattest in-room response or do as I do and tune it based on the music you listen to that has the lowest audible LF output since you never want to unload the driver below the cab's tuning. If this includes pipe organs, then tune to ~27 (4" dia. x 10" long) Hz and move it near a wall or corner to keep BSC to a reasonable amount, but for most music ~37 Hz (4" dia. x 4" long) is fine with a rising rate suspension driver such as the FE207E.

GM

[sadhill]

Thank you for your advice, GM.

Does that mean I should play lets say an organ disk and choose the lowest freq continuous sound and tune the length of the port during that sound ? not very easy but I'll try. And it's difficult to now if you really play the lowest possible freq you will find in your disks...

Is there no "technical" way to do it ? with impedance measurements, or LF generator or what do I know ?

It's more a matter of "fine tuning" than "tuning".

[Scottmoose]

Not exactly. What I believe GM is saying is that if you are running Martin's MLTL for the 207, and happen to listen to pipe organ music, then you'll need to tune the cab to ~27Hz, for which you would need a 4in diameter x 10in long vent. If such material isn't in your collection & isn't going to be, then a 4in x 4in vent making for a ~37Hz tuning should be fine for most acoustic material.

Basically, it's a case of taking the type of music you listen to most & establishing what the lowest average tone is going to be (you can do that easily enough based on the instruments). Then tune to, or just below, that, so the cab. never unloads on this material.

[GM]

You're welcome!

Well, if you have 16 Hz pipe organ symphonies and you play them loud, then of course there's no way a typical 'FR' driver can handle such signals, but at modest levels the low notes will be very attenuated, though not its 32 Hz harmonic when combined with the 32 Hz pipe's output, so the tuning needs to be in the high 20s.

If you listen to Pink Floyd or other types of music with significant output to ~30 Hz, then a low tuning is desirable, but the trade-off is a reduction in dynamic headroom an octave above Fb, so placing them where they will get some room gain is required. Otherwise, tune them in the high 30s and use some form of EQ to roll them off as required to keep distortion low.

If you want to be 'technical' about it, then get a RTA and map the < 200 Hz BW of each CD, etc. to find its dynamic headroom requirements and tune the cabs for the one with the lowest F6 gain BW with the understanding that you don't want to tune any BR or MLTL lower than ~0.707x the driver's Fs.

GM

[sadhill]

Hi GM, Hi Scottmouse,

Well, I tried a 10" long port, doesn't work well at all, but thre is only something like 1" left at the end of the pipe before the back of the enclosure...

Bass have gone away quite a lot.

So I did enventuelly come back to a +/- 4" diameter and 4" length port, much more satisfactory as far as sound is concerned, but with no "fine tuning" at all...

Although this is not intellectually satisfactory for me, it gives an acceptable result. I do not intend spending tens of hours adjusting this pipe, wich would be, furthermore, difficult without damaging the already finished enclosure...

It looks finally as if there is no real need for a fine adjustment in the MLTL enclosure, at least less than for the BR ?

I will now have to jump in the pleasures of BSC adjustment, as the recommended 2mH + 7ohms (with a Trends T-amp) doesn't hide the mid range enough for my taste, it looks as if this also something tricky ...

Many thanks anyway for your adivces !