T39 20" High Pass Setting

[Ollie]

I am about to let loose my pair of T39 20" fitted with single BP102s.
A number of members have mentioned running these to 38 v which I intend to limit at.
However what high pass settings are recommended? I think from reading past posts, a V plated pair can pass at 40 Hz, while singles or a pair need to be a bit higher at 45 Hz. Is this correct and why the difference?
Cheers

[bgavin]

If Bill will chime in and give us the voltage/Hz limit, I'll calculate the HP for you.

[gdougherty]

v-plate extends the horn path and increases mouth area, two factors that contribute to horn loading. IIRC, the T39 is a 45Hz horn, so no matter how many you stack together it won't run flat lower than that point as you increase total mouth area. When you v-plate, it may push to a 40Hz horn, either that or you gain additional extension with smaller numbers by virtue of the larger horn mouth. I recall mention of diminishing returns as you add cabinets for the v-plating, so it may be only the later. HP point may also depend on the slope you're using. I HP my T48's at 35Hz with a 48db slope meaning there's really not much going in below 35Hz. With a 12db or 18db slope I'd push that up to 40-45Hz.

[bgavin]

With the 3012LF driver, HP the T39 at 46 Hz, 24dB/octave slope.
This gives the requisite -3dB point at 42 Hz.
3012LF requires -3dB (half power) at 42 Hz.

Dunno about the BP102, but HP at 46 Hz is a good starting point.

[Ollie]

Help me to understand a couple of things...
"the T39 is a 45Hz horn, so no matter how many you stack together it won't run flat lower than that point" What in the design makes the T39 a 45 Hz horn?

And if one is to pushes lower than that there is dire results, correct? Why?

[bgavin]

Link to T39/BP102 Chart

The horn path length determines the cutoff frequency.
The mouth area determines the amount of ripple in the produced sound.
More mouth area (more horns) = smoother response.

The cone excursion rises significantly around 42 Hz in the T39.
This is why you HP filter at 46 Hz using 24dB/octave or better.
An active management box like the DBX 260 is a must to run at high power.

If you need something that goes lower, build the T48.
You get an extra 10~15 Hz bass extension with T48, but a lot larger box.

[gdougherty]

Help me to understand a couple of things...
"the T39 is a 45Hz horn, so no matter how many you stack together it won't run flat lower than that point" What in the design makes the T39 a 45 Hz horn?

And if one is to pushes lower than that there is dire results, correct? Why?

Like bgavin said, jog over to the SPL chart forum and look at the T39 charts where 1-8 boxes are compared. You'll notice the flat area extending lower as you add boxes. That's the larger mouth area bgavin mentioned.

[Ollie]

Thats a great help. I'm am quite happy with my decision to build T39, it was a matter best combintaion of size v's output v's cost at the time I started building... before 3012 was available. I just wanted to understand the limits of the design and the reasons why.
One final thing that you could help me with then, the maths on the output of the said 20" BP102 V plated at half power and safe full power (38v)
Cheers

[bgavin]

For me, I would use the recommendation from the plans (without V-plate) as the maximum limits.
The V-plating adds +2dB to the noise level, and somewhat extends the horn path (and lowers the Xmax limit).
Being conservative is the best way to insure your gear runs well over the long haul.

Think of V-Plating as added headroom for Xmax, rather than enabling a more risky lower limit.

If you need more noise, add more boxes.
Four T39/BP102/20" will make a whole lot of noise.