When does a a tall BR enclosure have quarter wave reinforcement

[abn1701]

Hi
I was looking irto the TRS-80 project by Paul Carmody .
I like very much the tower stylish .
But I'm wondering if it has some kind of quarter wave reinforcement , even if the internal shelf is placed.
Link below
TIA

http://sites.google.com/site/undefinition/diy-trs-80

[Sydney]

Re: When does a a tall BR enclosure have quarter wave reinforcement

The sound path is not being directed in the same fashion as a T/L; with the terminus at the end of a line.

[diy speaker guy]

Re: When does a a tall BR enclosure have quarter wave reinforcement

When does a a tall BR enclosure have quarter wave reinforcement

Always. But let's back up a moment because "reinforcement" is probably not the best word to use here.

All enclosures are best described by their 1/4 wave behavior, even the smallest boxes. Most people simulate them as helmholtz resonators instead of 1/4 wave resonators though. What's the difference? Sometimes not much, sometimes a lot. There's 2 issues here, the tuning frequency and the response above tuning.

In this case, with the shelf in place the length is only about 20 inches. That's not long enough to do much to the ~40 hz tuning, so a helmholtz simulation is going to be fairly accurate under 100 hz. Even without the shelf it's still not long enough to make a helmholtz simulation patently inaccurate below 100 hz. BUT above 100 hz is a different story. Depending on where the woofer and port are located in relation to the line, you could get anywhere from +/- 0db to +/- 5db (or even worse) above 100 hz, even with heavy stuffing.

I can tell by looking at it that the woofer location is fine (and it pretty much has to be where it is anyway). The port location also looks fine assuming the shelf is in place. (It might be in the perfectly optimal position but it might not be, at least it's in the right ballpark as far as location goes.) If the shelf is removed the port might not be in the best location but you never know until you simulate it, it might be fine. I'm guessing it's not ideal but that's just a guess.

I'd change the stuffing though, based on my previous mltl simulation experiences. I'd recommend using as much as you want (need) in the first half of the line, with none below that point. Stuffing is most effective at what we need it to do when it's up in the first half of the line.

[Paul K.]

Re: When does a a tall BR enclosure have quarter wave reinforcement

I've sorta established a rule of thumb for my uses, for better or worse and subject to differences of opinion, that an unimpeded internal height of a floor-stander needs to be 36" or more in order that the 1/4-wave resonant frequency from that height has a noticeable effect and contribution to the system's overall tuning frequency. Longer (taller) is better in that the height and its 1/4-wave resonance contributes more and more to the system tuning, and less comes from the mass-loading port.
Paul