Its been a while since we've argued along these lines =)
There are two things I'd like to add....Port CSA and the consequences of small vents...just like a driver...a larger port generates higher amounts of direct energy...essentially a larger port sounds better before even considering velocity issues..
Tuning...from a strictly personal (and techincal) point of view any vented design TL/BR that is tuned with the resonate within the passband is incorrect...if you look at the performance of either tuned to 15-22hz you'll see the best performance of either....
For this reason I am wondering if a fully stuffed vented design tuned to 15hz is better than a stuffed sealed box of the same size...being that the sealed resonant note is within the passband....
There are two things I'd like to add....Port CSA and the consequences of small vents...just like a driver...a larger port generates higher amounts of direct energy...essentially a larger port sounds better before even considering velocity issues..
Tuning...from a strictly personal (and techincal) point of view any vented design TL/BR that is tuned with the resonate within the passband is incorrect...if you look at the performance of either tuned to 15-22hz you'll see the best performance of either....
For this reason I am wondering if a fully stuffed vented design tuned to 15hz is better than a stuffed sealed box of the same size...being that the sealed resonant note is within the passband....
I have wondered about this myself. I like the bass of vented boxes when the tuning frequency is 30 Hz or below... Above that, the bass seems muddy and indistinct.
I have modelled boxes where the sealed box roll-off is 12 dB per octave, but venting it at a very low frequency results in a 8 dB per octave roll off... Theoretically this would be lower group delay. I have never built one like this, but i have wondered about it.
Irrelevant to the discussion. The quarter-wave forms over the box length — 750mm complicated by the vent.
I would like to see a model.
dave
Maybe. Joe uses that sort of technique in Elsinore, and i have helped some poorly aligned vented boxes that way, as well as just full-on aperiodic design simlar to the well engineered PEARL PR-2.
dave
So what I understand so far is that Dave and GM are suggesting that just because we don't see the 3rd harmonic of a 1/4 wave pipe in the example I linked to doesn't mean the cabinet still isn't acting like one because the location of the driver on the baffle is possibly suppressing that harmonic. Perhaps that explains the absence of that harmonic but I'm just not getting how that can possibly explain the presence of the huge peak at ~225Hz unless the column is acting like a 1/2 wave pipe showing a strong response at the fundamental standing wave frequency, which is what markbakk and tmuikku have suggested. In terms of the math, for a 1/4 wave pipe, the first 2 possible resonances would be F(1) at ~111Hz and F(3) at ~ 335Hz. So nothing generated at ~225Hz.
I don't for a second doubt Martin J King's work and that a ported enclosure may act like a column open at 1 end (1/4 wave pipe) but the evidence also seems to suggest that it can also act like a column closed at both ends (1/2 wave pipe). And in both cases, the vertical placement of the driver(s) and port on the front baffle can aid in mitigating or enhancing any one of the applicable resonances. That would inform the OP's question about port placement too.
What I'm interested in I guess is if anyone can explain what the primary variables might be that would distinguish between the cabinet acting like 1 case or the other? That is if my supposition is correct.
I don't for a second doubt Martin J King's work and that a ported enclosure may act like a column open at 1 end (1/4 wave pipe) but the evidence also seems to suggest that it can also act like a column closed at both ends (1/2 wave pipe). And in both cases, the vertical placement of the driver(s) and port on the front baffle can aid in mitigating or enhancing any one of the applicable resonances. That would inform the OP's question about port placement too.
What I'm interested in I guess is if anyone can explain what the primary variables might be that would distinguish between the cabinet acting like 1 case or the other? That is if my supposition is correct.
Sort of maybe, given that the box is of dimensions that are too short to be a TL and too tall to be a strict BR. A “hybrid"
It is in a zone where a model could help. Just as Martin developed his software, him and others, were bringing new insights into TL/QW behaviour (as well as opening up a huge new design space). Some of these were surprising. I wouldn’t be surprised to see some of the same in the "hybrid” zone.
Scott & i have published a design that is such a “hybrid" so he has some experience.
The plan/pdf attached is the hybrid, the family can be seen here, to see how the details of the ML-TL & Voigts compare.
dave
Outdoor measurement of a 6 foot long ( external, minus end caps ) sonotube. Port at one end, driver at the other.
Precisely so. I built two MTM 'MLTL' systems in 1990-1992 and performed quite some measurements at the ports that were situated in the bottom panel. Things didn't behave like true BR, but acted just fine altogether. This of course was before the WWW and the mathcad-sheets were available. And I was too busy with other things to pull out my acoustics text books to reconstruct.
My explanation then was (without going the math way) that the normal requirements for a Helmholtz resonator weren't met inside the speaker. Instead, the load on the port (the impedance) was more complex due to the line interacting with it. Must admit I designed quick and dirty then: build something, measure it's output and enjoy the music, but I always liked the results of such systems. Floor coupling of the port played a role too though, and that aspect (the elephant IS the room) one shouldn't omit in the equation.
After reading this thread start to finish, several times, I am not any smarter than when I started.
So help me out on this with a simple example:
Let's say we have a 60 liter box with a traditional shape of 1:1.6:2.6 (internal dimensions 9.6" x 15.3" x 24.9"). I want to tune this box to 30 Hz with a 3" diameter vent located very near the woofer. My calcs say I need a 7.8" long vent.
Now let's say I have a 60 liter box that is very tall and slim with a shape of 1:1.3:5.5 (internal dimensions of 8" x 10.4" x 44"). I want to tune this box to 30 Hz. The woofer is at the top, the vent is at the bottom. What length should I use? If you rely on my simplistic calculations and utilize a 7.8" long vent, what will be my actual tuning frequency?
So help me out on this with a simple example:
Let's say we have a 60 liter box with a traditional shape of 1:1.6:2.6 (internal dimensions 9.6" x 15.3" x 24.9"). I want to tune this box to 30 Hz with a 3" diameter vent located very near the woofer. My calcs say I need a 7.8" long vent.
Now let's say I have a 60 liter box that is very tall and slim with a shape of 1:1.3:5.5 (internal dimensions of 8" x 10.4" x 44"). I want to tune this box to 30 Hz. The woofer is at the top, the vent is at the bottom. What length should I use? If you rely on my simplistic calculations and utilize a 7.8" long vent, what will be my actual tuning frequency?
Been writing as time permits, so hopefully all will become clear from a technical POV once I finish, though markbakk's experience pretty much 'tells the tale'. 😉
Your answer and probably what I should have initially posted is that these elongated BRs can be viewed as internally extending the vent, ergo for a given vent area/length, the higher the box aspect ratio = the lower its tuning [Fb].
Your answer and probably what I should have initially posted is that these elongated BRs can be viewed as internally extending the vent, ergo for a given vent area/length, the higher the box aspect ratio = the lower its tuning [Fb].
I wasn't. The information the worksheets give does not seem to demonstrate whether there is a difference between the two cases.