I'll keep that in mind next time I make a rocket powered subI've been thinking about this for the last 24 hours...
There are too many variables to consider for a common DIY'er to even bother with. I may be able to sneak a design into the physics lab supercomputer but I doubt it's worth it. I'd get one answer for one set of variables, then you take the box out in the real world and temperature variations and humidity would probably have more effect on the sound than what the computations would predict. I modeled variations of between 20m/s and 40m/s over wood using a friction coefficient of .5 (smooth) and the layer grew from .25" at 20m/s to 2.8" at 40m/s. But as we are considering a pressure wave moving through the already somewhat turbulent flow in a horn throat....pressure, viscosity, bla bla bla (skip to end) I would predict variations on the scale of +- 3-7db. in the HF range of the horn.
I think a good rule o thumb is above 30m/s below 70"^2 strange things happen.
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I was wondering if we can extend the 30m/s rule of thumb to smaller throats as well (for smaller horns with smaller drivers).
Also, how much difference does the aspect ratio of the port make? (A very elongated rectangle vs square?) Just in general. No need to pull the supercomputer out.
That would be a cool thing to see.LoL! Then you would have "fast" bass.
Had a thought, Has anyone ever heard of a test where someone replaced the side of a horn with Lexan and introduced smoke into the cabinet.
I was thinking of doing this and recording it with a high speed camera.
Mylar tape, like VCR or cassette tape, also responds well to air movement, might be easier to see and implement than smoke.
A bunch of streamers taped to wires in various places throughout the throat area would give some idea of flow, might lead to some small changes that would make larger sonic differences.
* the aspect ratio make a lot of difference, a squared port is optimal and will have the least friction / compression because of the ratio of port surface area to volume. Circular ports have even less surface area per volume.example: Three ports with 160in^3 volume and 16in^2 mouth area.
4"h x 4"w x 10"L has 160in^2 of surface area
1"h x 16"w x 10"L has 340in^2 of surface area
circular port with 4.512" diamater opening and 10"L has 140.179in^2 of surface area
*No supercomputers where used in the computation of this answer.. well one, but it was a small one.
edit: forgot about the first question,
"extend the 30m/s rule of thumb to smaller throats"
No. Do you have Flair-it? Flare-it - Free Speaker Design Software This will give you Basic guidelines for how fast air can move in small spaces.
But remember, this gives values for very smooth surfaces in a circular port. A square port has ~13% more surface area.
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The specs on the BC18SW115 are amazing but well out of my price range. Do you have any experience with the Dayton Ultramax UM18? That was my first choice but I am not sure of the cone stiffness. My TH is a little compact which might increase the compression ratio.
In my first job interview for a loudspeaker design position I made the "mistake" of referring to something like that. The VP of Engineering said "there's no such thing, because the woofer is not reproducing the high frequencies." I replied "well, that's true, but it's a kind of shorthand, we're really talking about the perception thereof, which is the transient response." He hired me
That seems intuitively correct to me-there is a lot of pressure in there, and if the cone is not strong it will not be pistonic. This is with the caveat that the airflow is not so dramatic that it becomes turbulent, or nonlinear.
I wouldn't worry about cone stiffness with the UM. What I would worry about is that it's a very low fs driver that likes really big boxes. It would not be my first choice for a compact TH.
No.
But after a subdural hematoma, my head now has a titanium patch, which from my experience should be quite stiff.
Cheers,
Art
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