Hello everyone
I need to calculate the area of a rectangular aperture (or Vb needed for a given Fb and port area) for an old style Bass Reflex box.
The port will not be ducted nor will it be close to any wall of the cabinet, i.e. just below the woofer. Consider a 10-by-4-inch square opening under a 12-inch woofer, for example.
Now I'm sure there is some final correction and the virtual length will be more than just the wall thickness (air viscosity and other factors).
So what would be an estimate to add to the wall thickness?
Thanks
JJT
I need to calculate the area of a rectangular aperture (or Vb needed for a given Fb and port area) for an old style Bass Reflex box.
The port will not be ducted nor will it be close to any wall of the cabinet, i.e. just below the woofer. Consider a 10-by-4-inch square opening under a 12-inch woofer, for example.
Now I'm sure there is some final correction and the virtual length will be more than just the wall thickness (air viscosity and other factors).
So what would be an estimate to add to the wall thickness?
Thanks
JJT
There is no need of correction for wall thickness in Gilbert Briggs' traditional reflex tuning formula contained within his "Loudspeakers" book:
f² = (2700A) / V(L + 0.96√A)
where:
f = enclosure resonance frequency
A = area of vent or pipe in square inches
V = volume of cabinet in cubic feet
L = thickness of cabinet wall, or length of pipe, in inches.
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For the record, I'll add that in traditional tuning the enclosure resonance frequency was intended to equal the free air resonance frequency of the driver.
However, that was one of those 'rules' which could usefully be broken in order to obtain the best bass response from a particular driver.
Of course, we're talking of the days before we were blessed with the knowledge of a driver's T/S parameters.
However, that was one of those 'rules' which could usefully be broken in order to obtain the best bass response from a particular driver.
Of course, we're talking of the days before we were blessed with the knowledge of a driver's T/S parameters.
It doesn't have to boom. Remember those guys did basically @GM's oft-repeated battery/momentary switch "click test" damping with fabric over the port and/or over the rear of the driver to "de-boom" things.
Can't recall source, but I once found a sort of "boom limit" max size swag in one of the old papers somewhere (perhaps someone else recalls?)...that still lives in one of my spreadsheets.
Vb > 10VasQt^2 [L]
It's not a black/white hard delineation, but often it's a useful guideline for these type of designs.
Love your initiative--big-efficiency, big-cab, big-port boxes are still big-fun--hope yours is, too!
Can't recall source, but I once found a sort of "boom limit" max size swag in one of the old papers somewhere (perhaps someone else recalls?)...that still lives in one of my spreadsheets.
Vb > 10VasQt^2 [L]
It's not a black/white hard delineation, but often it's a useful guideline for these type of designs.
Love your initiative--big-efficiency, big-cab, big-port boxes are still big-fun--hope yours is, too!
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@Mosquito
I have found a note I made many moons ago relating to traditional reflex tuning using a tube or rectangular tunnel.
This time a length correction is included.
The information is from the book "High Quality Sound Reproduction" by James Moir:
Le = A (2150²/f²V - 1/√A)
L = Le - 0.41D
where,
Le = effective length of tube or tunnel in inches
L = physical length of tube or tunnel in inches
D = diameter of tube or equivalent diameter of tunnel in inches
f = enclosure resonance frequency in hertz
A = area of tube or tunnel in square inches
V = enclosure volume in cubic inches
(The volume of the tube or tunnel was not to be subtracted from the volume of the enclosure to obtain the effective volume.)
I'd be interested in knowing how the result of Moir's method compares with that of Briggs.
I have found a note I made many moons ago relating to traditional reflex tuning using a tube or rectangular tunnel.
This time a length correction is included.
The information is from the book "High Quality Sound Reproduction" by James Moir:
Le = A (2150²/f²V - 1/√A)
L = Le - 0.41D
where,
Le = effective length of tube or tunnel in inches
L = physical length of tube or tunnel in inches
D = diameter of tube or equivalent diameter of tunnel in inches
f = enclosure resonance frequency in hertz
A = area of tube or tunnel in square inches
V = enclosure volume in cubic inches
(The volume of the tube or tunnel was not to be subtracted from the volume of the enclosure to obtain the effective volume.)
I'd be interested in knowing how the result of Moir's method compares with that of Briggs.
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At least on this side of the Atlantic, a lot of the old papers (Planer & Boswell included) funnel down through Hoekstra on the way to Thuras. For posterity, here's Moir's derivation and citations...and Planer & Boswell's while I'm at it.
Attachments
Yeah, ostensibly. This "old school" reflex region of one-wall-long, Sd-sized Ap's (actual "ports" as opposed to ducts) shakes-out to need very large cabinet volumes and they ALL boom--that's the desired resonance working for us to gain the efficiency which was (at least part of) the original intent. They ALL then, need extra means of damping to knock-down the low peak (and shallow-up the rolloff, potentially, as damping approaches critical). The "boom limit" guideline is as arbitrary as any, but for some drivers has proven a reasonable place to start. So what everything becomes is the same old juggling exercise, just in a slightly different "realm" than is more common these days. As always, then, it's about what your priorities and goals are and how big of a box you can accept. Do you prize efficiency? Sd-sized ports no matter what? Equal impedance humps (resistive/less-reactive load for amp)? Damping? Box size? Ports (apertures) instead of ducts? Notice none of those even concern extension. Your answers become foundation for how you choose to treat the "boom" and what you think of the value of the guideline, IMO. The other thing is that these all get so big that the standing waves are in the passband, too. If were making tall boxes anyway, those'll need damping so it's best to ensure that you don't treat it all just as a big "dumb" compliance like the old papers did (even Moir's later papers, with the benefit of hindsight from 2024), omit those.
It's a different thing if you just want to check-out the "type" and see what it's about. Then I say build 'everything
OK, so it's how/where you choose to "cheat" to make an acceptable sized box. If you stay Sd = Ap, it's easy to approach 700L for a one-wall thick port. Faced with that, I'd probably compromise and set a 0.75*Sd max for Ap, personally, and also "cheat" by converting the port to some amount of short "duct" (like you're referring to, if I'm following right). I wouldn't feel happy about it, either, because I like big short ports Others will make different choices. It's how big a box you can stand which means what else it has to go with and where...or what size lumber you already have. Brace the crap out of it and it'll all come down to manual/physical damping, then. Play around with filling in HR and see what you think. You can get some 20's out of that thing with a big box...which wouldn't be the slam but it would bring some effortlessness to the energizing. Overdamping it a bit might get more of the other presentation. Any thoughts on the size yet?
Others will make different choices. It's how big a box you can stand which means what else it has to go with and where...or what size lumber you already have. Brace the crap out of it and it'll all come down to manual/physical damping, then. Play around with filling in HR and see what you think. You can get some 20's out of that thing with a big box...which wouldn't be the slam but it would bring some effortlessness to the energizing. Overdamping it a bit might get more of the other presentation. Any thoughts on the size yet?
Not exacltly, I dont think I can deal with any box bigger than 5 cu ft. And I want to avoid any duct other the 3/4 inch wall material, so if it's to cheat it will be tolerate some peaking (2dB max ), be happy with 45/47 Hz F3, calculate for Ql 5 instead of Ql 7, use a good 10" woofer if 12 inchers would fit not. The area of the port choosed also as a matter of visual likeness, not ridiculous small, nor exagerated big respect to the woofer size/box aspect ratio. Of course bracing is considered, we dont want big back panels screwed at the edges acting as a sort of passive radiator
The first loudspeaker I built comprised a 5 cu ft enclosure tuned to the 40 Hz resonance frequency of a 12" Fane driver.
I used a rectangular port of large area and of decent length - I can't remember the exact dimensions - and the internal walls were lined with carpet felt.
The fully rounded bass notes went way down low with no trace of boominess. It was the kind of bass you can feel in your chest.