When does a normal vented box become a "Big Vent Reflex?
I'm considering a design for my Eminence Kappalite 12" that looks like it requires a "huge" vent, 550 cm^2, which is slightly more than the Sd of the driver. Would this qualify as a BVR?
I've noticed during the design process that there's an "optimum" vent CSA and length to achieve the widest passband and extended response. Go larger and the usable bandwidth is reduced due to the increased output at the first harmonic resonance. Go smaller and particle velocity in the vent increases and the first harmonic resonance can start to be a problem as well. In this case, it seems that a vent with a csa of 550 cm^2 is the best fit for nulling out that first harmonic resonance (with suitable positioning of the driver, of course).
BTW, this is a design that I'm considering building to answer a hypothesis of mine - that vented boxes don't sound as good as THs or other 1/4 wave resonators with same or similar Fb and F3 because of the csa of the element radiating at low frequencies (be it the mouth of the horn or the vent). I tried using a larger vent in this model, but that resulted in the bandwidth starting to be restricted because of the first harmonic resonance impacting the overall response.
I'm considering a design for my Eminence Kappalite 12" that looks like it requires a "huge" vent, 550 cm^2, which is slightly more than the Sd of the driver. Would this qualify as a BVR?
I've noticed during the design process that there's an "optimum" vent CSA and length to achieve the widest passband and extended response. Go larger and the usable bandwidth is reduced due to the increased output at the first harmonic resonance. Go smaller and particle velocity in the vent increases and the first harmonic resonance can start to be a problem as well. In this case, it seems that a vent with a csa of 550 cm^2 is the best fit for nulling out that first harmonic resonance (with suitable positioning of the driver, of course).
BTW, this is a design that I'm considering building to answer a hypothesis of mine - that vented boxes don't sound as good as THs or other 1/4 wave resonators with same or similar Fb and F3 because of the csa of the element radiating at low frequencies (be it the mouth of the horn or the vent). I tried using a larger vent in this model, but that resulted in the bandwidth starting to be restricted because of the first harmonic resonance impacting the overall response.
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Ports 0.75*Sd on up weren't uncommon for the first ~30 years of the Phase Inverter (based on a radiation resistance argument). Jensen trademarked the term Bass Reflex but enforcement would seem insufficient. Openings were called ports and ducts were called ducts. It seems succeeding generations need to rename everything they "discover"... Plach & Williams wrote when more or less than Sd might be useful.
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What doeas a bvr do better than a br apart from managing particlevelocity better? (Idealy a br should do this as well). Seems there should be some difference for the two to merit their own abbreviations right?
What does the flow shift look like (laminar to turbulent), is this a rapid shift or a gradual shift, the reason for asking is that if it is a gradual shift it would be interesting to see the at which input power the system deviates from the idealised behaviour.
This could be (speculatively) one benefit of going bvr instead of br, if the flow starts to gradually shift to turbulent before we reach the particle velocity normally stated to be "to high" (17m/s?)
What does the flow shift look like (laminar to turbulent), is this a rapid shift or a gradual shift, the reason for asking is that if it is a gradual shift it would be interesting to see the at which input power the system deviates from the idealised behaviour.
This could be (speculatively) one benefit of going bvr instead of br, if the flow starts to gradually shift to turbulent before we reach the particle velocity normally stated to be "to high" (17m/s?)
I previously did a quick checkup on the underlying theories commonly used to simulate vented systems in a thread over at speakerplans, and it seems that the commonly used decomposition (Helmholz-Hodge) is derived from the far more comprehensive Navier-Stokes equations.
A decomposition is basically a simplification, but in this case it also comes with a set of prerequisities that sets it apart from Navier-Stokes, that it is only valid for describing an incompressible, irrotational and harmonic flow, which can only be the case if we steer clear of to high (non laminar) particle velocity, which a bvr should at around Apt=Sd.
I'm not sure I got this right though so please correct me if I'm wrong about it.
A decomposition is basically a simplification, but in this case it also comes with a set of prerequisities that sets it apart from Navier-Stokes, that it is only valid for describing an incompressible, irrotational and harmonic flow, which can only be the case if we steer clear of to high (non laminar) particle velocity, which a bvr should at around Apt=Sd.
I'm not sure I got this right though so please correct me if I'm wrong about it.
Well... I can certainly agree that the common "Bass reflex" very frequently feature underdimensioned ports.
But the lines get blurry with all these names for vented design where you do take into account things like turbulence and air speed.
Would just like to have it noted that my experience is the polar opposite, well, as long as the ports are properly dimensioned, properly placed and "front firing".
A well made vented box provides significantly better sound quality, and often with less need for corrections and filters.
A poorly made vented box however...
But the lines get blurry with all these names for vented design where you do take into account things like turbulence and air speed.
Would just like to have it noted that my experience is the polar opposite, well, as long as the ports are properly dimensioned, properly placed and "front firing".
A well made vented box provides significantly better sound quality, and often with less need for corrections and filters.
A poorly made vented box however...
With fresh memory of a different day, I think the BVR term was found useful to convey the true operation of such designs in a gray-area-space where many things were (still are?) called "horns".
Historically, big vent reflex [BVR] is TTBOMK a moniker I coined to differentiate between BR and the traditional ~1/8 WL horn vented reflex designed to boost the [mid] bass and the traditional wide BW 1/4 WL BLH that folks were [mis]labeling on all these types.
Since Thuras's BR patent used a vent area of Av = Sd, I've never considered this big a vent to be a BVR, though with the pipe lengths required for today's low Vas, tunings, one can make an argument for calling them a big vent transmission line [BVTL] 😉, though personally prefer morphing the summed net volume [Vb] into an inverse tapered TL [TQWT] or with added stub vent for fine tuning, MJK's ML-TQWT.
Since Thuras's BR patent used a vent area of Av = Sd, I've never considered this big a vent to be a BVR, though with the pipe lengths required for today's low Vas, tunings, one can make an argument for calling them a big vent transmission line [BVTL] 😉, though personally prefer morphing the summed net volume [Vb] into an inverse tapered TL [TQWT] or with added stub vent for fine tuning, MJK's ML-TQWT.
Right, the pipe/horn's 3rd harmonic dip is the upper limit, ditto for end loaded TH, so basically a two octave device whereas an offset driver TL/TH can do an extra ~ half octave and full on compression horn, ~ three octaves.
If the Hornresp ODOV sim is correct, and I have no reason to doubt it, it should be possible to get the same cancellation of that 3rd harmonic dip in a simple vented box, by careful choice of vent dimensions and driver location, which I've done with this particular design. It might be possible to suppress it even further, but I figure pushing it 15 dB below Fb (and in a Ql=7 model too) should be enough to get a nice smooth 40~200 Hz response, in a box that's less complicated to build or fit into a particular shape than the ODTL and TH designs. However, go much larger or smaller with the vent, and it becomes impossible to null out that 3rd harmonic by repositioning the driver.
IMO it's vent placement that can be the trickiest bit, the driver is usually moved about to get in a relatively good position, vent location is more like an afterthought.
what a great thread!! (edit/ i can't get pics to upload here) i think (i'm positive ) it's hidden in this but it's all based on the odd harmonic /standing wave that traverse..
each parts of a sphere/circle which in math can be seen as platonic solids just like the old greek and egyptian maths did it all long ago. it's so clever yet nostalgic. ironic when you find the age old ancecetry math is the modern answer. not a sores shert or calculator.. just horn respond and graph osier with isometric views in 1/4 as 30/60/90 degrees is on that. it's the math in much easier for digestion . and it's visual.
. i found the high order qw answer to everything this way. it's perfect! over and over.. but it's different if massloaded and the ratios are just outta my grasp. i've had a little bit of luck as a bow tie or bevy tutu shape , but nothing consistent like straight pipe and the pipe physics says why.. node abd harmonics w only pure ones not the taper or flare endless umbrella of them. it's a hard pill to swallow. but it's just the plain view to see..aand simple is always perfect ..
i cannot build /design a flawed dual compound parallel /series TL now. but i cannot quite get the mass loaded varieties into this.
typing sucks but the math is all in platonic solids as it then stages into the circle. nodes and anti are a hexagon overlaid on a circle you see this as a hexagon is 6 circunferece(perimeter) the circle is 5.236 per 5 of the hexagon sides (6 parts each). doesn't sound like much, but next compare the volumes? and also add up the parts as degres in the circle (trig, calculus whatever it's just the unit circle. google it . pinterst has pics . you'll see. i promise it will help and you'll grab it..
4/3 pi r^3. volume spherre.
c = pi r
compare the cube and sphere as 5.236.
now look at 6 parts of a perimeter of a hexagon. now look at 12 individual parts of a circles. there's 3.143 at 30 degrees, 31.43 at 300 and there the hint . because its .2618 .5236 and 1.0472, and keep go in g at 180 , at 330... it's all there
in a cube or hexagon(overlaid. this is the start of the answer. it's too much to type ..tilt the second hexagon at the velocity pressure .
each parts of a sphere/circle which in math can be seen as platonic solids just like the old greek and egyptian maths did it all long ago. it's so clever yet nostalgic. ironic when you find the age old ancecetry math is the modern answer. not a sores shert or calculator.. just horn respond and graph osier with isometric views in 1/4 as 30/60/90 degrees is on that. it's the math in much easier for digestion . and it's visual.
. i found the high order qw answer to everything this way. it's perfect! over and over.. but it's different if massloaded and the ratios are just outta my grasp. i've had a little bit of luck as a bow tie or bevy tutu shape , but nothing consistent like straight pipe and the pipe physics says why.. node abd harmonics w only pure ones not the taper or flare endless umbrella of them. it's a hard pill to swallow. but it's just the plain view to see..aand simple is always perfect ..
i cannot build /design a flawed dual compound parallel /series TL now. but i cannot quite get the mass loaded varieties into this.
typing sucks but the math is all in platonic solids as it then stages into the circle. nodes and anti are a hexagon overlaid on a circle you see this as a hexagon is 6 circunferece(perimeter) the circle is 5.236 per 5 of the hexagon sides (6 parts each). doesn't sound like much, but next compare the volumes? and also add up the parts as degres in the circle (trig, calculus whatever it's just the unit circle. google it . pinterst has pics . you'll see. i promise it will help and you'll grab it..
4/3 pi r^3. volume spherre.
c = pi r
compare the cube and sphere as 5.236.
now look at 6 parts of a perimeter of a hexagon. now look at 12 individual parts of a circles. there's 3.143 at 30 degrees, 31.43 at 300 and there the hint . because its .2618 .5236 and 1.0472, and keep go in g at 180 , at 330... it's all there
in a cube or hexagon(overlaid. this is the start of the answer. it's too much to type ..tilt the second hexagon at the velocity pressure .
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i'm not picasso but this is math with colors. it's solid , follow it as 90 degrees instead of 360 (qw instead of compounded 360). i think it'll fall into place . those are nodes at every other hexagon corner and this is 30/60/90/130/150/180 and trig is in .349/90, /180, /270, /360. it's pi (31.43 ) at 30, for a ~30-32 hz tune. tricky.. break it down to 1/7,7/7, 11/7, 22/7.. then you find 1/4, 3/4, 1 1-1/4... 28/7, 14/7, 7/7... 44/7, 49/7.... 13.9 billion/7 (14/7)is still a node. and so is 7.02969.., only 11 orders of operation later . this works all the at to sound .
