I need help understanding this equation
L effective = 7.5” + 0.6 x r effective
7.5" is the depth of the cabinet wings. what is (or why use) 0.6? and What is r effective mean?
in the example r effective is defined as 9". I don't see where this number comes from, but then again there is the distinct possibility that i may be an idiot.
Thanks in advance.
L effective = 7.5” + 0.6 x r effective
7.5" is the depth of the cabinet wings. what is (or why use) 0.6? and What is r effective mean?
in the example r effective is defined as 9". I don't see where this number comes from, but then again there is the distinct possibility that i may be an idiot.
Thanks in advance.
The equation is taken from the design of a port for a bass reflex speaker. When you have a port in a bass reflex speaker it has a physical lenght and a slightly longer effective length. End corrections are applied to the physical length representing the acoustic impedance of the air load at the port opening at each end. The same correction is used when designing the open end of a TL. I borrowed this equation to add an end correction to the U and H frame lengths. For a port open at only one end the equation I used is shown below.
L_effective = L_physical + 0.6 x r
where r is the radius of the equivalent circle that has an equal area. So for the U or H frame
Area = 16" x 16" ~ 3.1415 x (9 in)^2
L_effective = 7.5 in + 0.6 x (9 in) = 12.9 in
Fortunately the MathCad U and H frame worksheets do this correction automatically so all you enter is the physical length that you measure with a ruler.
Hope that helps,
L_effective = L_physical + 0.6 x r
where r is the radius of the equivalent circle that has an equal area. So for the U or H frame
Area = 16" x 16" ~ 3.1415 x (9 in)^2
L_effective = 7.5 in + 0.6 x (9 in) = 12.9 in
Fortunately the MathCad U and H frame worksheets do this correction automatically so all you enter is the physical length that you measure with a ruler.
Hope that helps,
Thanks Martin for the quick answer I have an idea for an OB project and was going to figure out if I can cross it over where I would want given the baffle shape and size but couldn't figure out what those numbers were.
Since I am not versed in all of the math the equation didn't make sense to me where it for others probably made perfect sense. Or in other words my suspicion that I'm an idiot was not that far off.
I haven't gotten around to buying your worksheets yet so plugging them in wasn't an option either. I will eventually though.
Since I am not versed in all of the math the equation didn't make sense to me where it for others probably made perfect sense. Or in other words my suspicion that I'm an idiot was not that far off.
I haven't gotten around to buying your worksheets yet so plugging them in wasn't an option either. I will eventually though.
MDF vs PLY
It may well have been mentioned before ...
Gilbert Briggs, in the 1950s when OB's were popular in the UK, recommended a sand-filled baffle. Thin ply front and back with a one inch gap filled with fine, dry sand. Heavy and well damped. Commercially not a brilliant idea (all those transport costs) but ideal for DIY.
It may well have been mentioned before ...
Gilbert Briggs, in the 1950s when OB's were popular in the UK, recommended a sand-filled baffle. Thin ply front and back with a one inch gap filled with fine, dry sand. Heavy and well damped. Commercially not a brilliant idea (all those transport costs) but ideal for DIY.
MJK, I am currently building the open baffle design with an Alpha 15A and FE108E Sigma.
Would you recommend adding a U or H frame as discussed in your latest article?
A person here has given it a try and reports positive results: http://www.audiocircle.com/circles/index.php?topic=52633.0
Would any changes to the crossover be required?
Would you recommend adding a U or H frame as discussed in your latest article?
A person here has given it a try and reports positive results: http://www.audiocircle.com/circles/index.php?topic=52633.0
Would any changes to the crossover be required?
I think pairing the FE-108EZ and the Alpha 15A would be my first choice if I were going to use a Fostex driver. The only thing to be concerned about is that the FE-108EZ may be a bit too efficient and require padding down for the OB design.
Adding a H or U frame, I would use the H frame, will extend the bass from the Alpha 15A down lower but at the expense of efficiency. You would need to pad down the FE-108EZ even more and this might require a crossover redesign.
The combination of these two drivers has real potential but would require measurements and some additional designing to get the system just right. The resulting crossover would probably be a slight variation on the one I originally proposed.
Adding a H or U frame, I would use the H frame, will extend the bass from the Alpha 15A down lower but at the expense of efficiency. You would need to pad down the FE-108EZ even more and this might require a crossover redesign.
The combination of these two drivers has real potential but would require measurements and some additional designing to get the system just right. The resulting crossover would probably be a slight variation on the one I originally proposed.
MJK said:
IMO, the Fostex FE108E Sigma is not suitable for your OB design unless you use active EQ. The 108 has a terrible frequency response and has a big hole right at the crossover frequency. Perhaps that’s why Ron prefers the 87 to 108. Anyway, I’m currently using the FE108E Sigma actively crossed over to a pair of Lambda Dipole 15 inch drivers. I couldn’t find any combination of crossover frequencies where the baffle hump was worse than the FR of the 108! The smoothest crossover was with both drivers set just below the baffle hump at 238 Hz. And with active EQ, the sound is very, very good.
-Rich
Hi James,jameshillj said:
I'm using the LR24 crossover set at 238 Hz on my Behringer Ultradrive DCX2496 crossover. The DCX2496 is overkill; the EQ on the DCX2496 didn't have fine enough EQ adjustment nor did I need independent crossover points. I am now using a Behringer Ultracurve DEQ2496 for the EQ in addition to the DCX2496 crossover. If I were starting over, I'd probably just buy a straight LR24 crossover without all of the "bells and whistles" and a DEQ2496 for the EQ.
-Rich
What sort of an L-pad would i need for padding down FE208EZ's in the MJK baffle?
Which of the three at the link below would be suitable?
http://www.soundlabsgroup.com.au/c/Level-Control/L-Pads.html
Which of the three at the link below would be suitable?
http://www.soundlabsgroup.com.au/c/Level-Control/L-Pads.html
wixy said:
Hi Wixy, yowza! The 97db FE208E-Sigma in the MJK OB seems like it goes against the spirit of the design.
One of MJK's key insights is to choose a fullranger that is a good efficiency match for the woofers and overall design. (This is why he chose the FE103E at 89db efficiency.) His earlier OB's 98db Lowthers were almost "too efficient" making it harder for the woofers to keep up, necessitating 2 woofers per side, active crossover etc. (though he could have opted to pad down the Lowthers but that seems wasteful).
Won't you need a tweeter with the FE208E-Sigma? So now this seems like you're taking another hit here -- padding down a super-efficient driver, but then still having to add a tweeter.
I can't recommend an LPad except to say you probably should get one rated at high watts, because you'll be pushing more watts through them (because you're throwing away 8db of efficiency, hence more watts will be needed -- disclaimer: I'm new at this and might be completely wrong.)
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