1-Time alignment?
2-Slot vent port?
3-frequency speed/second?
1-I am looking at putting a JBL 2/2245 and 2/2118 in a Dunlavy style design.
I would like to step the drivers forward/back for there proper frequencies...
EG,-if the 2245 is crossed over at 100hz and the 2118 at 400hz(or 2nd scenario...2245 crossed at 160hz and 2118 at 800hz) on 12db or 24db slopes,should the 2245 be a inch more forward or is all revelant to the VC being aligned??.
2-If a 2245 is in a cabinet with inside dimensions of 20"wX28"dX36"h,how does one calculate a front or rear firing slot vent ?Obviously it will be 20"w,but if we want the frequency to be 20hz,what is the width n height?
3-How to calculate 20hz/s and its arrival time at 12ft?
or
800hz?
thanks
opps-just noticed the title
2-Slot vent port?
3-frequency speed/second?
1-I am looking at putting a JBL 2/2245 and 2/2118 in a Dunlavy style design.
I would like to step the drivers forward/back for there proper frequencies...
EG,-if the 2245 is crossed over at 100hz and the 2118 at 400hz(or 2nd scenario...2245 crossed at 160hz and 2118 at 800hz) on 12db or 24db slopes,should the 2245 be a inch more forward or is all revelant to the VC being aligned??.
2-If a 2245 is in a cabinet with inside dimensions of 20"wX28"dX36"h,how does one calculate a front or rear firing slot vent ?Obviously it will be 20"w,but if we want the frequency to be 20hz,what is the width n height?
3-How to calculate 20hz/s and its arrival time at 12ft?
or
800hz?
thanks
opps-just noticed the title
2) The info needed for a port are: box volume, surface area and length of the port. If the surface is rectangular or circular doesn't matter at all. Find a calculator for a tube and do the math to find the other dimension. What I found in the past however is that the port needs to be shorter than calculated.
3) The sound velocity in air is a constant(*) whatever is the wavelength.
(*) It depends slightly on the pressure and temperature of the air.
3) The sound velocity in air is a constant(*) whatever is the wavelength.
(*) It depends slightly on the pressure and temperature of the air.
#1 use measurement is better but you can use the distance between the front and the voice coil and make the difference of the 2 drivers.
I like this picture from wolf : Woofer Acoustic Center Offset, 25-30mm typical. photo by wolf_teeth_speaker | Photobucket
Yes it is better to time align
I like this picture from wolf : Woofer Acoustic Center Offset, 25-30mm typical. photo by wolf_teeth_speaker | Photobucket
Yes it is better to time align
The locations of the acoustic centers that can be time-aligned are very hard to predict. You can try to guesstimate, but for a project of this scale you should really measure on a test baffle to determine the real offset. In any case, there's nothing to calculate until you know the offset you're starting with.
The first question, though, is how you're trying to align them. You can do it so the centers act like an arc shape that is perfectly aligned at a preferred listening distance, on a plane that gets closer to perfect the farther away you are, or somewhere in between.
And then there's crossover phase shift... You could always just time align with active delay, too.
Slot ports, unless you hang them out in the middle of a panel, will tend to act much longer than they actually are. They sort of extend themselves out along walls they share with the cabinet, and also the floor if you put them at floor level.
The first question, though, is how you're trying to align them. You can do it so the centers act like an arc shape that is perfectly aligned at a preferred listening distance, on a plane that gets closer to perfect the farther away you are, or somewhere in between.
And then there's crossover phase shift... You could always just time align with active delay, too.
Slot ports, unless you hang them out in the middle of a panel, will tend to act much longer than they actually are. They sort of extend themselves out along walls they share with the cabinet, and also the floor if you put them at floor level.
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#1 use measurement is better but you can use the distance between the front and the voice coil and make the difference of the 2 drivers.
I like this picture from wolf : Woofer Acoustic Center Offset, 25-30mm typical. photo by wolf_teeth_speaker | Photobucket
Yes it is better to time align
Voice coil and acoustic center won't always match. I still use the gated single pulse measurement method.
mclsound,
If you use symmetrical acoustical Linkwitz-Riley slopes then you only need to align the diaphragms (not the voice coils, but the actual membrane that will produce the sound) and you should get a perfectly phase-coherent acoustical crossover (which is the best thing you could obtain with an IIR filter: forget about impulses alignment if you don't use FIR, as it will lead to poor phase coherency and bad behavior at and around the crossover point...).
No need for a very precise alignment at these frequencies tho: even if misaligned by several centimeters you should still get an in phase summation at most listening angles: at 400Hz a wavelength is almost 1m.
The 2245H is well behaved and remains quite flat up to ~1khz, so the electrical slope of your crossover will result in an almost identical acoustical slope.
The 2118H will not be as easy to deal with: it will have some rolloff (depending on the volume of the closed enclosure) that you will have to take into account when choosing an electrical slope in your crossover. You will typically choose a shallower electrical slope to get the acoustical slope you where looking for.
You can also play will Linkwitz transforms (in one direction or the other, as long as you know what you get at the end!) or EQ.
Ultimately you would need to measure to known exactly what you get (and this is not an easy task at these frequencies...).
If you use symmetrical acoustical Linkwitz-Riley slopes then you only need to align the diaphragms (not the voice coils, but the actual membrane that will produce the sound) and you should get a perfectly phase-coherent acoustical crossover (which is the best thing you could obtain with an IIR filter: forget about impulses alignment if you don't use FIR, as it will lead to poor phase coherency and bad behavior at and around the crossover point...).
No need for a very precise alignment at these frequencies tho: even if misaligned by several centimeters you should still get an in phase summation at most listening angles: at 400Hz a wavelength is almost 1m.
The 2245H is well behaved and remains quite flat up to ~1khz, so the electrical slope of your crossover will result in an almost identical acoustical slope.
The 2118H will not be as easy to deal with: it will have some rolloff (depending on the volume of the closed enclosure) that you will have to take into account when choosing an electrical slope in your crossover. You will typically choose a shallower electrical slope to get the acoustical slope you where looking for.
You can also play will Linkwitz transforms (in one direction or the other, as long as you know what you get at the end!) or EQ.
Ultimately you would need to measure to known exactly what you get (and this is not an easy task at these frequencies...).
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