Hi.. Sorry if this is a common question (or a daft one), I had no joy from searching on the subject (although the flameproof jacket is already on just in case!)
I see a lot of designs for speakers with multiple drivers, and discussion over box dimensions and types, but I've not come across much information about how far apart the driver units should be placed (other than what is prescribed in someone's drawings).. I've noticed, however, that this is roughly (or maybe exactly??) as far apart as multiples of the radius of the drive unit.. so if I had a 6" mid driver the distance from the centre of the tweeter to the centre of the mid is 6", or 9".. or similar..
Does this make sense, and is there more to it? (like positive/negative reinforcement of the wavefronts) Does this apply to MTM configs? ..to Port placement?
Any general rules....
...or am I barking up the wrong floorstander...
Thanks in advance..
G
I see a lot of designs for speakers with multiple drivers, and discussion over box dimensions and types, but I've not come across much information about how far apart the driver units should be placed (other than what is prescribed in someone's drawings).. I've noticed, however, that this is roughly (or maybe exactly??) as far apart as multiples of the radius of the drive unit.. so if I had a 6" mid driver the distance from the centre of the tweeter to the centre of the mid is 6", or 9".. or similar..
Does this make sense, and is there more to it? (like positive/negative reinforcement of the wavefronts) Does this apply to MTM configs? ..to Port placement?
Any general rules....
...or am I barking up the wrong floorstander...
Thanks in advance..
G
It's the wavelength at crossover
I've always heard that multiple drivers (especially line arrays) need a center-to-center spacing of 1 wavelength of the crossover frequency or less.
So if you have multiple 6' woofers spaced edge-to edge, you would need to cross over to the tweeter at 2k or less. (roughly).
If you take the crossover frequency too high, say 3.5K, the woofers will begin to interfere with each other and you will get increasingly complex patterns of "lobing" with little thin jets of sound shooting out at certain angles, while other angles have basically silence coming out.
Of course in the far field, in a reverberant indoor room, it could be argued that a lot of the lobing smears together and balances out, so I think you could "cheat" these figures a bit.
I've always heard that multiple drivers (especially line arrays) need a center-to-center spacing of 1 wavelength of the crossover frequency or less.
So if you have multiple 6' woofers spaced edge-to edge, you would need to cross over to the tweeter at 2k or less. (roughly).
If you take the crossover frequency too high, say 3.5K, the woofers will begin to interfere with each other and you will get increasingly complex patterns of "lobing" with little thin jets of sound shooting out at certain angles, while other angles have basically silence coming out.
Of course in the far field, in a reverberant indoor room, it could be argued that a lot of the lobing smears together and balances out, so I think you could "cheat" these figures a bit.
I'm on my 6th line array project and I can tell you that the effects of lobing and comb filtering are vastly over stated. Scientifically they are there. Maybe using OB's with swept back wings makes a difference, but I've yet to attentuate the top end of the wide range mids that I use which go up to about 10Khz. The extreme top end may be attentuated somewhat by the effects of the array, but I add a super tweeter to fill in the top end. I use 4" drivers and on the 3 big arrays I've made, I used 4.25", 4.5", and 5" center spacing with only a slight difference in off axis response. From now on, however, I will put the drivers as close together as practical, so I can fit as many as possible in each array, since they're only $1.50 ea.
To me the very slight reduction in image focus of an array is more than offset by the "big" sound they create. Since you ask each driver to do very little, you can use very economical drivers and have super efficient speakers with great sound at a fraction of the cost a point source speaker with quality drivers.
To me the very slight reduction in image focus of an array is more than offset by the "big" sound they create. Since you ask each driver to do very little, you can use very economical drivers and have super efficient speakers with great sound at a fraction of the cost a point source speaker with quality drivers.
In David Weems' "Great Sound" book he describes discovering during the build of an MTM that the FR was better with the lower woofer farther away from the tweeter than the top one. I'm sure that was just for this particular design but it points out to me that it depends on the design and the drivers being used.
Drivers should be spaced on center no more than one wavelength apart at the highest frequency being reproduced, but there are exceptions. The main deviation would be for an MTM configuration; in that case the outboard elements should be spaced no more than one wavelength on center at the crossover frequencywith respect to the axis of the center driver.
As to the line-array question, this still holds true. Spacing further than 1 wavelength doesn't particularly affect horizontal dispersion and comb-filtering concerns, that being addressed by the vertical driver placement, but it does affect the integration of the various point sources into a coherent line source. That doesn't properly happen with spacing over a wavelength, which means that the nearfield/farfield radiation pattern will be fouled up.
As to the line-array question, this still holds true. Spacing further than 1 wavelength doesn't particularly affect horizontal dispersion and comb-filtering concerns, that being addressed by the vertical driver placement, but it does affect the integration of the various point sources into a coherent line source. That doesn't properly happen with spacing over a wavelength, which means that the nearfield/farfield radiation pattern will be fouled up.
OK.. so in a practical scenario, would you place the drive units close as possible and then tune the crossover accordingly?
I'm thinking of a set of folded transmission line speakers similar to the Capellmeister as a project, and was planning on an MMT configuration (i.e. inverted - with the tweeter below 2 mids), so there's more 'line' behind the mids for them to 'transmit' down.. Probably a daft idea but I can always remove the baffle and try something else I spose!
I'm thinking of a set of folded transmission line speakers similar to the Capellmeister as a project, and was planning on an MMT configuration (i.e. inverted - with the tweeter below 2 mids), so there's more 'line' behind the mids for them to 'transmit' down.. Probably a daft idea but I can always remove the baffle and try something else I spose!
Keep in mind there's some debate as to how to measure the actual spacing distance. Edge to edge is not 0 spacing and it's not cone edge to cone edge spacing nor is it center to center. Theoretically it's the spacing between where the sounds originate and what that is exactly is a matter of some debate.
If only it was that simple. The latest thinking is that it should be measured from the edge of the pressure zone where the impulse vectors (the direction the air is compressed/expanded in) are fairly similar. Nobody agrees on what constitutes similar though.johninCR said:
For a large rounded dustcap, this region is quite wide, and with a phase-plug it could be all over the place
Stick with centre to centre, it seems to work
Unless dealing with planars or flat pistons
Don't have digital pics of the best one so far that was built for a bar. Each channel consisted of:
16 BK drivers- 4ohm, 20watt, 4", $1.50ea, Fs 95, on OB they roll
off steeply below 150hz and above 9800hz and are reasonably
flat in between except for a gradual rise between 150 and 300
I let them run full range. Very close spacing.
1 Pyle Pro PDBT28- 1" Titanium Super Bullet Tweeter, 300watt,
104db, harshness tweaked out with 6 pencil eraser sized
pieces of foam rubber stuffed in the horn around the bullet plug.
Used the included capacitor and an Lpad. It's mounted in the
middle of the 16 driver array.
2 Nippon 12" woofers - 235watt, 6 coats of PVA glue to bring
the Fs from 31hz down to 28hz. They're set up in a W baffle
baffle at the bottom with push/pull alignment and a 1st order
roll-off above 125hz.
The baffle has a narrow front profile, 5" at the top and 9" at the
bottom. Baffle wings are swept back at 55 degrees. One is 12"
at the top and 22" at the bottom and the other is 4" shallower
to avoid cavity resonances. I have 3 2"x2" cross braces in the
back to minimize baffle vibrations and make nice handles for
moving. It's made out of 3/4" plywood over which I glued a
textured vinyl. I made a rounded grill out of thin expanded
steel that runs the length of the array painted with black epoxy
appliance spray paint, which fits into pre-routed grooves. After
putting on the vinyl, I ran an exacto knife the length of the
grooves and the vinyl holds the grill in place plus a few black
screws for security. I always put a layer of fiber fill and 1" foam
rubber in back of the drivers to damp the highs a little and cover
the ugly backside.
Hints- It's a fairly easy 2 day build. Cut and assemble the baffles
the first day. Wings at angles are tricky since you can't use
clamps, so get a helper to hold them and use screws. Solder
driver wires before mounting because it's a pain afterward.
Vinyl or another material can make an attractive easy and quick
finish, so you aren't tempted to leave it unfinished for months
or years. Do a test run with the drivers mounted before covering
just in case you have a cavity resonance and need to cut a little
off of the shorter wing.
16 BK drivers- 4ohm, 20watt, 4", $1.50ea, Fs 95, on OB they roll
off steeply below 150hz and above 9800hz and are reasonably
flat in between except for a gradual rise between 150 and 300
I let them run full range. Very close spacing.
1 Pyle Pro PDBT28- 1" Titanium Super Bullet Tweeter, 300watt,
104db, harshness tweaked out with 6 pencil eraser sized
pieces of foam rubber stuffed in the horn around the bullet plug.
Used the included capacitor and an Lpad. It's mounted in the
middle of the 16 driver array.
2 Nippon 12" woofers - 235watt, 6 coats of PVA glue to bring
the Fs from 31hz down to 28hz. They're set up in a W baffle
baffle at the bottom with push/pull alignment and a 1st order
roll-off above 125hz.
The baffle has a narrow front profile, 5" at the top and 9" at the
bottom. Baffle wings are swept back at 55 degrees. One is 12"
at the top and 22" at the bottom and the other is 4" shallower
to avoid cavity resonances. I have 3 2"x2" cross braces in the
back to minimize baffle vibrations and make nice handles for
moving. It's made out of 3/4" plywood over which I glued a
textured vinyl. I made a rounded grill out of thin expanded
steel that runs the length of the array painted with black epoxy
appliance spray paint, which fits into pre-routed grooves. After
putting on the vinyl, I ran an exacto knife the length of the
grooves and the vinyl holds the grill in place plus a few black
screws for security. I always put a layer of fiber fill and 1" foam
rubber in back of the drivers to damp the highs a little and cover
the ugly backside.
Hints- It's a fairly easy 2 day build. Cut and assemble the baffles
the first day. Wings at angles are tricky since you can't use
clamps, so get a helper to hold them and use screws. Solder
driver wires before mounting because it's a pain afterward.
Vinyl or another material can make an attractive easy and quick
finish, so you aren't tempted to leave it unfinished for months
or years. Do a test run with the drivers mounted before covering
just in case you have a cavity resonance and need to cut a little
off of the shorter wing.
Hi,
In order to avoid lobing at crossover frequency you need to space the drivers much closer than 1 wavelength. At a half wavelength you'll get the first null at 90 degrees, to totally avoid lobing you have to have even less spacing. This means you have to cross very low for any practical 2-way, below 1khz.
So, lobing can't be avoided in an ordianary multiway design. Going for MTM or TMM will worsen the lobing (MTM beeing worse)
You should make sure that you have no lobe tilt by time alingneing the drivers (execpt for MTM which don't have lobetilt no matter what).
My point is, choose crossover frequency according to what is pracical for the drivers at hand and then place the drivers as close together as possible. Don't bother so much about the actual wavelength spacing at crossover frequency.
/Jesper
In order to avoid lobing at crossover frequency you need to space the drivers much closer than 1 wavelength. At a half wavelength you'll get the first null at 90 degrees, to totally avoid lobing you have to have even less spacing. This means you have to cross very low for any practical 2-way, below 1khz.
So, lobing can't be avoided in an ordianary multiway design. Going for MTM or TMM will worsen the lobing (MTM beeing worse)
You should make sure that you have no lobe tilt by time alingneing the drivers (execpt for MTM which don't have lobetilt no matter what).
My point is, choose crossover frequency according to what is pracical for the drivers at hand and then place the drivers as close together as possible. Don't bother so much about the actual wavelength spacing at crossover frequency.
/Jesper
Actually lobing is not a problem with line arrays. It can be problematic with MTMs if the presence of the tweeter forces wide spacing of the outboard elements, which results in they're acting as separate point sources. With closely spaced multiple drivers, however, what is critical is the space between their radiating planes. ie, their frames. At frequencies where that space is less than a wavelength the multiple point sources will merge into a single coherent cylindrical wavefront and lobing does not occur.
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