Return of the Triffids

Dedicated to those large boxes at one end of the room
Post Reply
vinylnvalves
Old Hand
Posts: 1074
Joined: Sat Jan 26, 2013 12:08 pm

#1 Return of the Triffids

Post by vinylnvalves »

Ian has been kicking around ideas on what to do with his 8 12” AE dipole drivers. This concept was liked, but with the drivers facing the listener.
27850386-877D-4A11-8F62-45A9C7D9A944.jpeg
27850386-877D-4A11-8F62-45A9C7D9A944.jpeg (210.17 KiB) Viewed 5723 times
. The logic for facing the drivers the other way other than to compact the design, would be to bounce the LF off a boundary. I stand to be corrected.

Well using Ian’s drivers, some horns and he some 18” BMS drivers he has kicking around, I have come up with this concept. Midrange horn centre 1100mm off the ground.
F3E612AC-CE30-4449-BDE2-4E873184195F.jpeg
F3E612AC-CE30-4449-BDE2-4E873184195F.jpeg (182.08 KiB) Viewed 5723 times
This is the first cut at the idea, I would probably trim the frame around the drivers to a minimum. Feedback appreciated.
iansr
Old Hand
Posts: 401
Joined: Thu Nov 08, 2007 10:44 pm
Location: West Riding

#2 Re: Return of the Triffids

Post by iansr »

Thanks Steve. Looking forward to seeing the comments and observations.

It might be worth mentioning that my idea is to low pass 2 of the AE 12s at c.500hz (where they will cross to the big horn) and the other 2 at c.100hz so that they help to compensate for the low end roll off.
"Its good enough for Government work."
User avatar
Scottmoose
Needs to get out more
Posts: 1802
Joined: Sat Sep 08, 2007 11:03 am
Contact:

#3 Re: Return of the Triffids

Post by Scottmoose »

vinylnvalves wrote: Fri Apr 21, 2023 2:06 pm The logic for facing the drivers the other way other than to compact the design, would be to bounce the LF off a boundary. I stand to be corrected.
It's a dipole, so the LF emits in a figure 8 radiation pattern, i.e. equally (albeit with a 180 degree phase difference) front & rear of the cone. So it makes no difference to 'bouncing it off the boundary' which way around the cone faces (assuming it even does reflect rather than get leached off -that's a separate issue of room construction though). The main acoustic advantage, if advantage it is, of having the reverse-face toward the listener would be to gain some small low-pass effect with the motor structure disrupting shorter wavelengths. Since the original in the image have a relatively small motor & open basket structure, I would assume the primary reason was aesthetic rather than functional.
'"That'll do," comes the cry of the perfectionist down the ages.' (James May The Reassembler)
Website www.wodendesign.com
Community sites www.frugal-horn.com & www.frugal-phile.com
iansr
Old Hand
Posts: 401
Joined: Thu Nov 08, 2007 10:44 pm
Location: West Riding

#4 Re: Return of the Triffids

Post by iansr »

When you look at other photos of the Reflector you can see that the horn is recessed into the void between the baskets, so I reckon it’s to reduce the overall width and height. I think the AE Dipoles look great from the front which is why I want them forward facing.
"Its good enough for Government work."
vinylnvalves
Old Hand
Posts: 1074
Joined: Sat Jan 26, 2013 12:08 pm

#5 Re: Return of the Triffids

Post by vinylnvalves »

I am unclear if they need to be angled in, or not. I can understand when you’re using the rear of the cone the dispersal will be different. I have tried a cone upside down to get an omni spread with success in the past. The crossover will be above the beaming frequency for a 12” driver, so shouldn’t be a concern.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#6 Re: Return of the Triffids

Post by IslandPink »

I agree with Scott's comments above. and of course the bass frequencies are more or less omnidirectional. What frequency do the cones have to work up to ?
I still don't really know how all the bass doesn't just cancel, without a baffle, irrespective of how much E.Q. you apply !
"Once you find out ... the Circumstances ; then you can go out"
iansr
Old Hand
Posts: 401
Joined: Thu Nov 08, 2007 10:44 pm
Location: West Riding

#7 Re: Return of the Triffids

Post by iansr »

The woofers will go up to c.500hz.

The sound waves of an OB do cancel at the sides, which as I understand it is why you get the figure of 8 pattern.
"Its good enough for Government work."
User avatar
Scottmoose
Needs to get out more
Posts: 1802
Joined: Sat Sep 08, 2007 11:03 am
Contact:

#8 Re: Return of the Triffids

Post by Scottmoose »

Right.

The focusing will be partly for aesthetic reasons, since assuming a 500Hz crossover frequency you'd need a c.27in separation for the four LF units to have obvious integration issues if mounted on a flat panel. That said, crossovers are not brick walls, so you have to consider the slope and the width of the transition band since they'll still be contributing part of the overall system response over that, i.e. for an octave or so above the nominal crossover frequency, which is where a modicum of directionality / focusing may help. Strictly speaking, the front of the driver cone, given its profile, may be better suited in that regard, but the small disruption provided by the motor & basket structure may claw some of that back, so 'name your poison' as always.

Mark -you're right, the LF does cancel. But like [most] filters, it's not a brick wall / lightswitch where when you get to the point where the wavelengths are long enough to wrap around the 'baffle' (in this case mostly formed by the driver structures themselves). You get a deep lateral null in the polars, hence the figure 8 dipole radiation pattern under free-space / half-space conditions, and due to the cancellation, the overall system response nominally rolls off 1st order below that. This is why dipoles are the most inefficient of all load types since you either need EQ & power, or a lot of cone area to compensate for the progressive LF losses.
'"That'll do," comes the cry of the perfectionist down the ages.' (James May The Reassembler)
Website www.wodendesign.com
Community sites www.frugal-horn.com & www.frugal-phile.com
vinylnvalves
Old Hand
Posts: 1074
Joined: Sat Jan 26, 2013 12:08 pm

#9 Re: Return of the Triffids

Post by vinylnvalves »

The drivers are about 32” C to C across the diagonals in the configuration shown. So close to limit with regard to 1/4” wave length then at the x over frequency.
User avatar
Scottmoose
Needs to get out more
Posts: 1802
Joined: Sat Sep 08, 2007 11:03 am
Contact:

#10 Re: Return of the Triffids

Post by Scottmoose »

1/4 wavelength spacing out to the upper limit of the transition band is the 'ideal', especially for something like a Unity horn as they effectively sum as one unit; for roles where conditions are a bit less demanding as in something like one of these baffles, up to 1 wavelength is generally fine, although 1/2 wavelength spacing is best avoided as you can run into cancellation issues. Since the precedence effect (to say nothing of room acoustics, hearing damage etc. helps out) a bit of additional leeway is generally available -as is the case here. Assuming an SoS of 13560in/sec - 13680in/sec then full wavelength @ 500Hz = roughly 27 1/4in.
'"That'll do," comes the cry of the perfectionist down the ages.' (James May The Reassembler)
Website www.wodendesign.com
Community sites www.frugal-horn.com & www.frugal-phile.com
iansr
Old Hand
Posts: 401
Joined: Thu Nov 08, 2007 10:44 pm
Location: West Riding

#11 Re: Return of the Triffids

Post by iansr »

I’m appreciative of your input Scot, very helpful.
"Its good enough for Government work."
Post Reply