Apogee ribbon questions...

[george a]

Hi All
I wonder if anybody may be able to answer a few questions behind the design of some of the apogee ribbons .
I have been review their patent no.4550228 and find the following puzzling:

a) The midrange ribbon is noted as being attached at the edges along its length to the frame /diffraction thingos etc. as opposed to being limply suspended in space.
Whilst this would help with mechanical centering of the ribbon, I would have though that any such fixing would have restricted any ribbon movement and even lead to very high local stresses in the membrane leading to failure at the point of attachment .
I also note that the ribbon appears to be obliquely corrugated as opposed to perpendicular to its length - ie as the treble ribbon is. =perhaps this allows for some movement without mechanical failure.
From memory some of the magnepan treble ribbons were also attached at the edges ..??)

b) The treble ribbon seems freely suspended but has what appears to be a significant gap at the edges near the face of the magnets to each side .
I would have thought this to be a no-no for sound cancellation - or is this gap not an issue at the frequencies in question? (This gap is also plainly visible in some of the images of the ribbon found on the net.)

c) There also appears to be gaps between the magnets in their vertical positions - I would have thought to have them touching was desirable - or maybe the gaps are a concession to the difficulty in positioning the magnets at time of manufacture??


Any comments ?
Cheers

[a.wayne]

Is that setup for a specific model, i have repaired a few diva's and don't recall the Mids looking like that . I have to assume the patent does not have all the details , not unusual ..


regards,

[LineSource]

Hi George,

This Apogee patent is for their Full Range Ribbon Speaker.

The midrange ribbon is a true ribbon made from 10 micron thick aluminum foil with horizontal corrugations over the 2" wide, 84" long ribbon. The ribbon is clamped at the top and bottom by copper bars. To control resonance, the ribbon is physically divided into three equal lengths by soft foam rubber pads that contact both the front and rear of the ribbon to physically center-clamp the ribbon in the frame. Hence, the ribbon is one aluminum strip, but it is physically divided by the foam rubber clamps into three separate physical segments. The ribbon is 0.1 ohms, and almost pure resistance. A metal box holds a step up transformer that converts the 0.1ohm to 4ohm. The speaker comes with a passive front end bi-amp crossover box which provides 1st order crossover slopes at 320Hz and 10K Hz.

The magnet structure for the midrange is designed to provide a self-centering force. Two rows of ceramic magnets with about 0.1" separation are glued along the length of the iron frame. This 2-magnet-gap motor structure provides a more uniform magnetic field over the front-rear ribbon displacement. A second method is used to help self-center the ribbon. At the top electrical connection, two aluminum foil strips are run along the magets on both the left and right sides down to the floor, where the input started. Plastic tape on top of the magnets electrically isolates the foil, and a second strip of plastic tape on top of the foil provides electrical isolation from the ribbon. Remember that this ribbon is only 84 db/watt efficiency, so large currents flow through these side aluminum foils. A more efficient NdFeB ribbon would not generate enough current to make this side-foils arrangement worth considering.

The tweeter ribbon is constructed from a 0.5" wide, 2*84" long 10 micron aluminum foil with horizontal corregations. The two ends are both electrically connected at the bottom of the ribbon, and the top is a non-conductive clamp. The 2x length is used to create a bipole tweeter with about 0.5" air gap between the front and rear tweeter foil. When the drive current pushes the front ribbon forward, the rear side is pushed backward, thus creating a bipole radiation. Using one long strip of foil increases the total resistance to 2 ohms, and increases the efficiency to where Apogee used just a 2 ohm series resistor instead of a step-up transformer. The tweeter ribbon also uses two sets of soft foam rubber pads to physically divide the ribbon into three equal pieces, over just one continuous foil.


For comparison, the Magnepan true ribbon tweeter ribbon uses extremely thin 2 micron thick aluminum foil to create a 4 ohm resistive load that is easy and cheap to direct drive by an amp. The ribbon is horizontally corrugated, and a small blob of silicone between the ribbon edge and iron frame in several places along the length is used to physically divide the single foil strip. This protects the delicate foil from stretching from a puff of air and SPL abuse.

[Few]

Linesource:
Very informative post. Thanks!
Few

[george a]

Yes Linesource - a most informative and concise post!
Many thanks for going to the trouble to respond.

I can't help but wonder if the foam block restraints on the apogee ribbons or the silicone blob securing on the magnepan doesn't lead to some "work hardening" of the metal foil and premature failure at the points of attachment/ restraint.
Or maybe the forces involved just aren't strong enough to be an issue ..
any comments
thanks

[LineSource]

Originally Posted by george a

I can't help but wonder if the foam block restraints on the apogee ribbons or the silicone blob securing on the magnepan doesn't lead to some "work hardening" of the metal foil and premature failure at the points of attachment/ restraint.
Or maybe the forces involved just aren't strong enough to be an issue ..
any comments
thanks

The foam rubber "clamps" on the Apogee Full Range ribbons can be adjusted to better serve as "displacement stops" where the ribbon can move forward/backward about +/- 0.1" before touching the soft foam rubber. This is how I set up my speakers. Hence the "clamp" pad position protects the ribbons during transport, and "displacement stop" pad position protects the ribbon from having bass notes push the ribbon out of the channel to where the corrugations stretch. It is easy to see the Apogee midrange ribbon move during bass notes from the shallow 6db/oct 320Hz crossover. You cannot see tweeter ribbon movement. Hence, the 10 micron aluminum foil used by Apogee is strong enough to "stand alone" over the 84" length under reasonable SPL levels. To my ears, 10 micron aluminum is also thick enough to "sizzle" from resonances.

The 2 - 2.5 micron thick Magnepan ribbon can be broken by a strong puff of breath, and hence the silicon globs that attach the ribbon side edge to the metal frame are mandatory to maintain reliability, as well as maintain ribbon/frame centering. Magnepan forums mention a high ribbon failure rate due to the super thin foil. From what I have read I believe that the glue joint does work harden the ribbon foil, but it is mainly using usper thin foil that causes the frequent failures. Magnepan may use a 5056 Al alloy for greater strenth.

My DIY ribbons use 5.6 micron thick pure Al foil. This is thick enough for adequate physical strength, yet "soft" enough not to sizzle. Wiggle cheap household Al foil (12.5 - 25 micron thick) and it makes a lot of noise. Thinner foils seem to self-dampen these vibrations and are quiet.

There are many reasons why metal foil + plastic film are married by some magic to make strong, quiet ribbons.

[george a]

Hi Linesource and others

There is some talk about application of vaseline to some part of the ribbon foil of Decca / Kelly ribbons to control resonances (I think) but this may be an obscure art..?


May I ask what gap there may typically be between the edge of a ribbon and the magnets adjacent
In photos i have seen of the Apogee ribbons - it appears that this gap can be quite large...
Additionalyl in some photos there is a substantial length of ribbon at each end which does not have magnets either side - I wonder if this is a problem area subto resonaces as it is possibly not subject to any or much EMF...
Any comments anybody?
Thanks

[geraldfryjr]

On my Duette's, There is only a 1/2" of the tweeter ribbon outside of the magnet structure on each end.
And with the ribbon on center there is only 1/8" gap on each side of the ribbon to the magnets.
The ribbon itself is 7/8" wide.


I hope this helps you.

jer

[LineSource]

Originally Posted by george a

in some photos there is a substantial length of ribbon at each end which does not have magnets either side - I wonder if this is a problem area subto resonaces

Many ribbon designers attempt to absorb the resonant ribbon energy at the top and bottom attachment points. RAAL uses a cantelever mount with heat shrink plastic film tensioning. This construction provides a suspension without increasing height that can effect directivity control. Some long Al ribbon patents show significantly larger corrugations at the top and bottom ribbons portions where no magnetic field exists. Some long ribbons use soft foam rubber at the clamp points, both to reduce foil rip from vibration, and to absorb ribbon energy before it gets reflected back.

Some suspension is required for the ribbon. It is best to keep this non-linear-motion-suspension portion out of the magnetic field so it can effectively dampen out the ribbon's energy.

[george a]

Hi all

Many many thanks to you all for the considerable effort you've out into responding to my queries.
I will will be attempting to (re)build some 24" ribbons over Christmas . (wider ribbons this time)
This will be the second attempt as the first ones self-desructed when I carried one completed assembly across the room - too close to another assembly leaning against the wall.
Luckily fingers weren't caught - but oh what a mess!.....64 of 2" x1/2" sq neos

luckily only a couple broke....

had to completely dismantle the whole thing - magnet by magnet .
No way we could pull these things apart.....

Cheers and thanks again