Friday, April 06, 2007
Duralumin in ribbon microphones
B&O used what we refer to as "Duralumin" in their 1950's era ribbon mics. Here is a perfectly preserved example of the BEO-3 (BM3 B&O) with the original ribbon.
Duralumin was an early high strength aluminum alloy used in aircraft, such as zeppelins. Today there are many high strength aluminum alloys with various concentrations of zinc, silicon, copper and other constituent metals.
Here is a link to a table of aluminum alloys.
3 series and 6 series aluminum are common alloys. 3 series has high ductility and toughness, 6 series has medium strength and good weldability, and 7 series has very high tensile strength. Pure aluminum has high resistance to corrosion, but some of the alloys are much more prone to corrosion, which is a concern in aluminum ribbon microphones. That's one reason why B&O ribbon mics with original ribbons are scarce.
Notice that B&O has corrugated the ribbon at the ends, and made an offset convex portion through the middle. Nice idea, though it doesn't do much. The idea seems to have been to mimic the profile of a foil dynamic or crystal tympanic membrane and produce a stiff section supported by flexible sections at the end.
Things have improved a lot since the 50s. Magnets are much stronger, engineers are more careful about transformers, and the uniformity of modern aluminum foils is also much better than anything that was available in the 50s.
New and better materials are being developed all the time. Designers have a much larger choice of properties and can select from a wide range of strength, durability, uniformity, electrical and environmental attributes. Entirely new categories are also emerging, some from the so-called "nano" fields - nano is a buzzword, being misused of course by Apple and others - but what the nano scaled materials allow are properties that have not even been thought possible before.
Crowley and Tripp are working in the nanoscale to produce materials such as Acoustic Nanofilm TM, and other carbon nanotube and composite transducer technologies for a variety of applications including high quality recording microphones. In doing so, some of the old limitations of strength and durability are being completely eliminated, and this appears to be leading to the realization that, eventually, maybe soon, aluminum foils will become obsolete for transducer applications.
Posted by Bob Crowley at 8:57 AM