Monday, August 18, 2014

Pneumatic Tube Audio

You may have used one of these systems on an airplane as recently as the late 1980s. (Does that count as recent?)   This was not radio so to speak, as it contained no RF but it often included prerecorded radio programming. So file this under extinct radio obscurities...

Most sourced attributed the technology to AVID Airline Products who began marketing Airline pneumatic tube audio in 1963. The company had been founded as a division of AVID in 1951. AVID cites their first year marketing IFE headphones as 1961 and names TWA as their first customer. It's interesting that AVID picks that year as it was the same year Keith Larkin and Courtney Graham invented a similar pneumatic headset for pilots US3184556. They went on to found Plantronics. But the AVID patent I found actually gates to 1965, and there it gets interesting. It appears that the "Sound Tube Head Set" patent US3217831 was invented by Charles Scanlon Edward.  It was one of 4 patents he held, most of which relate to hearing devices. On the document he describes it's function simply
"In operation, in an airplane carrying a stereophonic tape, which may be a musical tape or a motion picture sound track, the tape is electrically connected to two separate transducers. The two transducers emit pressurized sound waves which will pass into acoustical passageway 63 and sound passageway 67 when acoustical socket 61 and sound socket 66 are, respectively, attached to a transducer. The pressurized sound waves from acoustical passageway 63 pass into and through inlet port 20, acoustical chamber 15 and ear port 23 to a human ear"
The legal problem with the patent is that the device had no new technology of any kind. It was effectively a stethoscope held against a speaker cabinet. In citing his patent he went way out of his way to mention no existing stethoscopes; instead citing awkward and disused stethoscope-like devices. They were all pneumatic headphones that conduct sound. In an act of patent attorney comedy, two other inventors cited him as a source for their own dubious stethoscope patents. 

Probably in an effort to defend the patent AVID went out and bought some more stethoscope patents including US3623571, US3539032, US3730290. Sometimes it's just cost effective you know? In fairness they did pad that first dubious patent with two more interesting ones. A self retractable sound plug US3721313 in 1973, and a passenger audio control box US3860139 in 1975. That later patent is actually the most original.

What made this whole system worth pursuing was not it's high fidelity. It was that the headphones were cheap to produce. AVID continued to make the units until at least 1979. By the early 1980s imported electronics so lowered the cost of traditional headphones that there was no remaining advantage in the pneumatic headsets. AVID and Plantronics continue to exist today. Sadly TWA does not.