Page 7 - Spring 2009

Fig 4. Photograph of Gerhard Sessler and Jim West in their lab holding Teflon foil with a production Western Electric EL2 electret microphone in the foreground at Bell Labs around 1977. (Source: Bell Labs Archive)
in scientific history, Sessler had also worked with the Sell
transducer. Gerhard used the reciprocal Sell transducer in his
Ph.D. work on sound propagation and absorption of gasses at
high and low pressures and temperatures. When Jim began
experimenting with the problem transducer, he accidentally
(but fortuitously) left the DC bias to the Sell receiver discon-
nected. To his surprise, the receiver started playing loudly
again with its original sensitivity—it had been restored by
removing the bias voltage! Kuhl, Schodder, and Schroeder had
observed this behavior as well but did not pursue this phe-
nomenon in their research. By this time Sessler and West were
on the trail and realized the sensitivity problem was due to the
fact that the Mylar® polymer had become slowly charge-com-
pensated. Charge compensation was causing the slow loss of
sensitivity in the Sell transducer. With this understanding of
the problem they went to the CRC Handbook of Chemistry
and Physics12 that was an encyclopedia of materials at the time,
and found that Teflon® had the highest volume resistivity of
any material they could find (greater than 1018 ohm-cm). With
from Dupont, the creator of Teflon . They metalized the Teflon® with a thin layer of aluminum and created the modern electret microphone by tensioning a charged Teflon® mem-
6 Acoustics Today, April 2009
brane over a metalized backplate. Like the condenser micro- phone, the principle of the electret was well known before a practical working system had been built. In fact, the name electret is credited to Heaviside in 1892.
It is interesting to note that AT&T decided that the electret invention was not commercially important. Apparently the folks producing telephones believed that the carbon micro- phone, whose invention was in the 1800’s, had been perfected and cost reduced to the point that no other technology could displace it. Thus it took almost 7 years after the first publica- tion of the electret before Sony in Japan produced an electret microphone for portable tape recorders. The low mass of the electret microphone diaphragm offered Sony a microphone that was much less sensitive to motor structure-borne noise. Once Sony started producing the electret microphone the growth in production was exponential. Conservative estimates place the total production of electrets to more than 2 billion annually. AT&T eventually produced their own electret micro- phones for their consumer and business telephones, but this production ended around 1986 when relatively high-quality low-cost electrets became available on the market. A photo- graph of Jim and Gerhard in their laboratory (circa 1970) is shown in Fig. 4. Both are holding a sheet of metalized Teflon® that was used for testing charge storage with different charging mechanisms, the modified JEOL U-3 electron microscope behind them was one of the preferred ways for precision space charging of electrets. In the foreground of the photograph can be seen a hand holding an EL2 electret microphone manufac- tured by AT&T Consumer products in Indianapolis.
Figure 5 shows a photograph of Jim and Gerhard in the
anechoic chamber at Murray Hill (the world’s oldest existing
anechoic chamber) holding a research prototype of a second-
order unidirectional microphone made from a single electret
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the launch of Apollo 17 can be seen in Fig. 6. Jim and Gerhard had successfully measured extremely low frequency acoustic signals from a previous launch of a Saturn V rocket from Cape Canaveral. The distance between Murray Hill, NJ and Cape
diaphragm.
Finally, a photograph of Jim, Gerhard, and Jim Flanagan at
Fig 5. Photo of Jim West and Gerhard Sessler examining a second-order differen- tial electret microphone in 1973. (Source: Bell Labs Archive)
this discovery, they managed to procure some sheets of Teflon® ®