"Our problem is analogous to having a deaf person sit in an orchestra audience watching someone play an instrument like a flute and, by looking only at the finger position [of the flute player], trying to figure out what music is being played," Jorgensen said.
"In our case, the finger positions are actually the signals that we're getting from muscles that control the speech articulators."
In preliminary experiments, the scientists used button-size sensors placed under the chin and on the throat to gather nerve signals.
The signals were sent to a processor and then to special software programmed to recognize words such as "stop," "go," "left," and "right" and the digits "zero" through "nine."
Initial word-recognition results were 92 percent accurate. But as of now the system can only recognize 25 to 30 words.
In the future the scientists would like to focus on detecting phonemes—units of sound that are the building blocks of words.
"When you can do that, the number of words [the software can recognize] becomes irrelevant," Jorgensen said.
Can You Hear Me Now?
According to Jorgensen, the idea for his research was initially spurred by rude diners.
"I was sitting in a restaurant with people talking on their cell phones and I was saying, If only their [conversations] were silent," he said.
Reducing background chatter from phone calls is only the tip of the iceberg when it comes to applications for subvocal speech recognition, Jorgensen added.
The technology could be used by astronauts, whose voices can become distorted inside a spacesuit, or by divers, who would be able to talk underwater even while wearing a mouthpiece.
Such devices could also allow firefighters working inside a noisy burning building to communicate with the fire captain stationed outside.
"The system is immune to [background] noise," Jorgensen said.
A separate team of scientists based at Carnegie Mellon University in Pittsburgh, Pennsylvania, is using subvocal technology to develop a wearable translator.
Their device uses electrodes attached to muscles on the face and neck to pick up voice patterns and convert the signals into text.
That text is then translated to a different language and synthetically spoken by the device (related interactive feature: Egyptian hieroglyphs translator).
Tanja Schultz, a researcher with the project, says the effect is like the real-life equivalent of watching a television show that has been dubbed into a foreign language.
It would really allow us to tear down language borders, Schultz told the Tartan, Carnegie Mellon's student newspaper.
In the future, the NASA researchers say, their device may be able to make out not only which words people are saying but also how they're being said.
"We call that speech enrichment," Jorgensen said.
"A lot of stuff comes down this nervous system channel besides a particular word," he said.
"We may be able to detect if you just had a cup of coffee or if you're feeling happy or sad."
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