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Extreme Machines

Hello, Is Anybody Out There? Scientists Make The Berlin Wall Talk

Tomas Kellner
February 11, 2016
Giving snowballs a chance in the hell of a steel foundry, catching lightning in a bottle and making a wall talk: Thomas Edison did none of these seemingly impossible things. But then, he never had the opportunity.
This year, GE is celebrating Edison’s birthday, which President Reagan proclaimed as National Inventors Day, by taking on the impossible challenges of lore. On Feb. 11, the company is showing that these feats are “unimpossible.”

Sara Underwood, a mechanical engineer at GE Global Research, was part of a team that made the Berlin Wall talk last fall. Listen to her story.

GE Reports: Tell us about the mission?

Sara Underwood: I am part of a team of researchers who work on projects for GE’s Power, Aviation, and Oil & Gas businesses. Our team specializes in vibration and noise measurement and mitigation. Our goal was to identify a sensor sensitive enough to detect the vibration in the concrete wall caused by a man reading a story on one side of the wall, filter and transport the vibrations to a speaker 160 yards away on the other side of the wall, and play the story back to a group of kids.

GER: What did you think when you joined the team?

WALL_4 (1)

SU: I thought the task would be much more challenging than it turned out to be.

GER: Really?

SU: We initially talked about the Great Wall of China, but decided that it wouldn’t be a good option because it’s not a consistent medium. It’s made from rocks and bricks, which makes it harder to carry vibrations. With concrete, the limiting factors were its thickness and ambient noise.


GER: How did you handle the challenge?

SU: The average human voice at close range is about 60 to 70 decibels and causes the wall to vibrate. We had to figure out how those vibrations would fade as they traveled through the wall. We were dealing with at least six inches of concrete, so there was some loss, but not so much that we wouldn’t be able to pick it up on the other side. We picked some sensors from our lab, attached those to a wall and gave it try.

GER: What type of sensors?

SU: We grabbed laser vibrometers and accelerometers, for example, which we use regularly to measure vibrations in jet engine blades, locomotive engines and MRI machines. The sensors can measure vibrations that can tell us things about the components like material properties and structural health. It was mostly off the shelf technology.


GER: What happened when you got to Berlin?

SU: We were there for a week and spent a lot of time at the Berlin Wall. But we only had one day to run the experiment when we had permits to work at the wall and attach anything to it. It was a pretty neat experience.

GER: How was it different from your lab tests?

SU: It was much harder. We were there in December and the day we did the experiment was cold and rainy. We planned to use commercial wax to attach the accelerometer to the wall but the cold made the wax rigid. We had to get a hair dryer to soften the wax, which is not something I’m used to doing at work.


The other big issue was background noise. There are streetcars, cars and people walking right by the wall. The wall goes deep into the ground and when a train went by, it generated more vibrations than our sensor could handle.

GER: How did you deal with it?

SU: We had to wait for a quiet moment for the speaker to read the story. We also used the same data acquisition software that we use in our lab to filter the measured vibrations to take the background noise out.

GER: How did the kids respond?

SU: I feel like they should have been more impressed than they were. I don’t think they grasped what we were doing. We also had an oscilloscope at the wall that was showing the vibrations as the speaker was reading. They were thrilled watching that.