Page 33 - Jul2009

Fig. 1. Some of the equipment. The Zoom H4 is not shown. Credit: China Blue.
(Fig. 1). That morning just as Dr. Horowitz and I were first setting up the equipment, a lead to one of the geophones broke. This was my worst nightmare because although I brought 6 sensors, that morning only two were on site and the rest were at the hotel a few miles away. Additionally, the one thing I didn’t pack was a soldering gun and I had no idea of how to say “solder” in French, much less where to find the equivalent of a Radio Shack in Paris. I looked down at the situation and then Dr. Horowitz and I just looked at each other in agony as attempts at wrapping the geophone in electrical tape were clearly not going to work. Dr. Horowitz happened to be chewing gum at that moment; I paused and then it came to me. I asked him for his gum. He looked a bit tentative but handed it to me and we used it to try to repair the connection. I didn’t think it would work but fortunately it worked perfectly for the entire duration of the recordings, and even as I am writing this today the gum is still connected to the sensor. Perhaps the lesson learned was to always travel with an extra pack of peppermint gum (Fig. 2).
With our recording gear working, we had to consider the best plan to capture the full range of sound and vibra- tions of the tower. We were expecting to record infrasonic and sonic vibrations based on the movement of the eleva- tors as they transport the visitors up and down, and the vibrations of the tens of thousands of feet that walk across it every day, as well as the whining of the wind through the tower and the human sounds of her visitors. These are the forces that transfer energy to the 2,500,000 rivets and 18,038 pieces of iron of the tower. We originally planned to record at each level using both the geophones and the bin- aural microphones, but my original plans to record with a geophone planted on separate pillars of the tower on the ground level was logistically impossible so it was not going to work out. Aside from construction going on at one of the entrances that blocked our access, the wind and rain on that September day was blowing at up to 10 km/hour, the tem- perature was a chilly 12oC and the rain was shifting from a constant mist to a near-horizontal deluge. However, breaks in the weather allowed us to carry out the alternate record-
ings designed. While members of our team walked about on the ground level with the in-ear binaural recorders and the additional shotgun microphones, Dr. Horowitz and I set about the first geophone recordings (Fig. 3).
We decided to set up the geophones on two separate legs that enter the stone at the base of the North pillar (Fig. 4). This seemed the simplest technique but did in fact cause some minor havoc—a miscommunication between the tower’s security manager and the local gendarmes led a squadron of startled French soldiers to start yelling at Dr. Horowitz in high speed French to come down and put down what clearly must have looked like a small wired bomb. The raised machine guns made the point and our recording ses- sion was over until the manager who fortunately was nearby came and cleared up the issue.
Once in position, via clambering on the stone supports with rolls of duct tape, the two geophones were calibrated and tested to see if we would get signal crossover from one leg to another. As soon as we started recording, we found that even with a best frequency in the near-infrasonic range, we were able to hear a rich acoustic signal. During the 20 minute recording session we captured both the wind’s force against the structure as well as the elevator’s intermittent movements as it passed from floor to floor. If you have downloaded the audio material (Editor’s note: see “Directions to download the interactive Audio Clips and Track Samples” at the end of the article) you will hear the ground-level near-infrasonic
Fig. 2. Peppermint gum used successfully to repair broken solder joint Credit: China Blue.
32 Acoustics Today, July 2009