BBC – Sound Waves: Big Ben
One thing I have learnt after 20 years as an acoustics and vibration engineer is that you have to expect the unexpected. Acoustics and vibration touch so many areas of our lives that often we do not even think about it. Over the years, I have been involved in projects that range from satellites to submarines; from sports bras to mushrooms; from violins to formula one cars; and all this I did via such unexpected things as artificial hip joints and subsea oil pipelines. So, when the phone rang in the spring of 2016 I have to admit that even I was surprised by the request that was made by the researcher from the BBC. Could we measure an item of incalculable and historic value to the nation without touching and risking any damage? I informed the caller that I was certain we could, our measurement technique using Laser Doppler Vibrometry was inherently non-contact and therefore it should be no issue. I asked what the item was and where we would be doing the tests, “Could you measure the vibrations of Big Ben?”
My immediate reaction, as with anyone involved in business development, was to say yes knowing that it would be down to the technical team to solve the delivery issues. Little did I guess the number of times I would be going up and down the 334 steps to the belfry of Elizabeth tower!
The request came because the BBC had commissioned a new science program for BBC Four that covered sound and was to be called “Soundwaves, The Symphony of Physics”. They wanted to use a bell to demonstrate how complex vibrations make up the sounds we hear, and if you are going to measure a bell then you might as well do, arguably, the world’s most famous bell…
After much discussion with the researcher and producer of the show, we agreed that a site visit was necessary to understand the obstacles that we would need to overcome to make the measurement. The outcome of the site visit by our Technical Expert, Martin Cockrill, was that it looked entirely viable to make the measurements in the belfry if we had enough time and were able to get our equipment up there. Not an easy task if you have seen the size and weight of the equipment that we use in our laboratory. We typically move a measurement system around in a small van, and we needed to get two systems to the top of the tower. If you are not aware, there is not a convenient lift to the belfry of Elizabeth tower. Just a narrow flight of 334 spiral stairs. The technical delivery team were going to have to carry the equipment up by hand. An additional constraint on this was the size of the team allowed up the tower. Health and Safety meant that only 12 people could be up the tower at any one time. By the time you include, the tower wardens, the producer, the camera operator, the sound recordist, the BBC presenter and runners we could only have a maximum team of 4 people.
After days of trials and development of the technique to reduce the equipment to its smallest possible size and weight and some phenomenal packing by our technician we were left with 6 journey’s worth of equipment to spread between our team of 4. On top of this, each trip for each person was around 30kg of equipment. It was no small undertaking that we had accepted and someone at least was going to be making multiple journeys up and down the tower.
After we had overcome the equipment challenge, we had to look to the time challenge. Parliament had given us a three-hour window to complete the measurement and time either side for setup and clearing away.
Our typical measurement process involves exciting a structure with a known and measured signal and then measuring the response at multiple locations to assess the dynamic behaviour of the structure. With the case of Big Ben we were unable to touch the bell so could not excite the structure. Handily, every hour the clock in the tower excites the bell and it was our plan to make measurements during a number of hour chimes of the bell. As the 10 o’clock, 11 o’clock and 12 o’clock strikes give the longest window of excitation we planned that these were the times to use. Our plan was to be set up in the belfry with one system just ahead of the 9 o’clock chime to undertake a system level check. Working back from this we decided we needed to start climbing the tower at 8am with the first system.
ASDEC, the BBC and the team from Parliament worked closely to arrange and overcome all these logistical challenges and to address the health and safety issues associated with the operation. All was finally planned for the 21st June.
The time and effort planning paid off when the test day arrived. We all arrived, slightly breathless, in the belfry by 8:45am and our 2 engineers, Martin and Max, started building the first system with assistance from Chris as a runner. It was at this point I regretted my stitching up of the technical team as I returned to the bottom of the tower for the remainder of the equipment. During my absence the engineers had everything ready for doing the level check at 9am. We were all set for the measurement.
The plan for the measurement was to acquire data from as large a portion of the bell as possible. We set up 2 Scanning Laser Doppler Vibrometers which allowed us a view of nearly 180 degrees of arc around the bell. We had further sliced up the surface of the bell to acquire a number of measurement locations during each set of hour chimes. This resulted, over the 3 sets of chimes, in measurements at over 500 points around the surface of the Great Bell.
Again, the many rehearsals completed prior to the test day paid off. The measurement process proceeded without hitch or hindrance. Time was taken between the chimes for Martin to be interviewed by the presenter, Helen Czerski, to explain the process, the science involved and the results that were expected.
Following the 12 o’clock set of chimes we finished a few more filming sequences the director wanted, packed away the equipment and began hauling back to the bottom of the tower. Another successful measurement campaign completed by the ASDEC team!
Martin and the engineering team processed the data acquired in Elizabeth Tower in the days following the measurements. We were quickly able to see that we had an excellent set of data that showed how Big Ben vibrated when struck in an unprecedented level of detail. The results can be seen below and in the BBC Four program shown on the 2nd of March 2017.
It was a great privilege to have had the opportunity to measure and analyse such an iconic national treasure.