Analysis: why St Paul's in London was the site for a fascinating experiment into acoustics in 1951 

By Fiona Smyth, Harvard University

Inside the cathedral

In late December 1951, the interior of St Paul’s Cathedral in London resounded to a series of shots fired from a Colt revolver. The cathedral was almost empty and, as the shots rang through the building, just a handful of journalists and scientists paid attention.

Inside the van

Outside the cathedral, parked amongst the skeletal shells of buildings torn apart in the Blitz, was a small olive-coloured van with the registration number HJH 979. This purpose-designed coach on a six-tonne, six-wheel chassis was a Mobile Acoustical Laboratory (MAL). The chassis had been sourced from surplus RAF stock in the wake of the Second World War and the specification for the interior devised at the Building Research Station, under the aegis of the Department of Scientific and Industrial Research.

Built in 1947, the MAL was a complete and state-of-the-art acoustic laboratory on wheels. It was amongst the first of its kind in the world. From information on environmental noise levels to the acoustical characteristics of buildings relevant to music, the primary purpose of the MAL was to collect data. The data was used in applications as diverse as designing construction methods and materials for sound insulation in housing to shaping musical tone in auditoria. By virtue of its compactness and mobility, it had visited myriad sites since it came into operation in February 1948. 

The tape recorder was roughly equivalent in size to a modern-day under-counter refrigerator

It wasn’t just the chassis which had emerged from the war. A magnetic tape recorder, also a war-time invention, occupied the entire right front corner of the laboratory space. The only semi-permanent piece of equipment in the MAL, the magnetic tape recorder was roughly equivalent in size to a modern-day under-counter refrigerator. The average cost of a tape recorder at the time was similar to that of a luxury car. 

Shots fired

As the pistol shots rang out through St Paul’s, the original shots and the building’s response to them were captured via the MAL, using moving-coil microphones and the state-of-the-art magnetic tape recorder. The resulting data was then analysed, separating the building’s response from the test signals (which is in effect what the pistol shots were) to determine how long sound was sustained in the interior. St Paul’s Cathedral, a giant musical instrument in its own right, reverberated for 12 seconds with variations to the effect discernible across the octaves of the musical scale. 

St Paul's Cathedral today. Photo: Fiona Smyth

"That little old echo"

At St Paul’s, data was used to characterise the acoustic interior in order to design and test a speech reinforcement system. In an interior where sound is sustained for up to 12 seconds, the effect on music is phenomenal. It is ethereal and rich. However, the degree of reverberation is less conducive to clarity of speech. "That little old echo", as the effect at St Paul’s was once described by an acoustic consultant in the 1930s, did little to aid speech intelligibility.

That little old echo did little to aid speech intelligibility

Various systems for improvement had been trialled at the cathedral, and various unsolicited suggestions had been made to the cathedral authorities in the previous decades. These latter included spraying the interior of the dome with asbestos and hanging a series of experimental (some would argue magical) devices from the dome. 

With little to recommend these solutions, the cathedral authorities had sought professional scientific and architectural advice to address the problem in 1939. Timing was unpropitious and the process of acoustic analysis and intervention at the cathedral was interrupted by the outbreak of the war.

However technological advancement in recording technology during the war, the subsequent availability of RAF-surplus apparatus and the potential for a fully-equipped acoustical laboratory to go mobile had changed the state of play in architectural acoustics. So too had the quantification of a phenomenon known as the Haas Effect in 1949. The Haas Effect was concerned with the psychoacoustic effect of time delays on the perceived spatial location of sound. 

At St Paul’s, a new acoustical system designed to maximise naturalness and intelligibility had been installed in the cathedral. With reference to the Haas effect, it used electronic means to manipulate time delays, directionality and loudness. The intention was to cut a line of clear, natural-sounding speech through the phenomenally reverberant cathedral interior. The pistol fire on December 19th 1951 was a public test. It was also a demonstration for the press of the new electronic system and the manner in which science and technology could be harnessed in analysing and addressing the acoustics of complex spaces. 

Hold the front page

Unsurprisingly, the event hit the headlines. "Murder in the Cathedral" proclaimed the Daily Mail, with more than a hint of levity. The London Times was more prosaic in coverage of the events, but nevertheless made reference to the drama of the occasion. Images of a scientist firing a pistol from the cathedral pulpit made the Daily Telegraph.

More than gathering data, testing a hypothesis and showcasing the state-of-the-art, the events at St Paul’s in late December 1951 provided tremendous PR coverage for a developing science. It brought public attention to the issue of acoustics, a field of endeavour which was often ignored or implemented behind the scenes. 

Dr Fiona Smyth is a Marie Sklodowska-Curie Actions Global Fellow (2015-2018) at Harvard University, (Dept of the History of Science 2015-17) and Trinity College Dublin (School of Engineering, 2017-18). This Fellowship is funded by the European Commission under Horizon2020. The article is based on research for a book on the musical, scientific and architectural origins of acoustics as a building science in early 20th century Britain and Ireland.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ