JOHN E. FFOWCS WILLIAMS
1935–2020
Elected in 1995
“For contributions to the theory of jet noise, and other aspects of aeroacoustics and hydrodynamics.”
BY ANN P. DOWLING
JOHN EIRWYN FFOWCS WILLIAMS, Rank Professor of Engineering at the University of Cambridge (1972–2002) and master of the university’s Emmanuel College (1996–2002), died suddenly of a brain hemorrhage on December 12, 2020, aged 85.
John, known to all his friends as Shôn, was born May 25, 1935, into a Welsh-speaking family in Llangadog, Wales. Eirwyn means “snow white” because it snowed the day he was born, a rare event in May—even in Wales. His father Abel was a preacher, poet, and writer. His mother Elizabeth died in 1940, and Shôn would later recount that he and his two brothers “had a lovely time running wild with dad looking after us.” After his father’s remarriage, Shôn was sent at age 10 to a boarding school, the Great Ayton Friends’ School, far away in North Yorkshire, some 2 days’ journey from his home. Shôn spoke almost no English when he arrived at the school. He soon learnt, ’though he then tended to speak Welsh with a Yorkshire accent.
He left school at age 16 and became an engineering apprentice at Rolls-Royce Ltd. in Derby, England, while continuing his studies at Derby Technical College (now part of the University of Derby). He won the Spitfire Mitchell Memorial Scholarship (1955–60), which funded his studies at the University of Southampton. His time there was transformational. He took
the aeronautical BSc degree in just 2 years, obtained a PhD in 1961, was president of the Students’ Union (1957–58), and met his future wife, Anne Beatrice Mason. They married in 1959 and went on to have three children: Awena (born 1966), an adopted son, Aled (1970), and Gareth (1980).
Shôn’s PhD, supervised by Elfyn John Richards, a Welsh aeronautical engineer, was on the noise of high-speed turbulent jet flows, which was increasingly important because of the high-velocity turbojets that were beginning to power commercial aircraft. He showed how the aircraft’s forward motion and the high-speed convection of the turbulent eddies in the jet modified the intensity and directivity of the generated sound, correcting an error in Sir James Lighthill’s 1952 paper (“On sound generated aerodynamically. I. General theory”). The error was subtle, the result of a reduction in the effective correlation volume of the turbulent eddies due to the variation in retarded time 1 over an eddy whose convection speed is non-negligible in comparison with the speed of sound.
Sir James supported and agreed with Shôn’s conclusions and communicated his paper describing this work for publication in the Transactions of the Royal Society. 2 The paper demonstrated what became characteristic of Shôn’s style—it addressed a technologically important problem, presented detailed mathematics but also gave a clear physical description of the underlying reason for the result, and backed up the theory by a comparison of its predictions with experimental data. The two men became close friends.
After leaving Southampton, Shôn continued his research in high-speed jets as a research fellow in the Aerodynamics Division of the National Physical Laboratory (1960–62) in Teddington, England, and at Bolt, Beranek, and Newman Inc. (1962–64) in Cambridge, Massachusetts. At BBN his research covered all speed ranges, from the noise of supersonic rocket
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1 The time from when sound leaves a source to be heard by a distant observer.
2 Ffowcs Williams JE. 1963. The noise from turbulence convected at high speed. Philosophical Transactions of the Royal Society of London, Series A (255):469–503.
jets to turbulent boundary layer flows over a submarine, whose pressure fluctuations could interfere with the sonar system.
Shôn and Anne returned to the United Kingdom in 1964 when Shôn became a reader in applied mathematics and subsequently the Rolls-Royce Professor of Theoretical Acoustics at Imperial College London.
He viewed aeroacoustics as a branch of fluid mechanics. Sound is usually a byproduct of unsteadiness in flow, perhaps due to turbulence or another flow instability. But the energy in sound is so small that the source flows are often unaffected by the noise they produce. Indeed the pressure fluctuations in the sound are typically many orders of magnitude smaller than local pressure fluctuations in the source flow, and can be expressed in terms of integrals of the flow field over the source region. Lighthill’s (1952) theory showed how that could be done for a turbulent flow in unbounded space, and Shôn, working with student David L. Hawkings, showed how that could be extended to the sound generated by surfaces in motion. The Ffowcs Williams-Hawkings (FfW-H) equation 3 expresses the far-field sound from moving surfaces in terms of integrals of pressure and velocity over the surfaces. Use of this equation has become ubiquitous as a means of predicting the noise from helicopter and propeller blades, turbomachinery, and many other applications where determination of surface pressure and velocities is a basic part of the design. The FfW-H equation enables the sound field to be readily calculated from this information.
In 1966 David G. Crighton joined Shôn as research assistant and PhD student, sponsored by the US Office of Naval Research to work on underwater acoustics. Sound travels long distances underwater and sonar is used to detect submarines by passively listening to the sounds they generate as they move through the oceans. Submariners would sometimes emit a screen of bubbles to mask their presence, thinking that the bubbles would scatter
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3 Ffowcs Williams JE, Hawkings DL. 1969. Sound generation by turbulence and surfaces in arbitrary motion. Philosophical Transactions of the Royal Society of London, Series A (264):321–42.
sound and conceal their noise signature. But the analysis by Crighton and Ffowcs Williams 4 showed that the high compressibility of the air bubbles could lead to strongly radiating monopoles in the inhomogeneous mixture not present in water alone. These could actually greatly enhance the sound generated by the turbulence—by some 50 dB for a 1 percent air/water concentration and by 70 dB for a 10 percent concentration. So rather than the cloud of bubbles disguising the noise of a submarine, it dramatically increased it. It is said that this result led to an immediate change in operational procedures!
As director of the Concorde Noise Research Unit and chair of the Concorde Noise Panel (1965–75), and an executive consultant to Rolls-Royce, Shôn led work to address Concorde’s noise problem. Concorde was the first commercial aircraft to fly supersonic, reducing the travel time from London to New York to just under 3 hours. But the design of its engines, powered by turbojets with afterburners needed at take-off, was inherently very noisy. Shôn led attempts to reduce the noise, but it was too late in the project to make radical design changes, and the ejectors and mixing lobes tested had only a marginal effect. The main improvements came through enhanced operations, such as a very rapid initial climb.
In 1972 Shôn moved to Cambridge as the inaugural Rank Professor of Engineering, taking the Concorde Noise Research Unit with him. Abbreviated as CNRU, it became known as the Cambridge Noise Research Unit.
The following year Shôn became head of the university’s Division of Energy, Fluids, and Turbomachinery. He used to say that his main achievement was to persuade very good research students to tackle important but interesting problems.
To his many PhD students he was an inspiring supervisor who brought his enthusiasm, curiosity, and creativity—and sense of fun—to every project. The research he led was wide ranging, and his former students went on to leadership roles across university disciplines that include mathematics,
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4 Crighton DG, Ffowcs Williams JE. 1969. Sound generation by turbulent two-phase flow. Journal of Fluid Mechanics 36:585–603.
engineering, computer science, and biomechanics, and in technological industries that range from automotive and aviation to electronics and chip design, bioengineering, and ship building.
Shôn’s work helped make “anti-sound”: the introduction of suitably phased sound to cancel unwanted noise—a practical option. In particular, he realized the power of feedback systems involving acoustic waves to stabilize aeromechanical systems. One branch of applications, pursued in collaboration with colleagues at the Massachusetts Institute of Technology, was the control of instabilities of turbomachinery, particularly compressors. Through a series of theoretical studies, complemented by laboratory and full-scale demonstrations, the teams showed the feasibility and benefits of “smart” engines. These are engines with modern sensors and feedback controllers driving actuators that enable the engines to operate in conditions where they would otherwise be unstable (for example, extending the operating range of compressors by artificially delaying the onset of surge or rotating stall).
Shôn enjoyed the interdisciplinary nature of the Cambridge community, which sometimes took him into completely different areas of research. An inquiry from clinicians who hoped that anti-sound might eliminate the annoyance of snoring led Shôn and his PhD student Lixi Huang to investigate the mechanisms that cause snoring. There are several. Anti-sound was not going to be effective, but preventing the sound source was possible, at least in the laboratory. The surgeons and their patients were ready to try out the engineers’ suggestions.
The first intervention was to drill a small hole in the soft palate to reduce its tendency to vibrate due to pressure differences on its two sides. It worked brilliantly—and continued to work even when the hole had healed and closed up! The remaining scar stiffened the palate sufficiently to prevent flutter. This led to a minimally invasive procedure in which a surgical laser beam was used to cause scarring and change the dynamics of the soft palate. 5 This was a huge step forward
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5 Huang L, Quinn SJ, Ellis PD, Ffowcs Williams JE. 1995. Biomechanics of snoring. Endeavour 19:96–100.
because before Shôn and Lixi’s work the surgical solution was a uvulopalatopharyngoplasty, an operation involving removal of much of the soft tissue in the palate.
Shôn was an early Cambridge entrepreneur and in 1979, with Jack Lang, founded the successful startup company Topexpress Ltd., focused on software development, research, and consultancy. The UK Ministry of Defence was a major customer, particularly AUWE (the Admiralty Underwater Weapons Establishment). Topexpress was eventually sold to the VSEL Consortium plc, essentially becoming its advance research lab. Shôn subsequently joined the board of VSEL as a nonexecutive director.
In 1977, for his many achievements in establishing the field, Shôn was awarded the Aeroacoustics Medal from the American Institute of Aeronautics and Astronautics (AIAA), followed by the Rayleigh Medal from the Institute of Acoustics (1984), the Silver Medal of the Société Française d’Acoustique (1989), and the Gold Medal (1990) from the Royal Aeronautical Society (RAeS). He received an ScD degree from the University of Cambridge in 1986 and was a fellow of the RAeS, the Institute of Mathematics and its Applications, the Institute of Physics, the Acoustical Society of America, and the AIAA. He was elected a fellow of the Royal Academy of Engineering in 1988, and received its Sir Frank Whittle Medal 2002 for “outstanding and sustained achievement” and “lifelong dedication to understanding the properties of sound, which has enabled huge innovation in international transport.” He was a foreign honorary member of the American Academy of Arts and Sciences since 1989 and elected a foreign member of the National Academy of Engineering in 1995.
Shôn had been a fellow of Emmanuel College since his arrival at Cambridge in 1972. He enjoyed its convivial community, which included playing bowls after a very pleasant lunch. In 1996 he accepted the invitation to became the college master and said it “changed my life completely.” Shôn, Anne, their schoolboy son Gareth, and their dogs moved into the master’s lodge to succeed Lord St. John of Fawsley, a very different character. As Shôn was to say, “The College
dominated our life, my interest then being much more to ensure the College’s academic success than in my own work.” He was a hard-working, convivial, much-loved master and, together with Anne, made the master’s lodge a welcome place for fellows and students. He placed particular emphasis on inclusive admissions, high academic standards, and generating a community spirit across the college. The college enjoyed his lively company and thrived under his leadership.
When Shôn retired in 2002, he was the longest-serving professor at the University of Cambridge. He and Anne moved to his family’s home in Eglwysbach. They were delighted when Gareth and his wife Ewa joined them in North Wales where Gareth, a keen cyclist, runs a cycle shop and tackles challenging mountain routes.
John Ffowcs Williams is survived by Anne, Gareth (Ewa), and two granddaughters. Awena, a psychiatrist, predeceased him, but he remained devoted to his son-in-law Andrew and grandson Ieuan, who is a musicologist. Shôn would be delighted that the study of some form of sound and vibration continues in the family.
He is greatly missed by family and friends throughout the world.
