History

Two Historical Impressions about Acoustic Photo and Cinematography

by Gerd Heinz

Looking for historical details, one of the early pioneers of sound localisation experiments, John Billingsley (Prof. Dr., University of Southern Queensland, Australia) told a nice story. He called his technique 'The Acoustic Telescope'. Although microphone arras had been in use for some time [2], he created a new quality of instruments.

The 'Acoustic Telescope'

Asked about the early beginnings, John wrote in July 2001:

"The 'telescope' was developed in Cambridge (UK) in the mid-1970s. Shôn Ffowcs Williams (right spelling) [4] had joined as Professor of Acoustics, supported by Rolls Royce Aero Engines. The concern was to identify sound sources in the wake of an Olympus engine to see if shrouding would be any use.

At the time, Polar Correlation was a favourite technique - centred on a team at Southampton University led by Mike Fisher [3]. Our system used an array of fourteen quarter inch B&K microphones - because that was the number of tracks we could record on our Ampex tape recorder! (Of course nowadays the digital processing would be 'live'). We had to redesign the amplifiers - the Ampex units were designed to give a flattish frequency response but were hopeless on phase. Also we were able to extend the response of the microphones down to 2 Hz - not bad for a device with a source impedance of six picofarads! Our output was a linear distribution of sources, with a spectrum displayed for each point. We were not trying 2D location, so a limited number of microphones gave acceptable results. We sold our telescopes to British Ship Research Foundation, SNECMA and Deutsche Luft- und Raumfahrtforschung, among others.

I suspect, however, that some were bought more for the 'personal computer' which was central to the system - our operating system employed a Computer Automation 'Alpha' LSI2 single-board computer. In Rolls Royce, for example, the computing division could veto the purchase even of a pocket calculator! This way, a research team could acquire their own computer, disguised as an 'instrument'.

Part of the team set up a company in Cambridge, Toltec Ltd. Shôn Ffowcs Williams founded a rival company 'TopExpress'. His company won. I moved to Portsmouth in 1976. We continued collaborating with Cambridge and Rolls Royce on telescopes, developing a 'coarse-fine array' technique which beat the resolution/aliasing conflict. We also looked at a number of 'sparse array' methods.

In some of the early data, running the results as a movie gave the impression of sound sources progressing down the wake, perhaps vortices in motion. If these had impressed the clients more, I think we would have developed wavelet methods for the data processing. Clearly the resolution of static features is enhanced by longer integration, while moving features are 'blurred'.

Good luck with your venture - and please let me know if I can get involved in any way. Even today some of our research is based on acoustic instrumentation - though more to detect when the blades of a sugar-cane cutter are misbehaving!"

Who invented the name 'Acoustic Camera'?

Like many other researchers, the author, G. Heinz, worked on informatic aspects of the nervous system:

"I studied wave interferences in nerve networks, a physical approach to neurocomputing. The "thumb experiment" (dec.16, 1992) indicated dramatically the importance of understanding nerve nets. So the book "Neuronale Interferenzen" was written in 1993, highlighting elementary properties of waves in networks. Interference circuits appeared now for example as a hyperclass over optical systems.

Starting with an initial project (3-dim. Neuronale Interferenzrekonstruktion, NEURO3D) in team with Sabine Höfs and Carsten Busch we simulated images of acoustics or images of thought in terms of physical properties (space, time, velocities, channel numbers). The intention was to characterise the effects of reduced channel numbers and over-conditioning, to study moving and zooming and other organisation principles of such networks.

We designed a special simulator "Bio-Interface", but it was impossible, to get high-quality multi-channel records from nerve system (EEG).

So I decided to demonstrate properties of interference circuits with an acoustic example. Instead of EEG-electrodes I coupled microhones to the data recorder. We obtained first acoustic wave films and photos in march 1996, see the "History"-page. I called it "Interference Transformation" (HIT), it was implemented in form of the so called 'Mask Algorithm' basing on a delay-and-sum algorithm in time domain.

With the first acoustic photos we asked ourselves about its importance. Appropriate methods of image- or film-like documentation for noise were unknown. It was surprising, because large companies offered lots of products for sound documentation (B&K "STSF", DLR ...). So we decided to try further experiments.

On the Cebit97-exhibition a science reporter of the Berliner Zeitung, Dr. Michael Ochel, discovered our 'Acoustic Interference Measuring Station' (Akustischer Interferenzmessplatz). "He can not clearly understand what we are doing but it seems to be interesting" he said. If there is a relation to optics he would propose to call the system "Akustische Kamera" (Acoustic Camera). So we did, although nowbody in the world did know something about properties of such 'acoustic images'.

To bring the story to an end: In February 1997 we got the first image with a noise reflection (airplane Boing 737-400 at SXF). GEO published this image in different countries on the title-page. Within months we become known worldwide. Since that date I have been answering a mountain of e-mails every day. Thanks for the famous response!

End of 2002 we had the feeling that our knowledge about image properties was complete. For months we received no industrial acoustical challenge which we could not analyse - although the results were sometimes far from the wish of the customer.

Our current task is, to open the market and to push our acoustic cameras."

References:

[1] The acoustic telescope, J. Billingsley, R. Kinns, Journal of Sound and Vibration, 48(4), 1976, pp 485-510.

[2] Soderman, P. T. and Noble, S. C., "A Directional Microphone Array for Acoustic Studies of Wind Tunnel Models," AIAA Paper 74-640, AIAA Aerodynamic Testing Conference, 8th, Bethesda, Md., July 8-10, 1974

[3] Fisher, M.J., Harper-Bourne, M., Sal. Glegg, "Jet engine noise source location: the Polar Correlation technique". Journal of Sound and Vibration 51, 23-54, 1977

[4] Shôn Ffowcs Williams: CV

[5] John Billingsley: first homepage, second homepage and CV