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Acoustic Emission

March 11, 2011

The Arthur Conan Doyle story, "Silver Blaze", features the famous fictional detective, Sherlock Holmes. Holmes was able to solve a crime through an interesting deduction:[1]
Gregory (Scotland Yard detective): "Is there any other point to which you would wish to draw my attention?"
Holmes: "To the curious incident of the dog in the night-time."
Gregory: "The dog did nothing in the night-time."
Holmes: "That was the curious incident."

Holmes realized a stable dog's not barking meant that no stranger had entered the stable at the time of the horse's disappearance. Holmes was using sound - In this case, a lack of sound - as a diagnostic tool.

In the early days of personal computing (Does anyone remember having an MFM hard drive?), computer owners generally had a sixth sense about when they needed to backup their data. The hard drive that usually went "click-click-click," started going "click-clack-click." Subconsciously, or not, you knew something was about to give at the next boot.

Nowadays, hard drives all have a faint "tick-tick-tick" sound, so it's hard to hear any differences. Solid state drives or MRAM will remove all sounds, except fan noise when there is a cooling fan, in future computers. Sound, however, remains as a useful diagnostic tool in many cases. This diagnostic tool is generally called acoustic emission, or acoustic emission spectroscopy.

Sound emitted by a crumpled plastic sheet.

Sound emitted by a crumpled plastic sheet. (Via arXiv, ref. 2).

Attaching microphones to the surface of a machine to detect acoustic emissions is a great way to monitor when something is starting to fail, such as a gear in a gear box. It's also a method to decide when preventative maintenance is required.[3] In one application, the wear of a tool in a single point turning machine was monitored to predict the point at which the tool should be changed.[4]

The automotive industry, which has always used "quiet ride" as a selling point for vehicles, has bundled acoustic emissions into a category it calls noise, vibration, and harshness, or NVH for short. Automobile manufacturers will effect redesigns of some parts after acoustic testing to eliminate unexpected resonance conditions. The older readers among us likely remember how their first car would buzz at a particular speed.

A paper published this month in BioScience by a research team from Purdue University proposes using the acoustic emission technique as an environmental diagnostic.[5-6] Such an idea goes back fifty years to the publication of Rachel Carson's influential 1962 book, "Silent Spring," which reported how pesticides were killing birds. In this way, sound (or lack thereof, similar to Holmes non-barking dog) was a first indicator of an environmental change. Says Bryan Pijanowski, lead author of the paper and associate professor of forestry and natural resources at Purdue,[5]
"The dawn and dusk choruses of birds are very characteristic of a location. If the intensity or patterns of these choruses change, there is likely something causing that change... Ecologists have ignored how sound that emanates from an area can help determine what's happening to the ecosystem."

The Purdue study used 35,000 recordings to ascertain how humans are affecting their environment. One finding was that human activity added a low, but constant noise. Humans have learned to block out this background buzz, but at the same time they've blocked out the sounds of nature. Pijanowski and his cohort are calling their new discipline, "soundscape ecology," and they are coining such new words as "biophony," for sounds made by organisms; and "geophony," for non-biological sounds, such as wind and rain. The authors propose the following soundscape ecology topic areas:[6]
(1) Measurement and analytical challenges
(2) Spatial-temporal dynamics
(3) Soundscape linkage to environmental covariates
(4) Human impacts on the soundscape
(5) Soundscape impacts on humans
(6) Soundscape impacts on ecosystems.

Research in this new field will be aided by the vast array of sensing and computing technologies that are already used in acoustic emission studies. References 7-14 are various arXiv preprints relating to acoustic emission, one going back to 1995.[7]


  1. Silver Blaze page on Wikipedia.
  2. R. S. Mendes, L. C. Malacarne, R. P. B. Santos, H. V. Ribeiro and S. Picoli Jr., "Earthquake-like patterns of acoustic emission in crumpled plastic sheets," arXiv Preprint, November 24, 2010.
  3. Miinshiou Huang, Liang Jiang, Peter K. Liaw, Charlie R. Brooks, Rodger Seeley and Dwaine L. Klarstrom, "Using Acoustic Emission in Fatigue and Fracture Materials Research, Journal of Materials (Online supplement for vol. 50, no. 11), November, 1998.
  4. F.A. Farrelly, A. Petri, L. Pitolli, G. Pontuale, A. Tagliani and P.L. Novi Inverardi, "Statistical properties of acoustic emission signals from metal cutting processes," arXiv Preprint, April 27, 2004.
  5. Brian Wallheimer, "New scientific field will study ecological importance of sounds," Purdue University Press Release, March 1, 2011.
  6. Bryan C. Pijanowski, Luis J. Villanueva-Rivera, Sarah L. Dumyahn, Almo Farina, Bernie L. Krause, Brian M. Napoletano, Stuart H. Gage and Nadia Pieretti, "Soundscape Ecology: The Science of Sound in the Landscape," BioScience, vol. 61, no. 3 (March, 2011), pp. 203-216.
  7. Paul A. Houle and James P. Sethna, "Acoustic Emission from crumpling paper," arXiv Preprint, December 7, 1995.
  8. J. Rosti, J. Koivisto and M.J. Alava, "Statistics of acoustic emission in paper fracture: precursors and criticality," arXiv Preprint, February 3, 2010.
  9. L.I. Salminen, J.M. Pulakka, J. Rosti, M.J. Alava and K.J. Niskanen, "Crackling noise in paper peeling," arXiv Preprint, January 11, 2006.
  10. L.I. Salminen, A.I. Tolvanen and M.J. Alava, "Acoustic Emission from Paper Fracture," arXiv Preprint, January 16, 2003.
  11. Jagadish Kumar, Rumi De and G. Ananthakrishna, "Intermittent Peel Front Dynamics and the Crackling Noise in an Adhesive Tape," arXiv Preprint, December 1, 2008.
  12. Jagadish Kumar, M. Ciccotti and G. Ananthakrishna, "Hidden Order in Crackling Noise during Peeling of an Adhesive Tape," arXiv Preprint, April 8, 2008.
  13. Vincent Gibiat, Eric Plaza and Pierre De Guibert, "Acoustic emission before avalanches in granular media," arXiv Preprint, June 20, 2009.
  14. Anders Johansen and Didier Sornette, "Critical Ruptures," arXiv Preprint, April 3, 2000.

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Linked Keywords: Arthur Conan Doyle; Silver Blaze; Sherlock Holmes; sound; diagnosis; diagnostic; personal computing; modified frequency modulation; MFM; hard disk drive; hard drive; five senses; sixth sense; backup; booting; boot; Solid state drive; magnetoresistive random access memory; MRAM; acoustic emission; microphone; machine; gear; transmission; gear box; preventive maintenance; preventative maintenance; tool; turning machine' automotive industry; noise, vibration, and harshness; resonance; BioScience; Purdue University; environmental; Rachel Carson; Silent Spring; bird; forestry and natural resources; noise; wind; rain.