Laser speckle imaging to measure surface motions

[Few]

I came across this paper on an updated method for using changes in laser speckle patterns to monitor surface vibrations. For speaker measurements, the issue of the source moving around isn't an issue (their method can handle moving musicians, but speakers or drivers can be held stationary). As a result, I think it might be possible to streamline their approach, and perhaps even get away with one camera instead of two. I'd be interested in a discussion that helps me identify flaws in my thinking before I waste too much time and energy! For those who prefer videos to old-fashioned reading, here's a video made by one of the paper's authors.

If there's no risk of the speaker or driver's location changing location during the measurement, then I think a single reference frame is sufficient for each measurement. A laser could be pointed at a speaker diaphragm, a reference image could be obtained, and then a video of the defocused speckle pattern could be obtained while the speaker's driving frequency is swept (probably logarithmically). After some image processing, the result would be a highly localized measurement of the local diaphragm displacement as a function of frequency. Repeating the measurements over the surface of a driver or enclosure could yield a map of displacement vs. frequency.

The trick of using a rolling shutter to overcome the need for an expensive high-speed camera is really intriguing. I've long been interested in this sort of measurement, but cameras than can record images at a rate that spans the audio range are out of my pay bracket. Perhaps somebody here has some expertise in a relevant area that can help assess the viability of this approach on a diy budget. I play with lasers and imaging technologies as part of my day-job, and so far, it seems like a simplified version of this could be fun. Perhaps I'm missing something fundamental...

Few

 

[restorer-john]

Celestion have been using laser interferometry for many decades, I think I remember it first used in the early 1980s.

Advanced Cone Measurement and Analysis - Celestion

Measurement and analysis of cone movement using LASER interferometry enables Celestion to perform in-depth evaluation of loudspeaker performance very early on in the design phase.

celestion.com

How is the 'speckle' pattern more/less useful?

 

[sam_adams]

Klippel has been using laser systems to measure speakers for quite some time, also.

Scanning Vibrometer System (SCN) - Klippel GmbH

Laser Displacement Sensor - Klippel GmbH

Scanning Mechanical Vibration - Klippel GmbH

Loudspeaker Cone Vibration - Klippel GmbH

Vibration and Radiation Behavior of Loudspeaker's Membrane - Klippel GmbH

Loudspeaker Sound Radiation - Klippel GmbH

 

[Few]

The speckle pattern approach can likely be made simpler and cheaper, and therefore more suitable for DIY. It also supports monitoring multiple spots on the vibrating surface simultaneously. The Klippel systems I have looked into use a specialized laser triangulation module, rather than interference effects.