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Laser Vibrometer Heightens Speaker Design

Photonics Spectra
Jan 1998
Ruth A. Mendonsa

The VPI 4000 scanning laser vibrometer produces a clear picture of what is happening on the surface of the speaker. Courtesy of Mackie Design.

The company's first loudspeaker product, the HR824, represents its commitment to quality through the application of solid engineering with state-of-the-art acoustic vibration measurements. In designing the HR824 studio monitor, engineers at Mackie used Ometron Inc.'s VPI 4000 scanning laser vibrometer technology to transform their theories for an active monitor, which is an internally powered rather than passive speaker, into a precise, powerful, completely neutral sound system.
Development engineers typically used the "tweak and listen" approach to system design: They would build something, listen to it, tweak it and listen to it again. Engineers at Mackie now use the VPI 4000 to get a clear picture of exactly what is happening on the surface of the speaker. The vibrometer employs a safe, low-power laser to scan the surface of a vibrating component. It produces full-field vibration maps at selected frequencies within seconds through fast Fourier transform analysis of the surface data. Although it is a laser-based technology, the VPI 4000 requires no special surface coatings and may even be used on
structures with complex geometries.
Greg Mackie, the company's CEO, likens the VPI 4000's application of laser Doppler vibrometry to speaker development to what the invention of the oscilloscope did for electronics design. He said that while the instrument is expensive, it "has proved its worth in selecting and optimizing the HR824's transducers." He said it produces accurate, instant images of the vibrations that occur in the transducer dome or cone at any given frequency. The combination of vibration and acoustic measurements enables the best understanding of the loudspeaker.



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