Figure 1. Three Similar Power Ultrasonic Actuators are depicted partly in cross sections to illustrate a progression of designs from a straight stepped horn to a folded inverted stepped horn.

Folded horns have been conceived as alternatives to straight horns used as resonators and strain amplifiers in power ultrasonic systems. Such systems are used for cleaning, welding, soldering, cutting, and drilling in a variety of industries. In addition, several previous NASA Tech Briefs articles have described instrumented drilling, coring, and burrowing machines that utilize combinations of sonic and ultrasonic vibrational actuation. The main advantage of a folded horn, relative to a straight horn of the same resonance frequency, is that the folded horn can be made shorter (that is, its greatest linear dimension measured from the outside can be made smaller). Alternatively, for a given length, the resonance frequency can be reduced. Hence, the folded-horn concept affords an additional degree of design freedom for reducing the length of an ultrasonic power system that includes a horn.

Figure 1 depicts an ultrasonic actuator that includes a straight stepped horn, one that includes an inverted straight stepped horn of approximately the same resonance frequency, and one that includes a folded stepped horn of approximately the same resonance frequency. The main role of the straight stepped horn is to amplify longitudinal strain at its outermost end.

Figure 2. The Overall Length of the 16-kHz Horn Is Shorter than the 20-kHz horn by virtue of being folded. The distance the acoustic wave travels has been designed to be the same. The lower frequency in the folded horn is due to reduced clamping and bending at the folds.

In the folded version, one can exploit bending strain in addition to longitudinal strain, and by adjusting the thickness of the folds, one can increase or decrease the contributions of bending displacements to the overall displacement at the tip. In this case, the folded-horn concept not only yields a shorter horn, but by enabling utilization of bending displacements, it also affords an additional degree of design freedom. Figure 2 shows an experimental folded-horn actuator of 16-kHz resonance frequency alongside a straight-horn actuator of 20-kHz resonance frequency.

This work was done by Stewart Sherrit, Stephen Askins, Michael Gradziel, Xiaoqi Bao, Zensheu Chang, Benjamin Dolgin, and Yoseph Bar-Cohen of Caltech and Tom Peterson of Cybersonics Inc. for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Mechanics category.NPO-30489.

This Brief includes a Technical Support Package (TSP).
Folded Resonant Horns for Power Ultrasonic Applications

(reference NPO-30489) is currently available for download from the TSP library.

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