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Spool horns

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A spool horn is an unslotted cylindrical horn in which the sides have been undercut behind the face to form a spool shape. This spool shape improves the face amplitude uniformity. Because a spool horn does not have slots, its stresses are much lower than comparable slotted cylindrical horns and machining costs are much lower. Spool horns generally about 1:1 gain, although somewhat higher gain is possible. Spool horns are used for plunge welding and, occasionally, for liquid processing.


The following example shows a 20 kHz 5" diameter horn. The axial resonance is the desired resonance. The horn is one half-wavelength long at axial resonance, as indicated by the single node that is generally transverse to the principal direction of vibration.

For all images, the output surface (face) is at the top and the input surface is at the bottom. The warmest colors indicate the highest amplitudes. The darkest color traces the axial node(s).

All results are from finite element analysis.

Original design

The following shows the original (unoptimized) design and the resulting amplitude distribution.

Ultrasonic horn -- unslotted cylindrical, unoptimized Original design -- No optimization

Ultrasonic horn -- unslotted cylindrical, unoptimized, axial resonance Axial resonance, relative amplitudes -- The amplitude is much lower at the outer face  than at the center. This will cause under-welding at the outside or over-welding at the center.

View actual vibration

Improved design

The following shows an improved design that has substantially better amplitude uniformity across the horn's face.

Ultrasonic horn -- spool, optimized

Improved design -- Uses optimized cavity, slots, and back extension

Ultrasonic horn -- spool, optimized, axial resonance

Axial resonance, relative amplitudes -- The amplitude is now very uniform across the horn's face.

View actual vibration

Design considerations

In order to preserve the face amplitude uniformity, spool horns cannot have any significant face cavities.

Size limitations

As the horn diameter increases, the face radial amplitude also increases quickly. If the horn is used for plastic welding, this radial motion can cause marking of the part.

Because of an adjacent secondary resonance (next image), the maximum horn diameter is limited to about 0.55 * wavelength (about 5.5" at 20 kHz). Above this size, the horn must be slotted. See slotted cylindrical horns.

Ultrasonic horn -- spool, secondary resonance

Secondary resonance

View actual vibration

At diameters near 0.4 * wavelength, an adjacent asymmetric resonance can cause poor face amplitude uniformity. Therefore, special design techniques are required for these diameters.


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Krell Engineering
212 E. Medwick Garth    Baltimore, MD  21228    USA

e-mail: info@krell-engineering.com