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Block horns

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A block horn is a rectangular horn that is slotted through both its width and  thickness. Block horns have low gain (generally near 1:1). Block horns are used for plunge welding.


The following example shows a 20 kHz 8" square block horn. The horn is one half-wavelength long at the axial resonance (the desired 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, no optimization -- block Original design -- No optimization.

Ultrasonic horn, no optimization -- block, axial resonance, relative amplitudes Axial resonance, relative amplitudes -- The amplitude at the corners of the face is much lower than elsewhere. This will cause reduced welding at the corners or over- welding elsewhere.

View actual vibration


Improved design

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

Ultrasonic horn, optimized -- block Improved design -- Uses optimized slots and back chamfer.

Ultrasonic horn, optimized -- block, axial resonance, relative amplitudes Axial resonance, relative amplitudes -- The amplitude is much more uniform across the horn's face.

View actual vibration

Design considerations

Because of their width and thickness, block horns must have longitudinal slots in order to reduce the transverse coupling due to the Poisson effect. The maximum distance between adjacent slots should not exceed about 0.3 * wavelength (about 3" at 20 kHz). Without such slots the horn will either have very uneven amplitude across the face or may even resonate in a nonaxial manner.

Although slots help to improve the face amplitude uniformity, additional horn refinements are often necessary to further improve the uniformity, depending on the particular application. Unfortunately, the required slots can introduce additional problems, although these can be reduced through careful design.

Secondary resonances

Slots often introduce additional secondary resonances. The following image shows a typical secondary resonance, although many others are possible.

Such secondary resonances may interfere with the vibration of the axial resonance. In some cases, the power supply may prefer to start on a secondary resonance or may jump to a secondary resonance during the weld cycle. The effects of secondary resonances can be minimized by designing the horn so that the secondary resonances are sufficiently far from the axial resonance.

Slot stresses

In block horns, the stresses are generally highest at the end of the slots, particularly where the slots intersect. The cause of this problem can easily be seen by watching the slots  deform as the horn vibrates (see the animation). High cyclic stresses can cause the horn to fail by fatigue.  This problem can be reduced by proper slot design and by machining the horn from high-strength materials.

The following image shows the stresses in a cut-away section of a complete horn where two slots intersect. The warmest colors indicate the highest stresses.

Ultrasonic horn -- slot stress



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

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