Deep Cavities: When the Horn Cannot Reach the Joint

Boxes, tubs and panels with a surrounding rim often carry the joint deep inside the cavity. Reaching it means a long, slender horn, and that fights the acoustic design.

Why slender horns are difficult

Length is tied to half a wavelength. A horn cannot simply be made longer; its length follows the operating frequency. Reaching deeper means a stepped design or an added booster, and every added section loses energy and adds uncertainty.

Slender horns develop flexural modes. At a high length-to-diameter ratio, lateral motion appears alongside the axial vibration, the amplitude distribution becomes unstable, and cracks start at stress concentrations.

Energy arrives attenuated. The longer the path, the greater the loss, and the real amplitude at the face falls below the design value.

Look at the design first

The cheapest answers all live on the drawing.

Move the joint towards the opening. In many structures the joint position has room; a few millimetres towards the opening lets a conventional horn work.

Reverse the assembly direction. A deep part with a shallow lid becomes a shallow part with a deep lid, and the joint moves from the bottom of the cavity to near the surface.

Enlarge the opening where the structure allows, so the horn can enter.

Use far-field welding. The horn contacts somewhere on the outside and energy travels to the joint. This depends on the material: amorphous resins such as ABS and PC transmit well and it is workable; semi-crystalline PP, PE and nylon attenuate fast and it essentially is not.

When the design cannot change

Accept a lower frequency. Lower frequency means a longer wavelength, a longer horn and longer transmission.

Use titanium. Slender horns run at high stress, and titanium is better than aluminium for both fatigue and acoustic loss, at the cost of machining and price.

Treat the horn as a consumable. Slender horns last less than conventional ones, and cracks start at stress concentrations — face corners, slot ends and thread roots. Inspect with a magnifier or dye penetrant on a schedule and carry spares.

Lower the amplitude demand. Where the material allows, replace the energy director with a shear joint, which asks less of amplitude because it works by sustained friction rather than instantaneous impact.

Another route: change the process

If a deep-cavity part only needs fixing and not sealing, ultrasonic staking is usually easier to realise than welding: it acts at a few discrete points rather than running a joint round the bottom of a cavity, and the tooling is far simpler.

Where sealing is required, another direction is to give sealing to a gasket and let welding or staking handle fixing only, solving the two requirements separately. On complex parts that is often the more stable plan.

Assessing feasibility

When sending drawings for assessment, state these: cavity depth and opening size, joint position and perimeter, resin grade, whether sealing is required, and which side is cosmetic.

The material decides whether far field is possible, the sealing requirement decides whether staking can substitute, and the cosmetic side decides where the horn may press. With those four settled, the approach is largely settled too.

Horn material selection, the relation between gain and profile, and the limits on slender horns are in the horn selection and design guide on the downloads page. Frequency options and maximum horn sizes are on the ultrasonic welding machine pages.

Send drawings to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.