Ultrasonic welding is an industrial technique whereby two pieces of plastic or metal are joined together seamlessly through high-frequency acoustic vibrations. One component to be welded is placed upon a fixed anvil, with the second component being placed on top. An extension ("horn") connected to a transducer is lowered down onto the top component, and a very rapid (~20,000 KHz), low-amplitude acoustic vibration is applied to a small welding zone. The acoustic energy is converted into heat energy by friction, and the parts are welded together in less than a second.

Ultrasonic welding is unique in that no connective bolts, nails, soldering materials, or adhesives are necessary to bind the two parts together. This saves greatly on manufacturing costs and creates visually attractive (i.e., unnoticeable) seams in product domains where appearance is important. Because ultrasonic welding is a largely automated process, all a technician needs to do is pull a lever and the welding is complete. The downside of ultrasonic welding is that it only applies to small components - watches, cassettes, plastic products, toys, medical tools, and packaging. The chassis of an automobile, for example, cannot be assembled with ultrasonic welding because the energies involved in welding larger components would be prohibitive.

The technology of ultrasonic welding appeared in the early 90s and has been under rapid development since then. As the technology improves, the range of materials that can be joined together using this technique increases. At first only non-flexible plastics could be welded because their material properties allowed the efficient transmission of acoustic energy from part to part. Nowadays, less rigid plastics such as semicrystalline plastics can be welded because large amounts of acoustic energy can be applied to the welding zone. As the technology matures and becomes more versatile, it is likely to obsolete large classes of historical techniques for joining materials together.