Boosters, as discussed earlier, are typically of standard design, whereas a horn (sonotrode), the vibrating component actually contacting the work, can be any of a wide variety of designs. These can be as as simple as cylinders or rectangles, possibly stepped to increase amplitude, hollow round horns, etc. Basic rules of horn design are simple. The horn (sonotrode) must vibrate most strongly at a frequency near the fundamental frequency of the machine, in an essentially linear fashion. It must contact the work with an acceptable level of marring of the surface (often this acceptable level is zero) and transmit a proper amount of amplitude to the workpiece. It must not wear excessively, it must not break too soon. It must not violate clean room rules or be subject to rapid corrosion in the work environment. Its manufacture must be at an acceptable cost. There are probably other rules but those are the main ones. There are three common materials for horn construction. Aluminum is relatively inexpensive on a unit volume basis, is easy to machine, responds acoustically very well, and can be plated or coated in many ways. Of the three common materials it is lowest in fatigue strength, wears easily if a protective coating cannot be used or is breached, and in applications where frequent tool changes are made is subject to wear and stresses at the interfaces and connecting threads. Common tool steel had been used for many years, but is rapidly giving way to sintered steel alloys. These newer steels have very uniform grain structure, can be made relatively hard to resist wear, and allow nearly as much freedom of design as aluminum or titanium. Because of the hardening process, they can be expensive to manufacture, and compared to tools of aluminum or titanium, they are amplitude limited. Titanium alloy tools have the highest fatigue strength of any common ultrasonic tools and so are durable and capable of high amplitude operation. They are about midway between aluminum and steel alloys in hardness, and are approved for food contact or contact with implantable medical devices. Titanium is somewhat freer machining than tool steel although special technique is required to avoid work hardening the surface or setting chips afire (burns in a similar fashion as magnesium). Large horns are not often made of titanium because of material expense nor of steel because the relatively high density would result in very heavy tools. Machinable titanium stock is often not available in very large sections. One common question is why larger horns have slots parallel to the direction of sound transmission: there is a rule in solid waveguide design that as a secion approaches 1/3 wavelength in the transverse dimension, the efficiency of the waveguide drops precipitously. In order to allow larger horn designs, the slots are added to break up the transverse dimensions such that no section has a transverse dimension approaching or exceeding 1/3 wavelength. A consequence of this, however, is that generally the section directly driven by the booster will have the highest amplitude, followed by the sections nearest outboard of it, and so on. In advanced horn design, various means are used to more closely balance the amplitude, de-stressing the tool and allowing for higher overall amplitude and more even welding. Generally, an even number of slots are used in each driection slotted, so as to avoid putting a slot directly under the stud hole and creating a weakened area where premature stress cracks can develop.