Strings are solids, and produce sound by the vibration of the string, which produces a soundwave through air. The resulting sound is very faint, which is why string instruments typically have resonant boxes that serve as amplifiers. These boxes move larger volumes of air than the string by itself. The strings act as initiators of the soundwave, but the air column is moved in the soundbox. The difference between air and solid instruments is that in air instruments the air movement is triggered either by human lungs, or mechanical "lungs" (bellows), while in solid instruments, some substance (the string) vibrates the soundbox, which then moves the air column.
The importance of the soundbox for string instruments can easily be tested by plucking, for example, the string of a bow (bow and arrow). Ancients cultures who began using the bow as a music instrument often use either mouth cavities to make the sound louder, or attach a gourd or calabash to the end of the bow. The structural design of a string or set of strings attached to a soundbox is thus a very ancient method.
Modern string instruments, ranging from the lute, harp, violin and guitar families to the piano all use sound boxes to increase the loudness of the vibrating strings. The orientation of the box in relation to the string set may differ, but the principle is the same. The 3D shape of boxes may differ, but again, the principle is the same. Different shape boxes will of course enhance different harmonics, and the cavity dimensions, and interfering objects (such as supporting beams) will play an additional role in modifying the baseline wave forms.
Many different materials have been used for the strings, from animal fibres to metals and plastic, and these alos have influences on the resulting soundwave, however minute the influences might be.
The triggering of the strings to initiate a soundwave range from plucking and hammering, to bowing, which on a micro-level might be regarded as tiny plucking fingers.
At some stage in the historical development of string music instruments, the keyboard was invented, which might be conceptualised as some kind of remote control triggering of strings. Instead of human fingers triggering strings, some artifical material is used, while the triggering itself is done mechanically from a set of keys (such as the familiar piano keyboard). Many instruments (such as clavichord, spinet, harspichord) of this design pre-date the piano, but as few people today know about these older instruments, the piano will be taken in this set of documents as generic type of this kind of string instrument. The organ's keyboard and construction may be similar to that of the piano, but the method of soundwave generation is totally different. The pipe organ is an air instrument, while the electric organ is obviously electronic.
The different sound textures produced by the many diferent kinds of strings instruments are due to the materials used to manufacture the strings, the triggering method, and the properties of the sound box (its dimensions and shape, and materials used in its manufacturing, including finishing touches such as the type of laquer used for finishing touches). Each of these components influence the base soundwave differently by their unique harmonics added to that base wave. Some experts claim they can hear the difference in sound quality between, for example, a modern violin and a Stradivarius violin.
Bar instruments may conceptually be regarded as solid strings. Membranes too may be conceptualy regarded as strings. The difference is in dimensions. A string has a relatively long length compared to its width or breadth (diameter), while a membrane, such as a square metal plate, has two larger dimentions. Another difference is that a string is tensed by the musician, by tuning it, while bars and membranes are "internally" tensed (by their nature), except of course the vellum of drums, which need to be tensed too, but typically fixed during the assembly phase.