The quadraphone is a massive brass instrument, played like the combination of a free-standing aerophone and a transducer. It combines the characteristics of a non-autonomous interruption-aeropphones with those of electric signals which are transformed into mechanical (i.e. acoustical) vibrations.
Unlike normal aerophone horns, it has a quadratic bell and rectangular sides. Colloquially known as an angular horn, the quadraphone is in fact a polygon sounder because the bell is, geometrically, a special polygon – a smooth, regular rectangle – which is quadratic. And this is itself a special case of a parallelogram and a trapeze, both rectangle and rhombus (diamond). For the construction of the bell only a single parameter is required, for example, the length of the sides or of the diagonal.

As the special case of a general n-dimensional form, the bell of a quadraphone is both a two-dimensional hypercube and a two-dimensional cross-polytope. The body of the quadraphone can be decomposed into a series of continuously diminishing cross-polytopes. Geometrically it displays a certain similarity to crystallography, which explains an important characteristic of its sound.

The way in which a quadraphone is played is so different from any existing instrument so that when it is played, it can combine vocal, instrumental, percussive and/or concrete sounds. When these are inserted (by a player and/or by a machine) they are, so to speak, rearranged and emitted with the quadraphone's specific sound. Thus the quadrophone is both instrument and instrumentalist, just as the player is like the combination of a person and a machine. This level has always evinced a certain philosophical peril for its user, since the border between control of the material being "played" and loss of control is very narrow.

Quadraphones are – pedending on location – played with or without an appendage, the so-called appendix vermiformis quadraphonis, in order to optimise architectonic and accoustic peculiarities. The initially spiteful reference to the common appendix, however, did not last as the appendix vermiformis quadraphonis constitutes a high-end addition to the body of the instrument.

The quadraphone was first constructed by artist and composer Rochus Aust in the year 2014 AD. In the same year, it was given its first large-scale performances at the Dusseldorf Quadriennal in 2014.

Its reputed relationship to the Neanderthals (homo neanderthalensis), discovered nearby, was never confirmed and can be regarded as a fable.

The name of the quadraphone should not be confused with the adjective quadrophonic.

The younger sister of the quadraphone is the mini-quadraphone. This is normally played mechanically, since its operation is subject to physical parameters, such as underground drains, letter boxes, dish washers etc.), which only machines can cope with.