A signal instrument is a musical instrument

Musical instrument

A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

 which is not only used for music as such, but also fit to give sound signals as a form of auditive communication, usually in the open air.
To make the message audible at a distance, percussion

Percussion instrument

A percussion instrument is any object which produces a sound when hit with an implement or when it is shaken, rubbed, scraped, or otherwise acted upon in a way that sets the object into vibration...

 and brass instrument

Brass instrument

A brass instrument is a musical instrument whose sound is produced by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips...

s, which are generally loud, are chiefly used for this purpose.

Environment

Before the introduction of modern technological communication, signaling over a distance was often a very good way to pass messages, especially in difficult terrain such as the mountains (e.g. Alpine horn, equivalents are still used in parts of the Himalaya) or sparsely populated plains or forests (the tam-tam type of drums as with American Indians and jungle drums), sometimes using rather elaborate code systems to pass even complex information.

Another ancient function, which has survived into modern urban life is to assemble or warn a whole population or congregation at large, usually not coded or just for a few common cases, as with a conch

Conch

A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

 or bells

Church bell

A church bell is a bell which is rung in a church either to signify the hour or the time for worshippers to go to church, perhaps to attend a wedding, funeral, or other service...

 in a church or belfry: a variation for more local use is the gong

Gong

A gong is an East and South East Asian musical percussion instrument that takes the form of a flat metal disc which is hit with a mallet....

.

Many types, especially the older ones, have also survived for ceremonial use, as in religion (often conservative in its forms) or gong ceremony

Gong

A gong is an East and South East Asian musical percussion instrument that takes the form of a flat metal disc which is hit with a mallet....

.

Hunting

While the types mentioned above are mainly used from one spot, signal instruments may also be useful to communicate on the move, when many alternatives were less practical. Naturally then instruments are preferred which are not too delicate to be moved, and often not too heavy (except when the musician is mounted or in a vehicle) to transport, or even better be played during march or even chase. Thus hunters traditionally use a hunting-horn to communicate, while drums (often just improvised 'percussion') were rather by drivers while shouting. When the social elite practiced hunting as a prestigious outdoors social activity, music was often present at any stage, e.g. the court of Versailles

Versailles

Versailles , a city renowned for its château, the Palace of Versailles, was the de facto capital of the kingdom of France for over a century, from 1682 to 1789. It is now a wealthy suburb of Paris and remains an important administrative and judicial centre...

 had composers such as the Danican

François-André Danican Philidor

François-André Danican Philidor , often referred to as André Danican Philidor during his lifetime, was a French composer and chess player. He contributed to the early development of the opéra comique...

 family and Philidor write special symphonies for a court orchestra to accompany the royal hunt party, prominently featuring percussion and winds but also including non-signal instruments, even strings.

Military, paramilitary and other uniformed services

In the (para)military and similar, mainly uniformed, corps such as police, the tradition of march music stems from the use of signal instruments, mainly metal winds (as the modern bugle

Bugle (instrument)

The bugle is one of the simplest brass instruments, having no valves or other pitch-altering devices. All pitch control is done by varying the player's embouchure, since the bugle has no other mechanism for controlling pitch. Consequently, the bugle is limited to notes within the harmonic series...

 and clarion

Clarion

-In music:* Clarion , a type of trumpet used in the Middle Ages* Clarion register of clarinets, ranging from B4 to C6-Places:* Clarion, Illinois, USA* Clarion, Iowa, USA...

), originally (and still) to pass standardized orders (often at a small tactical level), while percussion and flutes served mainly to march on the beat; especially when a larger force is gathered with ceremony (just drums often do, as in ruffles and flourishes

Ruffles and flourishes

Ruffles and flourishes are preceding fanfare for honors music .Ruffles are played on drums, and flourishes are played on bugles...

or accompanying formal administration of corporal punishment), both can be combined into a band.