Graduate Intern, Luke Wilding, discusses the term alarum, and suggests what it might have actually sounded like…
The term alarum occurs 89 times in Shakespeare’s first folio. The Oxford English Dictionary states that an alarum is “used as a call to arms or warning of imminent danger, esp. of being attacked.” Christopher R. Wilson and Michela Calore’s (2014) invaluable Music in Shakespeare: A Dictionary offers this:
This term literally means ‘to arms’ (from the Italian all’armi). The sounding of alarums by various instruments, especially trumpets, drums or bells is connected with military atmospheres, and is most frequently called for in historical plays and tragedies.
This does not, however, tell us much about what an alarum really is or sounds like. All we know is that an alarum is a sound which usually occurs in military situations in ‘historical plays and tragedies.’ Even Alan C. Dessen and Leslie Thompson’s A Dictionary of Stage Directions in English Drama 1580–1642 gives a limited definition:
A call to arms in the form of sound produced offstage before and during a battle helping to create an atmosphere of conflict and confusion; at Richard III’s call for “A flourish, Trumpets! strike alarum, drums!” is “Flourish. Alarums” (Folio Richard III, 2926, 4.4.151) and evidence indicates that a drum was usually used, although occasionally a trumpet of other instrument is indicated; the signal is most commonly simply alarum – the predominant spelling – with numerous examples in stage plots and playhouse manuscripts[.]
This is essentially the same definition given by Wilson and Calore (2014) with a little more detail. The reason that this is such a hard term to define is that it is so broad and general that it can cover a great range of military calls to arms.
To understand what an alarum is likely to have been intended to sound like in a certain passage from the Shakespeare corpus one must consider the context in which the alarum occurs. If we take, for example, the stage direction “Flourish. Alarums” which accompanies Richard III’s call for “A flourish, Trumpets! strike alarum, drums!” (Folio Richard III, 4.4.151) then it is likely that we are dealing with a relatively simple and short but melodic piece of music made up of a simple ostinato (riff) from the trumpets accompanied by simple rudiments played on a snare drum. It is tempting to think of the foxhunting bugle horn call or the military Assembly of Trumpeters For Reveille (recordings are available on the web, e.g. https://www.youtube.com/watch?v=pelDdFgajFk):
The alarum call was, however, most likely far less ornamental than these as it was designed for the simple purpose of calling for a unified rouse and advance. Though surviving transcripts of early-modern sheet music for plays which featured alarums are very rare (if sheet music for alarums existed at all and was not simply improvised), the stereotypical modern convention for an advance alarum used on the stage and on television from Game of Thrones to the BBC’s The Hollow Crown series is a sequential interval of a “perfect fifth” performed by a trumpet or horn followed by a marching rhythm performed on the snare drum (various marching rhythm examples can be found online, e.g. https://www.youtube.com/channel/UC0zRjYQSNxj86GtZrbZ03lg). Though it is unlikely that this is a truly synchronic representation of the advance alarum heard by Elizabethan theatre-goers, this simple compositional style fits the function of the alarum in this context.
In other situations, of course, this kind of melodically structured alarum makes less sense. The surprise attack on Le Mans in Act Two, Scene One of 1 Henry VI, for example, would allow the French army no time to assemble trumpeters and drummers and have them perform their rehearsed alarum. Sentinels, soldiers or guards keeping watch, upon seeing the English army scaling the walls of the city cry, “Arm! Arm! The Enemy doth make assault!” (2.1.38). This would most likely be accompanied by the tolling of an alarum bell rather than a figured piece of music. The bell would be struck as a kind of rude awakening to desperately rouse troops from their sleep and alert them to immediate danger. Even the call of the sentinel itself is a form of alarum in this context. An alarum is often used as a more functional device than as a musical element of military spectacle. The modern equivalents of the alarum are things like the military siren or the security alarm; automatic instruments which make hearers aware of danger and the need to prepare arms. Though no longer the tolling of the bell, the sounding of the trumpet, or the striking of the drum, these modern alarums serve the same function of rousing hearers through the same means of a loud, grating, rhythmic sound. Though somewhat artistic and ornamental in its advancement form, the alarum is, at root, a simple functional device for calling all to arm. It is an alarm.
 Christopher R. Wilson and Michela Calore, Music in Shakespeare: A Dictionary (Bloomsbury, London, 2014), pp. 2-3.
 Alan C. Dessen and Leslie Thompson, A Dictionary of Stage Directions in English Drama 1580–1642 (Cambridge: Cambridge University Press, 2001), p. 3.