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Swords, Cutting and Military History

Audible sharpness: swords and sounds

Audible sharpness:  swords and sounds

photo by c_liecht

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In cutting with live blades, practitioners quickly learn that real blades rarely make the sounds Hollywood so often attributes to them.  Movie audiences have come not only to expect certain sounds to be associated with swords in the movies, but most honestly believe the Hollywood version is what swords in use should sound like.

Indeed, within the film-making industry, there are a whole series of “tropes” ( in the industry, an over-used or stereotypical plot device ) which revolve around the sounds swords make on being drawn, used, etc.  One of these tropes is audible sharpness, from which this article draws its title.

Practitioners have long fought against these stereotypes;  some very aggressively



While others approach the same topic with a bit more background and explanation on how the industry may have come to this practice



One Hollywood sword sound  is very real:  the “swish” a blade makes while cutting through air.   This sound is created by the turbulence the sword causes as it moves.   Blade design and geometry help create circular vortexes of air along the blade, magnifying the disturbance, creating a loud and clear sound we perceive as a “swish” (scientists say it is more like a “humming” ) as the blade moves.


Blade with blood groove

photo by One Lucky Guy

Variations in sword design mean that some blades will be quieter than others.  The design feature to which the distinctive air-cutting sound can be most attributed is the blood groove or fuller; on Japanese blades this is most commonly called the bo-hi.

There is much fiction about the groove being cut into a blade so suction will not prevent the blade from being withdrawn from a wound, a myth which has long been disproved.  Instead, blood groves are areas where metal is removed so as to create a lighter, more maneuverable blade;  most often these are carved as dished or concave lines along the length of a sword.

A  secondary effect of this curved groove is to create perfectly circular air vortexes as the blade moves… provided the blade is held so the grooves are at right angles to the direction of travel.  In other words,  the more perfectly the edge of the blade is held in line to the direction of the cut, the more air is exposed to the geometry of the blade and the grooves on both sides, and therefore the louder and clearer the “swoosh” of the cut will be.  In the Japanese arts, this alignment of edge-to-direction-of-travel — indicating a clean cut through a target would likely be made — is known as hasuji.  The actual sound the sword makes is called tachikaze (Japanese:  tachi = sword, kaze = wind, thus literally “sword wind”)

The science and physics of this effect is perfectly explained in an excellent article from Scientific American:  The Science of Swords: The Sound of Approaching Doom

It’s particularly interesting to note the author commenting on something which  my own branch of Iaido uses in training:

The thickness of a sword and the geometry of its edge are also key factors regarding how easily a sword will generate tachikaze when swung. Intriguingly, the biggest factor behind tachikaze is whether or not a sword has a groove along its backside known as a bo-hi — swords with this groove are louder than swords without it. The blunt swords known as iaito that beginning students practice with typically have bo-hi, so the students can hear when they are getting hasuji right or wrong.

In other words, even in practice without a target being present, a student can know whether or not their cut would likely have been effective by the intensity and clarity of the tachikaze they create when cutting.

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