Chunk metal: Palm muting and spiccato
Spiccato lesson, cello
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Palm muting lesson, guitar
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To me, the signature sound of heavy metal is palm muting. Guitarists rest their palms on their strings near the bridge; that prevents strings from ringing out, and chug-chug-chug results. That "chunk" puts the "heavy" in heavy metal.
Palm muting has an analogue that I'm surprised isn't discussed more: spiccato. That is a classical string technique whereby the bow bounces off the string to create a chunky attack. Famous examples include the opening to Beethoven's 5th symphony ("power chords", if I ever heard any) and the 3rd movement of Vivaldi's "Summer". (You can see/hear the latter here with a nifty Ride the Lightning-esque graphic. YouTube also has many awful electric guitar covers of it; two that use palm-muting - a must to get the style right - are Alexi Laiho's and this dude's.)
Spiccato, which occurs in the lower half of the bow, is often confused with sautillé, another bouncy technique executed in the middle of the bow. Vigorous online discussions abound about these nuances; we need not get into them here. The takeaway is that classical string players play chunky chords and machine gun riffs, just like metal guitarists do. (Or is it the other way around?)
For demonstrations, see above. The first video is a brief cello lesson, which is appropriate since a cello occupies roughly the same register as a guitar tuned down to C. The teacher makes a comparison to drumming, which is both interesting and apt; palm muting and spiccato help make string instruments sound percussive.
The second video is a typically terrible-looking YouTube lesson that has me looking around to make sure that I'm not trapped in Guitar Center. (When I go to hell, it will be Guitar Center.) If you overlook the visuals, it's easy to hear that what the guitar and cello are doing isn't that different. Yes, the techniques are different, and the cultural differences are vast (I could care less about those), but the results are similar.
Basically, humans solved the problem of "making notes, often fast and repeated ones, on stringed instruments sound chunky and percussive" long before heavy metal. I happen to like chunky and percussive notes, often fast and repeated, on string instruments. If the execution is good, where they come from doesn't matter to me.
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