As discussed in the last post, the limited possibilities of musical instruments ensure overlap between musical genres. What is somewhat unexpected is the way in which some things particular to heavy rock have made their way into contemporary pop music.
Drop-tuning is the technique of tuning a guitar to a lower pitch for a heavier sound, which may include lowering the 6th string one whole step, which makes power chords playable with one finger, which makes riffing easier. Examples include Green Day's "Brain Stew" and System of a Down's "Aerials." One early example is Neil Young's "Cinnamon Girl," played in Drop-D tuning. This tuning is not uncommon in folk and classical guitar, but hard rock made really made something out of the one-finger power chord. Almost every Rage Against the Machine song is in Drop-D.
Here's an example where you wouldn't expect classicly heavy guitar:
I suspect that the trend towards lower pitches in music is being aided by the popularity of the two most common ways of listening to music: in loud clubs and bars where large sound systems can faithfully reproduce low pitches, and earbud headphones, which don't have to move as much air to deliver bass tones. As for country music becoming indistinguishable from hard rock, I think country had to drop tunings to compete with the hit-you-in-the-chest thump of rock and hiphop.
Stylistically, I present a comparison between Marilyn Manson's "The Beautiful People" and Katy Perry's "I Kissed A Girl."
Both tracks feature a swinging triplet drum beat at similar tempi. The main guitar riffs in each are sparse and punchy, and where the guitar is very reserved or even absent (the verses of each song), the vocals are buoyed by a synthesizer drone and a buzzing bass guitar, which is a pretty uncommon sound for basses (the only other example of that that I can think of offhand would be music by Muse).
To my ear, these tracks are very similar.
Incidentally, here's an excerpt from a Guitar Player magazine interview with the guitarists behind Movin' Out, the Broadway show based on the music of Billy Joel:
Although Byrnes and DelGaudio faithfully recreate signature lines from Joel’s recordings, there was still ample space for embellishment within the show’s rigid framework. “I use dropped-D tuning (D, A, D, G, B, E, low to high) to beef-up certain things,” say DelGaudio. “Suffice to say our ‘Angry Young Man’ is a bit more pissed-off than the original.” He notes that the most dramatic rearrangement sprang from an unlikely source: “Twyla wanted a nastier feel for ‘We Didn’t Start the Fire.’ We played her Marilyn Manson’s ‘The Beautiful People’ and she said, ‘That’s what I want!’ So the song’s I-V-VIm-IV progression mutated into power chords over heavy tom-tom triplets.”
The result, with inexplicable video from Cats:
Seems everyone wants the Manson vibe.