Which patterns, instruments, characteristics (or whatever) does a song need to have to be considered rock? Can this be objectively analyzed? If not, which subjective dynamics are involved?
Which patterns, instruments, characteristics (or whatever) does a song need to have to be considered rock?
Given that 'Rock' evolved (and encompasses) 'Rock n Roll', one defining aspect of rock music would be that it had to be created no earlier than the 1950s (when the term was coined).
This is a bit tougher, but I suppose we could say 'The Guitar' is typically the primary instrument associated with Rock. The catch is that not all rock songs may have a guitar, nor are all songs with a guitar 'rock'. So there probably is no one particular instrument that defines rock as a genre. It's just that there are a few instruments that are heavily associated with it (The Guitar, the Drum Kit, the Bass).
For this, I think Wikipedia actually does a good job. To summarize wikipedia, Rock typically:
- has a focus on the electric guitar, bass guitar, and drum kit
- has simple rhythms in 4/4 meter.
- Lyrically, rebellion or girls.
The catch is that there is so many exceptions to the above. This quote from the section linked to sums it up well:
Because of its complex history and tendency to borrow from other musical and cultural forms, it has been argued that "it is impossible to bind rock music to a rigidly delineated musical definition." -- P. Wicke, Rock Music: Culture, Aesthetics and Sociology (Cambridge: Cambridge University Press, 1990), ISBN 0-521-39914-9, p. x.
Can this be objectively analyzed?
In a lot of cases, sure. AC/DC? Guitar? Check. Drum kit? Check. 4/4 time? Check. Silly lyrics about drinking and getting laid? Check. Yep, that's rock. But in a lot of cases, not at all. The Postal Service? Well...now we have a good debate going on.
If not, which subjective dynamics are involved?
I think that list could be infinite as it's going to come down to a whole lot of personal opinion. And, again, the other issue is the broad umbrella that 'Rock' encompasses. For example, there are people that would say the 'Rap' genre certainly fits under the 'Rock' umbrella. And given how much cross over there has been between rap and and rock, it's hard to not accept that as a valid argument.
Theodore Gracyk (Rhythm and Noise, Duke UP, 1996) makes the interesting claim that what separates "rock" from jazz, blues, soul and other kinds of rhythm & blues (including 1950s rock 'n' roll) is the ontological primacy of the recording over live performance. A rock musician is more like a painter than a jazz musician: his "work" is the recording, which is a pseudo-object that can be copied, distributed, etc. Given a distinction at this level, any sound can, in principle, be on a rock record. (The example Gracyk gives is the bass line in Horace Silver's "Song for My Father." In the original, it is jazz; in Steely Dan's "Ricki Don't Lose My Number," it is rock.)
This distinction really only helps distinguish rock from closely related and previous genres. It doesn't mean that rock is the only music in which the record is the "primary text": arguably most contemporary pop works this way, as does some hip-hop. (American Idol and The Voice are counter-examples.)
Dance music is a strange case: the record is primary, but ideally serves as the basis for a "second order" performance when it gets played for dancers in a club.