Hey Friend,

There are many ways to fill out the sonic spectrum in music. By that I mean making sure the bass, mid-range and treble frequencies are represented in a song (as long as they serve the song).

Unlike an orchestra that can have hundreds of instruments in play, a rock band is usually limited to a handful of instruments. When you're moving from place to place on a nightly basis, you tend to have to travel light (unless you're some mega-star with a dozen semi rigs to haul tour gear around).

That's one of the reasons you see bands that are limited in size to four or five musicians. To get around and build an audience base, you want to travel with the minimum amount of gear and personnel that gets the job done. 

Fortunate for us, we can get by with fewer instruments because we have mics and amplifiers making our single instruments loud. But we still need to give the audience enough of the sound frequencies to satisfy the song and their ears.

How do we do that?

Ian S. wrote in and suggested down-tuning to get a deeper, more aggressive tone.

He's absolutely right there - dropped D and dropped C tuning are used in a lot of metal to achieve a dark mood. In fact, dropped D is a favorite tuning in a lot of Rock music (E flat, too). Combine that down tuning with the bass guitar, and you've got the low frequencies covered.

But guitars traditionally handle mid-range and high frequencies, and keyboards do the same. Most music needs to have those mids represented in a song to make it something we want to listen to. So we'll have a second instance of a guitar and perhaps a keyboard with additional parts written for those instruments.

Drums give us the sub-audible frequencies. That's everything at less that 20 Hertz (Hertz is the measurement of how many times a sound wave completes a cycle in one second). This is in the sound range that is below what we can discern as notes. So it comes across as thumps instead of a note like A or C. 

I know this is a real general discussion - is this something you'd like to know more about in the course of learning about guitar chords? I find it really fascinating (sound creation), but maybe you don't. Let me know...

Peace~

Dave