At 9/20/13 11:21 AM, PoonPun wrote:
But, what I don't understand is, why have two different time signatures, if the timing is the same?
They're not quite the same. The difference is what a beat in each time signature is made out of.
3/4 has THREE beats, and it's what is known as a "simple" time signature. This means that each beat can be divided into multiples of 2 (one beat = 1 quarter note or 2 eighth notes or 4 sixteenth notes etc). Here's a beat in 3/4: pic - audio. As you can see (and hear), a hi-hat is playing every half-beat, meaning 2 hi-hats for each of the three beats. This way you're able to notice that each beat is actually split into a multiple of 2, in this case 2 eighth notes played by hi-hats.
6/8 is what's known as a "compound" time signature. Its beats are split into multiples of 3 rather than multiples of 2 (one beat = 3 eighth notes or 6 sixteenth notes or 12 thirty-second notes etc). There are TWO beats in a bar of 6/8, and each beat is split into three eighth notes (making 6/8). Here is a beat in 6/8: pic - audio. There are three hi-hats playing for every beat, and there are two beats.
As Braiton said, mathematically they're the same. 3/4 = 6/8. However, rhythmically, they're "felt" differently. There are 3 beats in 3/4 and 2 beats in 6/8. Here's an example of a track in 3/4 (up till 1:16), and here's one in 6/8. The beat feels different. In the first the beats are 1 2 3 1 2 3, and in the second they're 1 - - 2 - - 1 - - 2 - -. It's hard to explain.
Stuff gets even more complicated with the less standard time signatures, like 5/4, for example. Songs in 5/4 have five beats as opposed to the three beats of 3/4. An example of music in 5/4 would be the famous Mission Impossible theme. Up till 0:19, the track is in 5/4. If you listen to the first 19 seconds of that theme, you can hear that there are 5 beats per bar. Then, at 0:19, the track does something spectacular and magical called a time signature change (and also a tempo change). It changes from 5/4 to 4/4, and to a slower tempo. If you're able to clap the beat to that song, you'll find that up until 0:19 you're clapping 5 beats in a bar, and then at 0:19 you're clapping 4 beats every bar at a slower tempo. Being able to clap the rhythm to a song is something you pick up naturally.
One thing to add about 5/4 - generally, songs in 5/4 have an accent somewhere in the bar which splits up the beats. 5/4 is usually 3/4 + 2/4 or 2/4 + 3/4, depending on where the accent is. If you hear this track for example, you'll hear that there's a snare on the fourth beat, which naturally splits the bar into 3/4 + 2/4 to make up 5/4. Try counting the beats to that track. Rather than counting it 1 2 3 4 5 - 1 2 3 4 5, try counting it as 1 2 3 - 4 5 - 1 2 3 - 4 5, etc.
There's a whole plethora of time signatures out there, and even I have trouble following some of them. It all comes with practice and experience listening to music in obscure time signatures, and once you start "understanding" an obscure time signature of the track, you'll start enjoying it much more and realise that it's a great change from all the 4/4 tracks we find everywhere.
Hope that helped!