Trying to determine the key of a song by ear requires a little ear training. Here's a tip-Try to find the main note ("root note") which the melody keeps resolving to. Everytime the song reaches this note, things will become relaxed and the music will seem to resolve. When you find a note, you can test if it is the right note or not by playing a major scale in the "root note" key on a guitar or a piano while the song is playing. If it sounds good, you've found the right note!
If you're playing to a blues progression, then you can easily determine the I, IV, and V chords by listening to the track for a while. When the song enters the I chord, pick up a harmonica and play the blues scale starting on the 2 draw. If it sounds right, you have found the right key harp. If not, keep trying other harps until it does sound right. Yes, it is a little time consuming at first, but it'll become easier with practice. Another way to determine the key could be to just hum the blues scale while the track plays. I think it is easier and faster to determine the key of a song by just humming along with it. If your humming sounds good with the backing track, you have found the right scale. Try to find the notes you are humming on a piano. Suppose you find out that the first note you are humming is an "E". This means that you can use an A harmonica and play along with the track in 2nd position. This is because the 2 draw note of an A harmonica is an E note.
For more information on harmonica keys and positions, read this article -http://www.harmonica.com/blog/19-harmonica-keys-positions.html
Another answer to a similar question:
Well, trying to figure out the key of a song by ear will require some experimentation.
If I want to figure out the key of a song..let's say "Old McDonald had a farm". What I would is I would listen for the main, "tonic" note. The tonic note is the note around which the whole song revolves. In Old McDonald, it's the first note. So, I would hum this note and try to find it on the 1 blow of a harp. After I find it, I can figure out what key harp to use for 2nd position or 3rd position to play in that key.
You might have to go through several harps to find the key, but I would start with a C harp..and if the tonic note is lower than the 1 blow of a C harp, I would try an A harp next....or a E harp (if it's higher). Then I would repeat the process. You can figure out the key of a song faster this way without having to try all 12 keys.
Here is another tip that I found on the Internet:
- Sing along with the song for one or two lines while it’s playing, and then:
- Sing DO-RE-MI-FA-SOL-FA-MI-RE-DO.
- The final “DO” you land on is the key you are trying to find.
It’s not an exact science, but it works. :)