#1
hey guys, im currently making a riff of a song i made, im trying to get this sad sound, so i was thinking what would be a good scale to apply to my riff..my chords are bm-g-d-a..
#2
B natural minor should work fine. I would maybe think about using major 7th chords on the D and A too. Major 7ths sound a bit more complex than major triads so they can help make a piece sound a little darker
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#3
It doesn't matter what scale you use, it's how you use it.

People usually say that a song in a major key sounds happy and a song in a minor key sounds sad. That's usually true, but you could write a song in a major key and make it sound sad if you wanted to...

Anyways, B minor would be a good scale to use
#4
Personally I tend to play more melancholic in A Major and B Major than in any other scale. Those two and D Minor are my preferred scales for playing sad stuff.

But yeah, as the other two said, scales are pretty redundant if you know how to work them.
#5
stay away from the harmonic minor. it tends to take the roll of the bad-ass minor.