Rapidly repeated notes on harp will result in a buzzing sound as fingers touch already vibrating strings. Some harpists call this “stomping.” This is a simple, incredibly important fact which is not in any orchestration manual I own, and it’s especially true with the lower, more widely vibrating strings. The orchestrator may score a harp part where a low octave is supposed go – bum BUM! What they will get is a bum-ZZZBUM. Unfortunately, sound sets complicate the problem, because you can repeat a sound as fast as you like without hearing any buzzing. And that can get you into heaps of trouble during rehearsal.
There’s a further problem – a quickly repeated note effectively kills the resonance of the first pluck in an unfortunate way. The composer may wish to hear two beautifully articulated notes played in rapid succession: dun-DUN! – but what actually happens is more like dutt-znDUN! This is why harp tremolos are often tuned in enharmonic pairs, like the right hand on D#-F# alternating rapidly with the left on Eb-Gb. That way, there is just slightly more time for the string to stop vibrating before being plucked again.
I fell into this trap myself recently while working on my concerto. The screen cap below contains two excerpts. In the upper excerpt from the first draft, you see the problem right away – I’m asking the harpist to repeat the upper and lower notes of each arpeggio. As cool as this sounds in my head, it doesn’t work on harp strings.* After working with the harpist, I came up with a better solution, more idiomatic to the harp (and frankly more interesting musically). In the excerpt from the second draft, you can see how I dovetail around the end of each arpeggios, to set up the strong first note of the next arpeggio as it changes direction.
My harpist liked this a lot, as it made the music sound spilling over rather than punctuated. We’ll see how it comes off in concert this April.