This is for all of you who would like to compose for the harp, but don't understand its numerous oddities. It’s important to note first that I am not a professional harpist. I have been learning to play the harp for the past two years, and therefore have two years’ more experience than most of you, but have no more than that.
The harp is part of the plucked strings family, the rest almost entirely belonging to the guitar family (lute, mandolin, etc.). This means that acoustically, a single plucked string on a harp sounds roughly like a single plucked string of the same pitch on a guitar. It furthermore shares with the keyboard instruments the polytonality of using a different string for each pitch, which allows it to provide accompaniment as well as accompany itself; it’s therefore notated on the grand staff.
Pedals
The pedals are the oddest thing about the harp and the least understood. Unlike the piano, which uses separate keys for chromatic notes and diatonic ones, the harp can only play diatonic notes, though it can change keys with the pedals (this is talking about a standard harp. There are, outside the classical music world, chromatic harps). The advantage, however, is that it can play any key (with seven notes per octave), and most importantly can play each one equally easily. A harp can do a 32nd note scale (through glissando) in C-flat melodic minor as asily as in C major, something a piano has trouble with due to it being “set” in C major.
Technically, you cannot simultaneously play C and C#. What isn’t realized, however, is that you CAN simultanesouly play C and D-flat. You can even play B# and D-flat and never touch the C-string! So the harp is not as limited as it may seem. In fact, every note except D, G, and A natural can be played by two different strings, which enables the above as well as certain tremolo effects.
The pedals work by restraining the top of the string, thus sharpening it. They cannot create flats. Instead, the harp is originally tuned in C-flat major, then every string is sharpened. So C# is actually C-flat-double-sharp. This is important because a string sounds nicest when unrestrained. For this reason, the harp’s “favorite” key is C-flat major, and it does not sound very good in C-sharp major. A minor point (the harp still sounds lovely in C-major), but one worth noting. Many harp solos are in C-flat major (remember, a harp doesn’t need piano accompaniment in solos).
Acoustics
This will not make anything technically unplayable, but can lead to bad sounds if you don’t look out for it. For those who don’t know, the piano has a special mechanism that stops the string the second you let go of the note and stops the sound (the damper). The harp does not. Instead, a loud note or a low octave note on a harp will last longer, and a shorter or higher one will be more staccato. Many dynamic effects, such as staccato, are rarely done on the harp. This requires the harpist to manually shut the string up every note, which is impossible if the hand is doing other things. Usually the harpist will just ignore the staccato. In fact, a harpist rarely pays any attention to the difference between an eighth-note eighth-rest and a quarter note. You pluck the string on the beat, then let it vibrate as long as it wants.
On a high octave, don’t play long notes. The sound doesn’t last. I don’t have a piano at home, but I seem to remember it having a similar problem, so I won’t elaborate. Worse is the low octaves. Because the sound continues for a while, it will interefere with other notes a little. If you’re writing an arpeggiated chord, this is good; you get the arpeggiation and the notes still manage to harmonize and create a chord. If it’s not, however… It’s minor, but you should look out for it if you plan to write sixteenth note melodies in the low register. This problem won’t happen if you keep playing the same notes (like in a repeating arpeggiated chord), but if you move around the low register a lot it’s a problem. So don’t do glissandos of the low octaves. Also, at the end of long phrases, the left hand (which plays the low notes) usually silences the strings, which stops the low hum that can still be heard. You can notate this or not ("