(Tip no. 65 from “100 Orchestration Tips,” Part 4: Harp)
Some harps do not possess a high Gb string, and both bottom strings of Cb and Db may need to be tuned by hand. The double-action of the modern harp has limitations, as each pedal will essentially create tension on 6-7 strings of varying lengths simultaneously. The one or two lowest strings are just too much for the action to handle, creating a problem by their extreme length and mass for the tuning mechanism.
Orchestration manuals tend to be unclear about this. Some manuals say that only the lowest Cb needs to be hand-tuned – others claim that both Cb and Db are stationary strings, but don’t mention that in quite a few harps there is no top Gb string. The truth is, it depends on the harp. These extremes are somewhat flexible, as that top string is almost never called for, and the bottom two are needed mostly for resonance (and tend to strongly cloud the tone if played frequently throughout a phrase).
The thing to remember is the old joke, which actually contains a germ of truth: “harpists spend half their time tuning, and the other half playing out of tune.” Because of the vagaries of temperature, moisture, flexibility of strings, and construction, harps need extra care in approaching tuning – and tuning 47 strings is quite a chore. The only fussier instrument I’m aware of is the harpsichord (one of which I used to possess and perform upon). In this case, the bottom two strings must be tuned by the harpist in advance of performance. That sounds obvious, but it’s not unheard-of for a composer to ask for a low Cb in one movement, then obliviously insist upon a low C# in the next. This is clearly impossible for the harpist, who must then decide which of the two notes is more important, and which should be left out. The composer has only themselves to blame.
Also remember these two words: “sympathetic vibration.” The lush, generous sound of the harp results from all of the strings responding to a plucked tone or series of tones. The extreme outer strings’ purpose is less to be played than to supply a sense of timbral space. What will always surprise a less experienced composer raised on perfectly sampled sound sets is how ineffectual and slightly off-key the top octave of strings actually sounds. The best sound is always in the middle four octaves – most easily understood by the listener’s ear, and characteristic to the technique and sound of the harp.
In the excerpt above from my harp concerto, I take the harp up to the very top possible strings – but notice how they’re supported by octaves from below. Also, see how that top Gb in bar 38 is bracketed, showing that it’s an optional note. If it doesn’t exist on a particular soloist’s instrument, then it should be left out. The overtones will carry the phrase in any case.