The harp is often used to accompany singing voices and solo instruments. As a thorough-bass instrument it performed bass and harmonic tasks.
In orchestral composition it has retained both these functions: the bottom strings are often used to play the bass voice. The weaving of delicate harmonic sound backdrops (arpeggio technique) is another of its quintessential tasks. In addition, it is the various types of harp glissando that lend the undulations of the orchestra the shimmer of brilliance which is one of the most fundamental sound experiences of them all.
The harp is one of the quiet instruments. In his revision of Berlioz’s Instrumentation Theory, Richard Strauss deals with the subject in the following terms: “The harp must always be treated as a solo instrument, even in the orchestra, if one wishes to avoid writing notes that will not be heard.”
The harp combines well with all orchestra groups. It has the function of fleshing out the sound and is often treated as a filling-in instrument.
Harp + string orchestra
The harp achieves a good blend with the strings because they are related to each other as stringed instruments, although in particular cases it depends on the playing technique used. The string orchestra can be used as a kind of giant harp (pizzicato technique), as often shown by Beethoven in his orchestral works, in which he uses the strings as a harp substitute.
Harp + violin
Played in unison or in octaves with the violins a blend with contours unfolds which is especially well suited for melodic or thematic high points. The effect achieved when the harp plays harmonics to the violins in unison or octaves is particularly appealing.
Harp + double-bass
When played in unison these two instruments reinforce each other through their resonance. When playing piano the harp provides a solid bass foundation even without the double-bass and can replace the double-bass’s pizzicato, albeit with less volume.
Harp + woodwinds
In the middle register the harp doubles the woodwinds’ and horns’ sustained chords in unison, which produces an extremely mellow effect. Woodwind chords in unison with the harp are one of the key combinations. If the harp plays an octave above the woodwinds in the upper register the harp sounds like an overtone of the woodwinds.
Harp + flute
In unison with the flute, harp harmonics blend particularly well. The harp and flute have always been an extremely popular combination, the harp as the accompaniment to the flute melody.
Harp + clarinet
The blend in unison with the clarinet is somewhat more acerbic than with the flute.
Harp + bassoon, contrabassoon and bass clarinet
In alternating combinations, whether in unison or in octaves, a dark and sustaining effect with clear contours emerges, especially in pianissimo and piano passages, which is well suited as a bass foundation and for bass lines.
Harp + brass wind instruments
Harp + horn
Both instruments blend well.
Harp + trombone
Both instruments blend in unison, although the trombone must play two to three dynamic degrees lower than the harp so as not to drown it.
Harp + singing voice
Here the aim is less a good tonal blend than the complementing of sound and substance. The harp and singing voice are traditionally partners that complement each other, one performing the role that the other is not performing so that two complementary tonal streams unfold. The singing voice has a higher dynamic level than the harp.
Harp + percussion
Harp + timpani
Good effect played pianissimo or piano in unison.
Harp + xylophone
The harp in unison and in octaves with the xylophone lends the latter’s short and sparkling notes the necessary resonance, while the xylophone lends the harp a certain edginess.
Harp + celesta, glockenspiel
In unison and in octaves in the upper register, chords played by these instruments produce a crystalline, gossamer-like, ethereal effect which is used to evoke fairy-tale atmospheres or to denote dream sequences in opera, ballet and film music.