Bisbigliando

"The term bisbigliando (Italian, 'whispering'; abbrev. Bisbigl. or bisb.) indicates an unmeasured rapid tremolo between two or more strings played quietly in the middle and upper registers of the harp."  [p.360, Gould2011] 

A bisbigliando is played with both hands by plucking the interval or chord, one note after the other, rapidly, lightly and continuously. The playing technique of a bisbigliando is quite different to the normal playing technique. Normally the harpist would first place the fingers on the string, then press them and then release to create a full sound. When playing a bisbigliando, the harpist hardly places the fingers on the strings before playing. He/She plucks the strings very fast and lightly to minimize buzzing and to create a continuous sound. Bisbigliando is a quiet effect, since its production does not allow enough muscle strength for a full sound.

Due to its production a bisbigliando is a quiet effect, however the whispering sound carries well, provided that it is not obscured by thick instrumentation from accompanying instruments.

A bisbigliando is notated as a tremolo chord with the term Bisbigliando written above or below. If the bisbigliando label is omitted, the harpist might play a measured repetition of the chord instead of a bisbigliando. [Gould2011] 

Bisbigliando, both hands play the same chord:

Bisbigliando, the hands play alternating chords:

The direction of the bisbigliando can be notated with arpeggio lines placed before each cord. If no direction is given the harpist will play each chord upwards, starting from the lowest note in each hand.

If a specific order of notes is wanted the bisbigliando must be notated out in full: 

Both Luciano Berio and Toru Takemitsu use the following notation to indicate a random bisbigliando. The notated pitches should be repeated rapidly and continuously in a random order.

A bisbigliando can also be produced with one hand, however, its speed will be diminished by a considerable extent. Due to the low speed the resulting sound is less continuous and the notes will be perceived rather as a fast repetition of a sequence than as a bisbigliando whisper.