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Walla'': In American radio, film, and television, a sound effect imitating the murmur of a crowd in the background. A group of actors brought together in the post-production stage of film production is known as a walla group. According to one story, walla received its name during its genesis in the early days of radio, when it was discovered that engaging several people to repeat the sound ''walla'' in the background was sufficient to mimic the indistinct chatter of a crowd. Nowadays, walla actors make use of real words and conversations, often improvised, tailored to the languages, speech patterns, and accents that might be expected of the crowd to be mimicked. Walla is called "rhubarb" in the UK and "rhabarber" in Germany, perhaps in part reflecting the varying textures of crowd noise in the different countries.
External links -
filmsound.org - Walla from filmsound.org
home.sprynet.com - Using Walla Walla Crowd Sounds in Radio Drama