In American radio, film, and television, walla is 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 to create this murmur is known as a walla group. According to one story, walla received its name during the early days of radio, when it was discovered that having several people 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 and rabarber in the Netherlands and Flanders (Belgium), perhaps in part reflecting the varying textures of crowd noise in the different countries. Similar phrases are "carrots and peas" or "watermelon cantaloupe, watermelon cantaloupe". The TV show South Park often parodies this concept by having angry mobs mutter "rabble rabble rabble," sometimes clearly and distinctly. While it is generally considered against actors' unions [SAG, etc.] rules to put distinct words in a specific background performer's mouth [as this would turn 'extras' into actors during the sound mix] this problem can be avoided by recording gibberish that syncs with the on screen mouth movements or 'lip flap' of a specific background performer [aka extra]. It is thereby possible to make it sound as though an extra is saying something, when in fact they are not delivering any actual dialogue. This gibberish is known as 'Snazzum' - named in reference to the way in which the cartoon character Yosemite Sam would swear when angry ie. "Yassin Sassin Snazzum Frazzum!"
The British comedian Eric Sykes wrote, directed and starred in the 1969 film Rhubarb, in which all of the actors' dialogue consists of the word "rhubarb" repeated over and over. This gives the finished movie the general feeling of a silent film because it has no coherent dialogue, but with the crucial difference that the "rhubarb" dialogue still conveys the characters' emotions and moods.