Sets the visual bell setting for this window. Omitting the parameter toggles the setting. If vbell is switched on, but your terminal does not support a visual bell, a message is displayed in the status line when the bell character (^G) is received. Visual bell support of a terminal is defined by the termcap variable vb (terminfo: flash).
Per default, vbell is off, thus the audible bell is used.

Ctrl+A and then Ctrl+G to switch between visual and audible bell.