How to disable start up sound in MacBook Air

It has puzzled me for a long time, especially at midnight.

$ sudo nvram --help

nvram: (usage: no such option as --)

nvram [-x] [-p] [-f filename] [-d name] [-c] name[=value] ...

-x                       use XML format for printing or reading variables

                          (must appear before -p or -f)

-p                       print all firmware variables

-f                        set firmware variables from a text file

-d                       delete the named variable

-c                       delete all variables

name=value      set named variable

name                 print variable

Note that arguments and options are executed in order.

$ sudo nvram SystemAudioVolume=%80

$ sudo nvram -p | grep SystemAudioVolume

SystemAudioVolume %80