How to disable iMac OSX start up sound

I schedule an automatic shutdown of my iMac at 19:30 every night and an automatic startup at 06:00 every morning. The loud startup chime is extremely annoying as it tends to wake other people up so I need to mute it or make it very quiet.

The startup chime (sound / noise ) cannot be muted via any of the current built-in Apple Mac GUI utilities, to do this you have to change a startup configuration setting stored in the non-volatile random-access memory or NVRAM by using Terminal or another command line interface.

If you are not comfortable with using Terminal this then Google for utilities to mute this sound. They do exist but they appear to only work with certain version of Apple iMac OSX.

Using Terminal to mute iMac startup chime
Fire up Terminal, it can be found by running Startup, pressing T and selecting Terminal. 

To check the current setting of the startup audio level enter  and see what setting is shown under SystemAudioVolume

To set the audio startup level to a mere murmur, nenter  and enter your password and return when propted.

Check that the setting has been made by entering  again and see what setting is shown under SystemAudioVolume

You can experiment with other values, %05 worked fine for me, I can just about hear it.

Do not attempt to make any other changes to the NVRAM as this could brick your lovely shiny Apple!

Resetting NVRAM
If you do manage to do something stupid and you need to reset your NVRAM:


 * Shut down your Mac.
 * Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
 * Turn on your Mac.
 * Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
 * Hold these keys until the computer restarts and you hear the startup sound for a second time.
 * Release the keys.