Disabling the Startup Sound on a 2011 iMac (iMac12,2) running Arch Linux

Lets disable the startup sound on an 2011 iMac running arch linux: 2011 27-inch iMac (iMac12,2).

The sound is controlled directly by Apple firmware via EFI NVRAM variables:

• SystemAudioVolume
• SystemAudioVolumeDB

These variables persist independently of the operating system and are still respected by the Mac firmware.

On this system, the values were:

SystemAudioVolume = 0x4c
SystemAudioVolumeDB = 0xed

The key variable is SystemAudioVolume, which controls whether the startup sound is audible (SystemAudioVolumeDB is for the intensity).

Since the system boots in UEFI mode, the variables can be modified from Linux using efivar. Apple firmware interprets 0x80 as a mute value for startup audio.

printf '\x80' > /tmp/SystemAudioVolume.bin
sudo efivar \
  -n 7c436110-ab2a-4bbb-a880-fe41995c9f82-SystemAudioVolume \
  -f /tmp/SystemAudioVolume.bin \
  -w
sudo poweroff

A full shutdown is required for the change to take effect. Then wait a few seconds and power the system back on.