Full-volume singing before your muscles are warm is like sprinting without stretching. You need a gentle entry point.
The closed mouth hum wakes up your voice without the complexity of vowels or big volume. It shifts your focus from throat to face.
The Sound
Close your lips. Teeth slightly apart (don't clench). Make a soft "Mmm" like you're agreeing with someone.
The Feel
You want a tickle on your lips. If there's no buzz, the sound is trapped in your throat. Adjust your jaw position and tongue until you feel vibration move forward into the mask (the area around your nose, cheekbones, and forehead).
The Drill
Hum 3-2-1-2-3 at low volume. Starting on 3 and descending to 1 lets you settle into your chest voice rather than climbing out of it. You're chasing sensation, not power.
Why This Works
Humming sends sound waves through your nasal cavity into the facial bones. That sympathetic vibration gives you feedback that your voice is resonating efficiently. Because your mouth is closed, volume stays naturally limited. You can't push or belt even if you wanted to. It sets a healthy template for the rest of your session.