Your mouth is an acoustic chamber and makes as much difference to your sound as singing in a grand echoey cathedral or a small squashy carpeted curtained room.
To encourage you to make the most conducive space in your mouth for singing, I could demand the following:
Raise your soft palate!
Relax your tongue root!
Open your throat!
Loosen your jaw!
Or, I could ask you to use your imagination to do the following:
Imagine an egg. A very special egg that you don't want to break. Maybe inside the egg is a baby chick, or a baby dinosaur?
Put the egg in your mouth.
The point is to encourage you to make as much space as you can in the internal area of your mouth so that, as the sound passes through, it can ricochet around, growing in lovely warm resonance rather than being swallowed up and absorbed.
The feeling and imagery of having an egg inside your mouth when you sing keeps your tongue flat at the root base, the roof of your mouth (soft palate) high, your jaw loose and your throat open.
For other tips about using the resonating chamber of the mouth (pharynx) have a read of:
Good Vowel Naughty vowel
The golden golf ball
Let me know if the egg imagery works for you!
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