Exodus 19:19

וַיְהִי קוֹל הַשֹּׁפָר הוֹלֵךְ וְחָזֵק מְאֹד מֹשֶׁה יְדַבֵּר וְהָאֱלֹקים יַעֲנֶנּוּ בְקוֹל

The sound of the shofar grew increasingly louder: Moses spoke and God answered him with a voice.

This verse is one of the Shofros – verses describing shofar-blowing that are recited as part of the Rosh Hashana musaf service. Rashi points out what we have all observed: when a person blows shofar, the longer he blows, the weaker the sound becomes. G-d, however, is not subject to such limitations. The longer His shofar sounded, the louder it became.