In the female voice, there is a major Passaggio (or transition) that occurs between D5-G5. Males sing these notes less often, but will also will notice the same phenomenon these pitches. At this part of the voice, singers usually benefit from a slight opening of the mouth if they are seeking a stronger sound. This opening aids the strength of First Formant (F1) resonance. Either a slight jaw opening or a small widening of the lips typically helps this resonance to intensify. In general, we don't want to open up the mouth or spread the lips to sing higher notes. Instead, we should learn to sing them with a neutral mouth first. But, if you're seeking more power through this particular Passaggio, try letting Acoustics work for you!