Here's a tip on the internet:

1) Breathe in, breathe out, controlled exhale; meaning, variable pressure, under control. (Note: tightening of vocal chords raises pitch, more air pressure raises volume.
2) Breathe out, holding notes, singing scale, especially 1-5. Sound vowels, ah -- la -- ee -- oh, and dipthongs, ohm -- ong -- amm.
Now, here is the tip:
While exhaling, engage vocal chords for a couple of seconds. Sound a note. Then, disengage vocal chords but continue to exhale in one continuous motion. That's the trick. Use this to build (or rebuild) vocal strength.

Link: www.soundcloud.com/ed_shaw (Feel Free to Use)
https://drooble.com/edward.shaw/hymn/index.htm
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