You do not need a melody to make a sound change. A short “oo,” “oh” and “ah” on roughly the same comfortable note can offer three different listening impressions. This is a tiny exploratory exercise, not a test of vocal range or a lesson in correct singing technique.
Keep the experiment easy to repeat
Use an ordinary, comfortable volume. Make each sound briefly, with a relaxed pause between attempts. Stop if anything feels uncomfortable; the exercise has no target worth straining for. You can also simply imagine the spoken vowels or listen to a recording of speech rather than sing.
Start with two vowels instead of all three. The smaller comparison gives you less to remember. Do not aim for a perfectly controlled laboratory result: notice when pitch or loudness drifts and include that in your description.
Give the change a concrete description
Does one sound seem rounder, narrower or more open to you? Those words are personal listening notes, not measurements. Add the pair you compared so that “rounder” still means something when you return tomorrow.
For example: “On my second try, the first vowel felt darker; I also sang it more quietly.” That second clause matters. It stops you from assigning every difference to the vowel when your own performance changed more than one thing.
Build a miniature piece from the sequence
Try “oo, oh, ah, oh,” leaving an equal pause after each sound. Then reverse the sequence. Without adding words, you have created a small pattern with a direction and a return. Which version feels more settled at the end? Either answer can lead to another experiment.
If you want to extend it, borrow the idea in changing only the final sound of a four-part pattern. Keep this version short enough to remember without recording. The point is to discover a choice you can hear and describe, then use that choice deliberately in your next sound sketch.