Winds and voice compete with strings, keyboards, and synthesizers that have equal or adjustable SLs. A digital chromatic tuner calibrated at 440 hertz verified the good intonation of the test tones. Other studies on the SLs of winds, voice have found differing results and conclusions. One study found decreasing SLs with rising pitch though the researcher metered the instruments SLs from audio recordings, but offers no other information about his test. In fact, it is sonically difficult to produce very loud low notes.
Most people get softer as they enter their lowest five or so notes.
The only way I know to increase volume down there is to add nasality, in which case I sound like a Tibetan monk. According to physics, high note resonance is heard as louder to our ears. This whole notion of real loudness vs. apparent loudness is interesting, but in the end, it is the apparent loudness that matters. Even the D tonic is quieter than the next whole tone as are all the subsequent tonal relationships. Likewise, the top fifth of the flute range is extreme because its tones are too loud in relation to the lower range, and its tones tend to be pitch unstable.