Read the curve left→right as low → high frequency. The filled area is energy the master channel keeps; the dashed red line is the low-pass cutoff — everything to its right is trimmed to avoid transients and overwhelm. Numbered dots mark the frequency we associate with each of this library's engaged brain regions: deep regulatory structures (brainstem, hypothalamus) sit low and are always retained in the foundation; cortical, auditory and language regions sit higher. A dot to the right of the cut line is engaged indirectly through that retained low-frequency foundation and downstream regulation, not a dedicated high band. These are design-reference associations based on structure depth and function — not measured tonotopic values.