Let's assume we want to detect a small bandwidth electrical signal from the brain (0-10 Hz). EEG is roughly 25 µV for that bandwidth.
If we apply unfocussed ultrasound to the entire brain at 1 MPa, we'd have an AE signal that is
$$ V_{AE} = V_{EEG} \cdot K_{AE} \cdot P = 25 \text{ } \mu V \cdot 10^{-9} \text{ Pa}^{-1} \cdot 10^{6} \text{ Pa} = 25 \text{ nV}