The acoustics of the palatal approximant

For [j], as for [i], there is a low F1, because the tongue body is close (i.e. raised), and a high F2, because the tongue body is front in the mouth.

The portion labelled ‘PAL approx’ is part of the spectrogram where the palatal approximation is clearly visible on the spectrogram. Notice the voicing throughout, seen in the vertical striations that continue throughout the marked segment.