Vijnana Bhairava Tantra · Dharana 91 · Verse 114
The sound without letters
Close your ears with your fingers, and gently draw in the muscles at the base of the body, closing its lower gate. Listen inward for a sound that has no vowel and no consonant: a hum that nothing is making. Rest in that listening. When even the hum seems to fade, stay as the listening.
Older line
Seal the ears, seal the lower gate as well, and attend to a sound with no vowel and no consonant in it: so you pass into the eternal Brahman.