The state of mind a person is in when s/he is so stressed or angry that s/he might scream a scream so high that only a dog could hear it. Also to be in a state of mind where every little sound is like nails on a chalkboard. The place one finds one's self after one's last nerve has been worked. Typically plural.
"Getting cut-off on the road three times today has sent me into the higher frequencies".