No idea about audio frequencies close to hearing, but I'm pretty sure it's common to manipulate the brain with ultrasonic frequencies these days.

That's a toughie, but if it were me and I had the energy, I'd start by looking at the following patents:

- US20030171688A1: Mind controller - Induces alpha/theta brainwaves via audio messages. - US20070084473A1: Brain wave entrainment in sound - Modulates music for desired brain states. - US11309858: Inducing brainwaves by sound - Adjusts volume gains for specific frequencies. - US5036858A: Changing brain wave frequency - Generates binaural beats to alter waves. - US3951134: Remotely altering brain waves - Monitors and modifies via RF/EM waves. - US5306228A: Brain wave synchronizer - Uses light/sound for entrainment. - US6587729: RF hearing effect - Transmits speech via microwaves to brain. - US6488617: Desired brain state - Electromagnetic pulses for mind states. - US4858612: Microwave hearing simulation - Induces sounds in auditory cortex. - US6930235B2: EM to sound waves - Relates waves for brain influence. - EP0747080A1: Brain wave inducing - Sine waves via speaker for alpha waves. - US5954629A: Brain wave system - Feedback light stimulation. - US5954630A: FM theta sound - Superposes low frequencies for theta induction. - US5159703A: Silent subliminal - Ultrasonic carriers for brain inducement. - US6017302A: Acoustic manipulation - Subaudio pulses for nervous system control.