I can get it pretty cheap. High frequencies are directional and relatively easy to reflect and absorb.
Hard materials reflect and soft materials absorb.
Place either type of material in the path of the sound especially in front of and behind windows. Place absorbent materials on all hard surfaces that might reflect stray high frequency waves. Low frequencies diffract or bend around obstacles in their path. Nevertheless, absorbent materials placed in the path of low frequencies will attenuate them somewhat. The greatest reduction in the amplitude of low frequencies occurs when you isolate your bed from the floor.