30 Khz to 300 Khz, although there are signals transmitted well below this region, principally the OMEGA naval navigation network.
This is usually defined as the range from 20 to 100 Hz. Because they cycle slowly, relative to mid and high frequencies, they have longer wavelengths in air. These long length waveforms easily diffract around smaller objects, making their way around any barriers of less size. We tend to perceive low frequencies as omni directional.