As you can see, low frequencies are not absorbed as well. This means low frequencies will travel farther. That graph comes from this extremely detailed article on outdoor sound propagation.extremely detailed article on outdoor sound propagation

Outdoor sound propagation or atmospheric sound propagation is of special interest in environmental acoustics which is concerned with the control of sound and vibrations in an outdoor environment. Outdoor sound propagation is affected by spreading, absorption, ground configuration, terrain profile, obstacles, pressure, wind, turbulence, temperature, humidity, etc. The subjects covered in this page are speed of sound in air, decibel scales, spreading losses, attenuation by atmospheric absorption, attenuation over the ground, refraction, diffraction and sound reduction examples.

Reflection from the surface of a solid [15] [edit] [15]edit When a wavefront comes in to contact with a solid surface, it is reflected away from that surface. The angle of reflection of the sound wave is equal to the angle of incidence of the wave. Reflected waves can interfere with incident waves causing constructive and destructive interference. This can cause a standing wave pattern and resonance because the incident and reflected waves travel in opposite directions. Near the surface of the solid, sound pressure intensity is enhanced because the pressure of the reflected wave adds up to the pressure of the incident wave. Figure 3 - Angle of incidence for a reflected sound wave

Diffraction is the mechanism by which sound can propagate and spread out beyond an opening and around obstacles. The traveling waves tend to bend around obstacles. Diffraction is related to the wavelength of the sound produced by the source. It is stronger for lower frequencies. High frequencies propagate in a more directional manner. This is why low frequencies can be heard better from behind obstacles and in shadows zones. If a wavefront travels towards a small opening, diffraction will cause the waves to spread out past the opening in a spherical manner. When a wavefront that passes through a small obstacle, compared to the wavelength, diffraction will cause the sound waves to bend and the wavefront will reconstruct past the obstacle. This means that one could not identify the presence of that small obstacle from sound measurement far from the obstacle and the source. See table next slide.

Different paths of sound propagation behind a barrier Figure 6 - Outdoor sound diffraction behind a small opening

Different types of noise barriers Vertical vegetation walls