The train noise at the out- door location contains quite strong low-frequency components, with the peak sound energy within the 50 to 160 Hz frequency range. Fig. 4 also shows that the train noise at this outdoor location also has significant energy at frequencies ranging from 160 Hz to 5,000 Hz.

Train noise is a complex mix of frequencies, but it is predominantly characterized by low-frequency sounds and vibrations. The primary source of this low-frequency energy comes from the heavy axle loads and the physical interaction between the steel wheels and the rails, often generating significant noise below 100 Hz. This "rumble" is highly effective at traveling through the ground and building structures, which is why people often "feel" a train before they hear it. However, high-speed trains also generate significant high-frequency noise, largely due to aerodynamic drag and the "pantograph" (the arm connecting the train to overhead power lines) whistling through the air at high speeds. Squealing sounds heard during braking or when the train rounds a sharp curve are also high-frequency. Therefore, while the "foundation" of train noise is low-frequency, the overall acoustic profile includes a broad spectrum depending on the train's speed and the surrounding environment.