Why are the first sounds of an approaching enroute airliner low frequency, booming ones?
The air attenuates high frequencies more than it does low. Thus, lower frequency sounds travel farther, so you hear them first. Also, longer wavelengths will appear to "bend" around obstacles more than ones with shorter wavelengths, so buildings around you will in many cases develop acoustic "shadow" zones for higher frequencies as those buildings' dimensions are significantly larger than their wavelengths.