The farthest I have heard a sound travel was 172 miles, roughly the width of Washington State. This occurred on May 18, 1980, when I was fly-fishing in North Cascades National Park near the Canadian border. Dynamite, I thought. But Sunday morning? Minutes later, other blasts arrived from different directions. Hours later, ash fell from the sky. Mt. St. Helens had blown! … The subsequent blasts were not, in fact, true echoes, but rather different sound paths traveling different paths at different speeds due to variations in temperature and atmospheric pressure.

Gordon Hempton, John Grossmann, One Square Inch of Silence-…, loc. 955