Bouncing off the sky

The effects of the atmosphere on sound transmission can be quite profound. I found this out recently when I was recording along the Carson River, near Carson City, NV. I first set up my recorder along the river on December 28, 2012. It was lightly overcast, the temperature was in the mid-20s, and the ground was covered with about 5” of snow. I was hoping to catch the sound of the slush floating down the river as it brushed against the ice that extended from the river banks. I also wanted to record some of the birds that spend the winter along the river – Canada geese, Magpies, American Robins, Bald Eagles, and a variety of ducks and smaller birds.

I set the recorder right next to the river, and set the record level fairly low, to try to avoid recording distant car and plane traffic. The geese left the river for a nearby golf course just as I was setting up the recorder, but I did get a few Magpie vocalizations, some drowned out by aircraft noise. So I went out to the same spot on January 8, 2013. It was a clear day, temp around 30, same old snow on the ground. The biggest difference was that the area was under the influence of a strong temperature inversion, which had kept temperatures well below freezing for almost 10 days. The increased roar from the traffic in Carson City was really noticeable. I set up the recorder in exactly the same spot as before, and set it at the same gain level. At once point in the recording, I could hear clarion bells from downtown, which was 3 miles as the crow flies from where the recorder was set up.

Sound propagates in waves, and like any wave, is subject to interference from objects in its path. Temperature, humidity, and sound frequency all affect the distance that sound can travel. Normally during the day, as warm air rises, sound is carried up into the atmosphere. However, during an inversion, the temperature differential between the cold air trapped at the surface and the warm air above causes sounds to bend back toward the ground, increasing the distance that sound can travel along the surface. Temperature inversions can occur as temperatures cool at night or during winter temperature inversions, which are common in the mountain valleys of western North America.

The following recording features the recording before the inversion and during the inversion. In the first part, you can hear the river and some Magpies. If you listen close (use either good speakers or earphones) you can hear a faint roar of traffic. In the second part, you can hear the loud roar of the traffic, as well as the river and Magpies. I normally would filter out the sounds of the traffic, but I was fascinated by the effects of the inversion.