Laughing Dog Sound is Calming to Dogs.

Patricia Simonet, an animal behaviorist at the Spokane County Regional Animal Protection Service in Washington found that the long, loud pant is the sound of dog laughing.

This dog laugh can affect other dogs:

"When they played the sound of a dog panting over the loudspeaker, the gaggle of dogs at the shelter kept right on barking. But when they played the dog version of laughing, all 15 barking dogs went quiet within about a minute."

Link (via Chapel Perilous)