Using a modulation power spectrum, David Poeppel of New York University and colleagues analyzed human screams of terror. They found, for instance that screams of terror produce a sound that is unlike any other made by humans. It has auditory qualities that no other human vocalization shares. They found for example, that unlike human speech that typically changes less than 5 hertz per second (meaning it stays around the same volume), the loudness of screams quickly fluctuates anywhere from 30 to 150 hertz per second. These fluctuations give the sound of a scream of terror a quality referred to as roughness. The scientists asked volunteers to listen to a variety of screams of terror. The rougher the sound—or the larger the variation in volume within a scream—the more fearful-sounding the volunteers ranked it. More tomorrow.