Loudness is how strong or intense a sound feels to your ears. When you crank up the volume on your headphones, you're increasing the loudness. When a teacher asks you to use your “inside voice,” she's asking you to decrease the loudness so you don't disturb the class.

Loudness depends on how much energy a sound wave carries. A whisper has very little energy, so it's quiet. A shout has much more energy, making it loud. A jet engine produces such powerful sound waves that it can actually hurt your ears if you stand too close.

Scientists measure loudness in units called decibels. Normal conversation happens around 60 decibels. A rock concert might reach 110 decibels. Sounds above 85 decibels can damage your hearing if you listen to them for too long, which is why construction workers wear ear protection and why your parents might worry about your headphone volume.

Loudness isn't quite the same as volume, though people often use the words interchangeably. Volume is what you adjust with a knob or button, while loudness describes your actual experience of how strong a sound feels. The same volume setting might sound louder in a small room than in a large auditorium because the sound waves bounce around differently.