No residential window blocks all sound, all the time. “Soundproof” is shorthand for noise reduction windows that block up to 90% to 95% of noise coming through windows. Sounds with low frequencies, like garbage trucks grinding trash, are harder to block than sounds with higher frequencies, like birds chirping. So when you shop for sound-reducing windows consider “what frequencies you want to soundproof against,” says John Storyk, an acoustical consultant and a founder of the Walters-Storyk Design Group based in Highland, N.Y. The acoustics industry makes soundproof window shopping easier by rating the sound-stopping quality of windows on a sound transmission class (STC) scale; the higher the number, the more a window inhibits sound. Your basic, single-pane window has an average STC rating of 27; a dual pane window has an average STC rating of 28. Soundproof windows, however, have STC ratings of at least 45, and some climb to the mid-50s, which block as much as 95% of noise.