The glass used in high-rise structures High-rise glass is composed of two 6 mm (a quarter-inch thick) panes separated by a half-inch of air space, resulting in a one-inch thick unit. The glass is manufactured with multiple seals that let desiccated air (dry air) inside to prevent fogging. These multi-layer panels are more resistant to heat and sound than single sheets of glass. The outer layer provides protection against damage caused by rocks falling from great heights or collisions with other objects. The inner layer reduces heating during cold days or when it is raining outside. Both layers contribute to reduce the weight of the window frame required to hold the glass in place. There are three main types of glazing: transparent materials that allow light but block noise (opaque); translucent materials that allow some light and sound but not enough to be fully opaque; and clear materials that allow all visible light and sound. Noise from roads, trains, planes, etc. can pass through solid walls and floors into apartments, where it can be heard by people upstairs or downstairs. Soundproofing is the process of reducing sound transmission through building materials. There are four main types of soundproofing: absorption, isolation, reflection, and insulation. Absorption means covering surfaces with acoustical materials such as wool or shredded tires. This method is most effective at reducing low-frequency sounds like those made by air conditioning units or vacuum cleaners.