Q.: Do you have any information about concrete mixes for better soundproofing? The contractor wants to use the smallest amount of concrete between building floors but wants to achieve maximum soundproofing benefits.

A.: Tests have shown that porous surfaces with interconnected pores (like ceiling tiles) from the surface to the interior work well for sound absorption. At the porous surface, the energy of sound is converted into heat. However, the same factors which make good sound absorption provide poor sound insulation.

Porous concrete absorbs sound but has poor sound insulation, or a low STC (sound transmission loss). Therefore, painting or plastering porous concrete reduces sound absorption and increases its sound insulating characteristics. The sound absorption coefficient for plain cast concrete is about 0.02, indicating that about 98% of the sound energy is reflected by the surface. The more the concrete weighs, or the denser the concrete, the more sound insulation (the greater sound transmission loss) rating.

Simply put, walking down a painted, dense (high psf weight) concrete hallway (floor, ceiling, walls) in capped boots will be very noisy, but the person in the next room may not hear you very well if there are no cracks or openings in the wall. Walk down a lightweight, unpainted, porous concrete hallway in capped boots, and you will not hear yourself making much noise, but someone in the next room will hear you walking by.