How building materials absorb, reflect, and transmit sound waves
hen sound impinges on a surface, like that found on a building's interior, some of the energy passes through.
Some of the sound energy is absorbed and reassigned as heat within the material. You can't make the wall hot by yelling at it; it's not that much heat.
There are molecules inside the building material, and when they are impinged upon by the sound energy, they move. As they rub together, they lose some of the energy through friction.
Finally, some of the energy is reflected.
All three are happening simultaneously—transmission, absorption, and reflection—and it's a function of the material mass, surface smoothness, fiber orientation, porosity, airtightness, and stiffness of a material.
Let's look at a material that has little mass and a surface with negligible pores, say glass. We'll put the glass on top and watch sound impinge on the glass.
- Lots of energy transmitted
- Not much mass to the glass
- Not much absorbed in the surface of the glass
- Few pores, and therefore a few corresponding air pockets
Simultaneously, we'll look at smooth concrete: much more mass and, provided the concrete barrier is airtight, absent penetrations, very little will transmit through the barrier.
Because the concrete surface is smooth, there is little absorption and high levels of sound reflection.
Finally, we'll add a third barrier. We'll start with the concrete wall and add a layer of fiberglass to it.
The mass of the concrete restricts the sound energy transmitting through, but the pores in the glass fiber absorb a good deal of the sound. Relatively little sound is transmitted, and relatively little is reflected.
Generally, we have little use for barriers that allow sound to travel into a building or through buildings, so usually less sound transmission is a worthwhile goal. But the proportion of sound that is reflected relative to the portion that is absorbed? That decision is the subject of the next series of animations.
—This video is from the YouTube channel of Amber Book, a channel about building physics, building technologies, and environmental building systems.
