There’s a common misconception that “acoustic materials” are some sort of specialty substance that make rooms sound better. It’s also often thought that cost is a factor in how much of an impact these materials can deliver.
This line of thinking is actually far from reality. To understand why, we need to build some basic knowledge of what an acoustically balanced space means and how acoustical treatment makes an impact to get that balance.
When sound is generated in a room it travels in the space and reflects off its surfaces and objects bouncing around repeatedly until that sound energy dissipates into heat energy. Listeners in the room hear a combination of the waves arriving directly to their ears from the source combined with thousands of reflections of those waves. The further a reflecting surface is from the source of the sound, the longer the time gap in perceiving the direct sound and the reflected sound.
When sound waves are heard in close time proximity to each other, we don’t hear them individually. Our brain fuses them together and we perceive them as one sound with a character that is combining the natures of the distinct arriving sounds. This is true until the time difference between these waves gets larger than about 50 msec; then we start recognizing them individually.
One example of how you might experience this is when you pick an instrument to buy. You choose it at the store, making judgments on its sound in that environment, but notice that it sounds different when you take it home. The difference in what you hear in these two places is the portion of the instrument sound that is coming from the room acoustics, typically significantly different between your room and the store. Yet to your ears it feels as if the instrument itself has a different sound.
In a nutshell, room acoustics refer to all the room reflections along with their loudness levels and arrival times at all frequencies. The efficiency with which a material absorbs sound energy can be different at different frequency ranges. For example, soft, fluffy material like fiberglass, deforms while resisting air flow caused by audio waves through its pores. The friction between the moving particles generates heat. This process of absorption is effective with high frequencies because they are low energy and have small wavelengths, and ineffective against low frequency since these frequencies are carried by high energy and large waves and will simply go through the material unless the material is several feet thick.
Another form of absorbing energy is when the reflecting surface is membranous. The cohesive yet resilient surface deflects and causes the wave to bounce back colliding with the incoming waves and cancelling out each other’s energy. This type of absorption is very effective at low frequencies but ineffective at high frequency since the high frequencies carry too little energy to move the surface so most of it reflects back to the room.
The capacity to absorb sound energy at each frequency for a material is called the Absorption Coefficient of that material. The Absorption coefficient is a number between zero and one at each frequency range, with one meaning completely absorptive and zero meaning completely reflective. These values are determined in labs by exposing a sample of the material to sound waves and measuring the energy before and after it interacts with a material and then comparing those two energy values to each other at each frequency range. There are many theories and principals that are applied to tailor this absorption coefficient by combining different materials. In fact, in many cases, the cost of the research and development to create these targeted combinations contributes significantly to the cost of the specialty products. There are many printed and online resources that publish the absorption coefficient of common materials. Also, all reputable manufacturers will publish this data for their products.
In summary, the absorption characteristic of a material and its relationship with the sound reflection pattern present in a room are what together determine the effectiveness of a given material/product in changing the room acoustics. Hopefully this makes it clear that anyone knowledgeable about acoustics, when presented with the commonly asked question “what is the best acoustic material,” will most accurately answer: It depends.