The problem with the sponge analogy is that a sponge will saturate because the water doesn't change. WIth sound absorption, the sound energy is actually converted to heat. Unless you are putting kilowatts of sound energy out, it's not going to "saturate" to the point that you can even detect the increase in temperature.
Instead think of it more in terms of light. Things will absorb specific wavelengths of light, and reflect others. Those absorbed are converted to heat. A black curtain will absorb the majority of the light. A medium grey will also absorb the same wavelengths, just at a lower efficiency. White will reflect all wavelengths back. A red curtain absorbs the light but reflects back red. Likewise, certain materials are going to absorb different sound frequencies, based on density, rigidity, and thickness. The glass that is used to make OC703 would probably make a nice reflective window if cast into a smooth, very rigid plate.
The biggest problem for most people is that measuring sound is not as easily done and interpreted as viewing a room and looking at the light. It's easy to see that the corner is darker, or seeing a light reflecting in a mirror or off a window. Hearing bass bloom in the corner is the same as getting the reflection of a lamp in a mirror.
Putting up a thin layer of low density fiberglass for sound control is about as effective as putting up sheer curtains to keep the sunlight out of your room. Putting up a wall with mass loaded vinyl and rockwool panels is like adding thick blackout curtains.