This is a common question that gets asked by the insightful, but not scientifically inclined members of the music community:
"So where does the absorbed sound go in acoustic treatment?"
That's a clever question. We know that the entire reason we need acoustic treatment is because sound bounces around off of the walls in the studio and makes a sonic mess. This leads to using absorption panels in specific places to keep this from happening while you suit in the mix position for critical listening.
There are the types of panels called diffusors that take the energy of a sound wave and spread it out thinly across the space of your room by bouncing the sound off of itself. But here we're still talking about sound waves bouncing around.
The real mystery is what happens inside of bass traps and absorbers?
That question holds a hint in it. Sound doesn't disappear. As we engineers know from our studies of thermodynamics, the law of the conservation of energy states that no energy can be created or destroyed... in a closed system (such as our studio rooms for the most part) there is always the same amount of energy no matter what.
This tells us that the sound can't disappear. It has to become something else.
Recall that there's no actual thing as "sound particles" or anything like that moving through the air. What sound really is is the medium of air itself waving. For instance, our mouths use our vocal cords to constrict the flow of air out of our lungs and we use our mouths to shape the waves further. So as we talk or sing, we vary the pressure and frequency of the waves coming out of our mouth which continues on into the air. It is the air that waves and our ears pick up the pressure (volume) and speed of the passing crests and troughs in the waves (frequency) and our brains translate that into sound.
So when these sound waves push into our acoustic treatment panels the wave starts to push and pull at the air that's already inside the treatment panel.
Acoustic treatment is typically made out of high-density fiberglass insulation, although some cheaper options will use high-density foam. When we say high-density, we're talking about the fact that there are tons of these fibers all smashed together into a small space with little cavities of air pocketed inside of it. When the air wave starts to "wave" those air pockets, there's not much space for the air particles to move around.
These particles of air begin to vibrate and pulsate due to the energy of the sound wave coming in. What happens is they are "excited" by this energy, begin to move around rapidly, but since there's not much space all they can do is rub against these insulation fibers. And when the air is rubbing on the fibers, friction occurs.
You know when you rub your hands together and it generates heat? That's precisely what's happening in your acoustic treatment, just like a microwave oven does with your food. Heat is generated by the air particles rubbing against the fibers, and since neither feature perfectly smooth surfaces there is friction.
The kinetic energy of the sound wave moves into the air of the bass trap or absorption panel, which then rubs against the fiberglass threads.
And when this happens, the energy is then transferred into the air as heat!
This is why it's important when building acoustic treatment to make sure the fiberglass and the cloth that you use to cover it is all flame-retardant. You don't want a candle knocking over or a cigarette flash catching all of that insulation on fire. I've never heard of a case of this ever happening, but that's because nobody takes the risk (major points to the music community for not burning down their studios through negligence!).
That's how it works! I hope that made sense and answers the question for you if you had it. Otherwise, you're a lost cause. Just kidding! Let us know what isn't clear and we'll clarify it. Thanks for reading!