Are we absorbing, or stopping sound?

To some, the distinction between absorbing sound and stopping sound is entirely common-sense and intuitive. But to the majority of the people (generally, musicians) out there, it’s not an obvious line that separates the two concepts, so let’s clear a few things up!

Absorption does not stop sound

This paragraph title sums it all up quite nicely, but joking aside, the explanation is a little more complex. Absorption does not necessarily stop sound. What it does do, is transduce sound energy to kinetic energy, ideally. To explain what this means—as this really gets to the core of the point above—let’s take a few steps back and understand how sound travels in the first place.

Visualizing sound waves is confusing..

Without having you relive your days in physics class, imagine a room that has 1 loudspeaker in it as a single sound source, and the rest of the room is empty. Of course, air particles fill the entire room to its brim, and it’s these air particles that make the transmission (in air) of sound possible. A couple of things happen when a sound is produced:

  • Air particles will move a certain times per second, and this will denote what frequency (or pitch) is audible
  • Air particles will vibrate back and forth further out depending on the amplitude (or loudness) of the sound

The second one is important to note, because while a particle moves forward and back (let’s assume space has 2 dimensions, not 3 for the sake of simplicity), it’s going to bump into its neighboring particles, which in turn will bump into its neighboring particle, which in turn will bump into its neighboring particle… -> you get the point. This is how sound travels; it’s the bumping-into-eachother of particles that allows for sound to travel in a longitudinal fashion.

Longitudinal wave animation. Red dot indicates single particle.
Longitudinal wave—red dot shows a single particle’s movement.

..back to the absorption materials

When we put absorption material in between these air particles, an interesting thing happens: sound hits the absorption panel, and goes right through. Mind that this has to happen in order for the absorption panel to be effective. Absorption materials have tiny fibers inside—small enough for air particles to bump into and set into motion. This is called transduction: sound energy is converted to another form of energy (movement, or kinetic in this case). The energy that initially was used to set all these air particles in motion, is now—in part—used to move fibers of this absorption panel. The result of this kinetic movement is essentially (almost non-detectable) heat.

The more sound energy is transduced, the more effective an absorption panel is. Again, it’s very important to remember that for an absorption panel to be effective in the first place, sound still needs to be able to travel through the panel. That said, a smaller amount of energy comes out the other end of the absorption material than initially came in. This simple fact is probably what’s confusing most people when mixing up absorbing sound and stopping sound.

Walls DO stop sound, absorption panels do not

And you can quote us on that incredibly controversial—but accurate—quote. In terms of acoustics, the two concepts serve two very different purposes (..walls serve a lot of purposes, but that aside). So when you try to convert your garage into a band rehearsal space, lining the walls with egg create foam (we’ll get back to this in another article altogether..) will do very little in terms of stopping sound from reaching your neighbors—it will also do very little in absorbing sound, to be frank. This is especially true when amplified bass guitars and a drumset are part of the equation.

What will be effective, however, is doubling down on the walls by beefing them up with extra materials. There’s no better way of increasing the transmission loss of a barrier than with sheer mass.

Even if the concepts described above are foreign to you, imagine two different types of wall. One wall a simple partition wall you’ll find in your average house between bedrooms. The second wall a heavy concrete wall you’ll, for instance, find in a parking garage. Intuitively, you’ll agree that if we had two rooms—one with walls made up of simple partition walls, and one room with 12″ thick concrete—very little sound would escape out of our concrete room. Mind that preventing sound from leaving an enclosed space will simply result in the sound being reflected back into the room with very little attenuation. This, in turn, is where absorption comes into the equation.

Match the tools to the problem

As with most other things, it’s important to pick the right tools for the right job. If we’re trying to increase the walls’ transmission loss by keeping inside-sound in, and outside-sound out, we need mass. Another important thing is that your room’s effective transmission loss is only as high as its weakest point. Unfortunately, nine times out of ten, when musicians build their studio or rehearsal space and did their due diligence when it comes to researching soundproofing, sound still escapes. Evaluate not only your wall, but also the doors, windows, electric outlet boxes, and/or conduits that pierce your walls. All of these ‘wall openings’ to have a transmission loss equal to—or greater than—the surrounding wall. This is in order to not become a weak point or sound leak in said wall. Also mind your gaps, by caulking them shut.

If, on the other hand, the goal is to control (as in ‘tame’) the inside-sound, then we need absorption. Absorption will allow us to make the room less reverberant, which is one of the most important factors in making sound (speech, or complex music) more intelligible. Specifically tuning a room involves using different types of absorption materials to target certain frequency ranges (low/bass, mid, and high), among other things. This is where looking at a material’s absorption coefficients comes in. A material’s absorption coefficients will show how effective a material is at absorbing sound in a specific frequency range.

If you need any advice in beefing up your walls or tuning your room, make sure to get in touch with us at Calico.