Understanding Soundproofing is Simple
Ever watch piles of large, sharp rocks on a shoreline break up the waves coming in? Foam panels dampen sound in a similar way.
The rocks “break up” the wave to reduce its force. Acoustic foam is porous inside (like Swiss cheese)—kind of like a giant box of sharp rocks! The sound bounces through all the air “tunnels” between the sharp rocks, reducing its energy.
And more is better! You needs a long maze of air spaces inside the foam for the sound to bounce around in and lose energy.
But a wave will just flow over a pile of smooth stones, so the rocks need to be sharp, and in acoustic foam that means lots and lots of tiny, sharp-edged airspaces.
Solid material doesn’t work well. It mostly reflects sound instead of trapping it. You don’t want the sound bouncing back, you want to trap it inside the material and reduce it.
But you do need something solid enough to resist water waves. Obviously piles of shredded plastic would just flow with the waves! Rocks resist the waves.
This is why cheap foam not made for soundproofing (such as that used in egg crates) doesn’t dampen sound well: it’s not firm enough. Air is a gas. Sound is a wave that moves through that gas, similar to wind, only sound is more like the water wave. The internal cell walls of cheap foam vibrate with the sound too much instead of resisting it sufficiently. You don’t want material that just ripples like ribbons in the wind!
You get an immediate improvement from acoustic dampening materials if you place them at right angles to each other, since the sound will bounce from one to the other. Aiming sound into a “V” shape is the most basic acoustic “trap”. This is why foam for acoustic dampening has a zig-zag or “egg crate” pattern in it. Besides the maze inside the foam that helps trap the acoustic wave, it forces it to bounce around as it enters the maze as well.
Why Bass Frequencies “Pool” in corners
Drop a large rock in the center of a square pond and a circular ripple will hit the four shores at the same time and spread out until it hits the corners. Picture the corner. Two shores of the pond connect to it. On each of those shores a wave hits, then spreads toward the corner. So you have two walls each sending a ripple toward the corner. Now picture rubber ducks bobbing up and down. Where do you think the ducks will bounce up and down the most? That’s right, the corners, because each corner has those two walls sending a pressure ripple into it.
Sound is like continuously dropping rocks into the pond—a continuous source of ripples heading for the walls. But a room is not a flat pond. Each corner has THREE flat surfaces sending ripples to it—two walls and the ceiling for upper corners, or two walls and the floor for lower corners.
Of all acoustic frequencies from high to low, the lowest frequencies (bass) behave the most like the waves in the pond. So corners of rooms tend to “pool” bass frequencies in them, where all the ripples are bumping into each other. And because lower frequencies have longer wavelengths, thin wave dampeners have less effect. You need THICKER material to affect bass frequencies.
The Bag of Laundry Trick
So to limit bass frequencies from causing sounds you don’t want, a minimum strategy is:
- Thick material (2-3 feet thick, not just inches thick);
- Material placed in corners where bass frequencies “pool”.
So: yup, if you hang a big bag of crumpled laundry in every corner of a room, you’ve created great bass traps! Picture a “cube” of air in each corner that is 2-3 feet square. This is the area that needs the most work.
One Problem with a Voiceover “Booth”
If you are listening to sound where your ear is far from the speaker, like a home theater space, bass traps are very important to normalizing the sound. It’s most important in smaller spaces—those 2-3 foot corner spaces make up practically the whole room in a 6x6x6 space! This is just one reason that small voiceover “booths” with parallel sides don’t always work well. But in large rooms bass pooling doesn’t effect the sound as much.
Also, if the “ear” is very close to the sound source, bass pools will have less effect on what you hear. If you are recording voiceover, your voice is the sound source, and the “ear” is the microphone, just inches away. So bass traps matter much less for voiceover than for a home theater space. And they matter even less if you are using a proximity sensitive microphone that warms up your sound when you get close to it.
Rather than try to trap bass in a small square or rectangular space, try to work in the corner of a larger space instead of trying to fix a voiceover “booth”-size space.
The simplest home studio is to stand with your back to the corner of a room, line the corner with acoustic dampening material, and put some acoustic dampening material behind your copy, or on your copy stand.
Here are a couple of articles on bass traps for further reading: