Bass Traps and Air Gaps

Today I’m answering a common question about DIY small room acoustics and bass traps, specifically: “Can an acoustic panel, when placed across the corner of a room, work as a bass trap?” Even more information on bass traps, and how to set up your room for high quality music production can be found here.

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A bass trap is a piece of acoustic treatment that is meant to absorb the lowest audible frequencies. The most efficient bass traps use a combination of two things: thickly laid sound absorbing material along with a small air gap placed behind the trap. A thin acoustic panel placed across a room’s corner does not have sufficient thickness of sound absorbing material to effectively absorb the lowest frequencies.

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Why bother with installing bass traps? Absorbing excess bass energy helps achieve a more even bass response in your music production room. If you don’t invest in bass traps, you will notice inconsistency in your mixes depending on where they’re played back. That mix you thought sounded perfect in your home studio will suddenly sound completely different when played back in the car. Not fun.

Bass traps are essential equipment for achieving consistency and good translation across playback systems, when you’re producing music. They are one of the first things you should set up, when designing a room for making music.

In the DIY home music production setting, bass traps are often constructed in-house, which leads to today’s interesting question:

If you were to leave an air gap behind a simple thin acoustic panel placed over a corner, like this:

Would you get greater bass absorbing effects then if you filled the entire space with acoustic absorption, like this:

Short answer: Kind of.

You should fill that air space with lots of acoustic absorption for best results, but don't pack it too tight, and leave a small air gap in the back of the trap if you’ve got the space.

The specific application of the science of acoustics is nuanced, in this case. Let’s learn more.

Sound Energy and Air Volume

If air didn’t absorb sound energy, every time you hit the kick drum low frequency sound waves would endlessly circle the planet forever. Instead, you move about 200 feet away and the kick gets a lot softer. So, the basic concept that air can absorb sound is correct, and we can indeed use this to our advantage when controlling room acoustics.

But:

It takes a VERY LARGE air volume to act as an effective absorber for the lowest bass frequencies.

Thousands of cubic feet of air. The air space behind that panel you placed over the corner is under 10 cubic feet in volume. It doesn’t take a lot of energy to move a volume of air that size. So, in this case, air is not a strong absorber of low frequency sound energy on its own.

However, a small air gap when paired with the correct thickness of rock wool becomes more effective than either tightly packed rock wool or just air. We need to use both thick rock wool and a small air-gap in combination, for the most effective bass trap.

A Structural Bass Trap Case Study

Bass trap setups that use only air to absorb low sound frequencies typically use a very large volume of air. Electrical Audio in Chicago, for example, uses a setup like this:

In this application, the main recording area shares an air space with a large basement void underneath. There’s a small gap between the floors and the walls of the two rooms. This allows the air in the recording room and the air in the large basement below it to vibrate as one single connected air volume.

Because the combined air volume of these two connected spaces is so large, it takes a lot of sound energy to vibrate. That massive volume of air is what allows empty air space to be used as a giant structural bass trap.

Tricks like these are very common to see in custom, high-end recording studio build-outs. And, they’re very effective in that context: a commercial studio with a large construction budget.

Bass traps in the DIY setting

However, a lot of these fancy kinds of acoustic techniques only work in the one highly specific context of a commercial studio build-out, and aren’t relevant if you’re not building new construction from the ground up with a big budget.

In the DIY setting, we need to use different strategies, to get the most out of the existing spaces we already have. I’ve got a whole series on how to do this called High Quality Budget Gear, if you’re interested in learning more about how to set up your home studio.

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In summary: air gaps placed behind acoustic panels and bass traps can be used effectively, and can improve acoustic performance!

But, remember, in the case of a bass trap we also want our volume of absorbing foam to be as large as possible, in order to absorb the very lowest frequencies most effectively. That means the thickness of material matters. The thinner the panel, the less effective it is at absorbing both mid range and low range frequencies.

So, when building a bass trap, keep the rock wool inside the trap as thick as possible, with maybe just a 3-4 inch air gap built into the panel in the back framing, if there's room to do that in your installation space. This is the reason why corner bass traps are so effective, relative to the volume of space they take up in a room. Using this corner design, we are placing the maximum volume of sound absorbing material right in the place where it will be the most effective at absorbing bass energy.

The source material for this article is F. Alton Everest’s Master Handbook of Acoustics, which is a fantastic resource for learning more about the science of acoustics.

If you’re interested in learning more about mixing your own music, check out my free Mixing Cheat Sheet! It’s a handy checklist that’s got every step to a great mix, all laid out in order for you.