Tame Unwanted Bass Frequencies In Your Room - Corner Slot Resonator Bass Trap
Oct 23, 2023Do you have mixes that alway have too much or too little bass? Does your low end sound muddy on your speakers? Most rooms have low end problems and even acoustic panels cannot fix them. We need low frequency pressure traps to attenuate low frequencies. This article focuses on one such bass trap known as a corner slot resonator. It is fairly easy to build and give fairly broadband absorption down to the low bass frequencies in that 20-50hz range.
1) Corners Are Where Bass Builds Up
The corners of the room are where most bass builds up and is therefore the best place to absorb all modal problems in your room. By putting a slot resonator in your corners you can target problem frequencies in your room.
Now, notice how I said target frequencies. These acoustic bass traps are built custom to your room, so you need to know what frequencies are causing issues. To figure this out you need to measure your room using a measurement microphone and a software like REW. I go over this in other posts, so check that out before continuing.
2) How To Build A Slot Resonator
Once you have measured your room and found some offending low bass frequencies also known as room modes, then you are ready to attack them! Okay, just absorb them, but let's look at how to do that.
A slot resonator is a form of Helmholtz resonator. These resonators use airspaces between wood slats and a depth behind that slat to attenuate bass. Here is the formula to figure out the depth of your corner trap, the width between your wood slats and the depth behind the wood slats.
f = 2160* square root r/((d*D)*((r+w))
f = resonant frequency
r = slot width
w = slat width
d = Effective depth of slot (1.2 times the actual slat thickness)
D = depth of box to the inside slat face
Now, the best thing to do is plug this equation into a spreadsheet to solve for the frequencies you want to attenuate. Another option is to use an online calculator like this one: https://www.acoustic.ua/forms/calculator5.en.html
For a diagonal trap in a corner your slot depth will very with each wood slat and you can vary the width between slats. 
The trap I have shown above has a center depth of 1' 7 1/4" with a 1/16" width between the 1x6" wood slats, which will attenuate at 46Hz. For my clients building home studios I like to build these traps custom so that each slot width and depth is targeting a specific problem frequency in their room. This will help get a smoother bass response at their mixing position.
Designing these traps in 3D CAD program will allow you to accurately measure and design your traps to the 1/16." Accuracy is important with these traps since, a 1/16" difference can changer your resonant frequency by 20 or more Hz.
3) The Overall Design
Start by building a frame using 2x4 lumber for the bottom, sides and top plate. Then put in a 2" thick piece of Cornings 703 Insulation. Place fabric over the insulation and wood frame. This should be fabric you can breath through and feel air on your hand on the other side of the fabric. I recommend Guildford Of Maine's acoustic fabric.
Last, you will attach the 1x6" wood slats to your frame on top of the fabric. The width between your wood slats and the depth behind the inside face of each slat will determine the resonant frequency your trap will absorb. Below is a diagram of how to build these traps.
Conclusion
If you want to reduce low bass frequencies in your room and have a fairly broadband trap than these slot resonator corner traps are a great cost effective solution. Just take care to measure carefully and do the math to get the trap as accurate as possible so it absorbs the frequencies you want it too.
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