- What are bass traps and how do I use them?
- What different types of bass traps are there?
- Find out all you need to know to control the low end in your home studio
Although the term ‘bass trap’ is used a lot in the world of room acoustics and home studios, the operating principle and the correct utilization of this type of absorber can be hard to understand.
This can be especially frustrating if you are sitting in a small home studio right now which is almost definitely not a room that is designed with music production in mind. You need to put in the work to stop the sound from being boomy and inconsistent.
Controlling or ‘trapping’ the bass in your studio by applying low-frequency absorbers can significantly improve your listening environment and thus the quality of your production.
(For a more general look at acoustic treatment, check out our roundup of the 7 Best Acoustic Foam Panels For Home Studios.)
Where Do I Place My Bass Traps?
Generally speaking, bass traps work best in the corners of the room or at least close to a wall. Depending on the way your studio is designed the corners behind your speakers are a pretty safe bet.
If that is not possible in your room, don’t worry! There are different approaches and solutions to make the low-end work for your studio space.
Why Do I Even Need Bass Traps?
Of course, building home studios includes compromises. There are restrictions like budget, space, roommates, and neighbors.
But you still want to create the best achievable listening environment and get as close to professional standards as possible. With proper room treatment, you have a more accurate idea of what your mixes actually sound like as they are not being obscured by environmental factors that are specific to your listening environment.
This means that reflections in your studio room are going to mess with your perception of what’s actually going on in your mix, and when someone listens to this same mix in a different environment, it won’t sound as balanced.
This is critical for bass which is an important frequency range in club and festival music. If the balance is out, your song just won’t sound good.
So there are acoustic parameters we need to adjust for in order to accurately monitor bass levels, namely:
- Frequency response
- Reverberation time
- Early reflections
Frequency and Wavelength
To get started, let’s go over the connection between frequency and wavelength really quickly. When we look at a sound wave, the frequency tells us how many cycles the wave will run through in one second.
When a waveform is produced by the speakers in your studio, it has a real length that we can measure, even if we can’t actually see it. As you may expect, that length changes with the frequency, and the lower the frequency, the bigger the wave length.
Bass frequencies are actually several meters long, and to give you an idea of this in terms of feet, a 90 Hz tone is 12 feet long under ideal conditions.
Let’s look at some exemplary frequencies and their respective wavelengths. Please note that those values only apply to sound propagation in air and can vary slightly depending on the temperature.
|20 Hz||17 m|
|100 Hz||3.4 m|
|1 kHz||0.34 m or 34 cm|
|2 kHz||0.17 m or 17cm|
Bass Frequencies and Room Modes
How does this relate to building a home studio? First of all, if we take the concept of a wave moving through a room a step further, it becomes apparent that in some places the sound pressure will be at a maximum, and in others, it will be 0.
So you could have lots of bass in one part of your room, but no bass at all where you are sitting (and trying to mix your kick drum).
Secondly, if the wavelength of a frequency and the dimensions of your room match, your whole room will resonate because of a phenomenon called room modes or standing waves.
The direct sound from your speaker and the reflections between the walls are in phase and superpose, creating a boomy, vibrating sound. You might even notice your walls moving depending on the material and volume.
The tricky part is that room modes don’t only develop between 2 walls (then they are called axial room modes), but also between 3 or 4 walls (tangential or oblique room modes), making it very hard to predict them.
On top of that, a lot of times not only the frequency of the room mode itself is making problems but also multiples (harmonics) of it.
Luckily, through sound absorption with bass traps or other absorbers, we can reduce the energy of those standing waves and achieve a more balanced frequency response in the studio. Let’s see how that works!
How Does Sound Absorption Work?
When a sound wave hits a reflective surface, there are three possible results (or a blend of them) that are interesting in the context of studio acoustics:
- Specular Reflection: the wall reflects the sound like a mirror. Those direct reflections make the listening environment less transparent and annoying phenomena like comb filter effects, echos, and flutter echos develop this way.
- Diffusion: when the sound wave hits the wall, it is scattered in many directions. The energy stays in the room, creating a natural and diffuse sound field without shortening the reverberation time.
- Absorption: the energy of the sound wave is absorbed and no longer in the room. This process is frequency-dependent and shortens the reverberation time.
If you’re wondering where that energy goes in the absorption process, we need to take a little detour into the world of physics.
Sound Pressure vs Velocity
In the propagation of sound, there are two amplitude-related parameters. There is the obvious sound pressure we measure in dB on the one hand and a value called velocity on the other.
The latter describes how fast and with which amplitude the particles in the air (that transmit the sound) move. Velocity describes a rate of change in speed, not speed itself. With this in mind, the curves of velocity and pressure are offset by 1/4 of a wavelength of the sound wave.
So, if the pressure is at a maximum, the velocity is zero and vice versa. And since the particles in the wall can’t move, the velocity there is 0, and therefore the sound pressure is at a maximum.
Furthermore, 1/4 wavelength away from the wall, the sound pressure is at 0, while the velocity is at a maximum.
Dissipation and Porous Absorbers
Now let’s see where that energy goes. One option is the transformation into thermal energy or simply put, heat. The process is called dissipation.
The sound wave is reflected many times inside a porous material, and the friction converts the energy into heat. It’s the underlying principle behind porous absorbers and many bass traps.
For this to work, the porous material (like rock wool or foam) has to have a thickness of at least 1/4 of the wavelength of the lowest frequency it is supposed to absorb.
Depending on where your problematic frequencies lie, this can lead to purchases of pretty thick absorbers.
Let’s say you measure the acoustics in your studio and detect a strong increase (maybe caused by a standing wave) around 60Hz. That is kick drum territory, so nothing you can live with.
Taking a peek at our frequency/wavelength table: 60 Hz has a wavelength of about 5.7m. So to have our bass trap be actually effective for a frequency this low, we would need an absorber with a depth of over 1.4m (1/4 of 5.7m).
This may sound extreme, but it’s what physics dictates.
However, porous absorbers have many advantages (most corner bass traps are based on this principle):
- They address all three studio acoustic parameters we introduced earlier. By altering the frequency response, cutting the reverberation time by the transformation of the energy into heat, and absorbing annoying early reflections.
- They give broadband absorption above 1/4 wavelength of the lowest frequency. And lastly, porous absorbers are pretty easy to build yourself.
- Even if you have a standing wave at 60Hz, a bass trap being effective around 120Hz will take away the first harmonic and other multiples of your problematic frequency, so you will already hear quite a difference.
Resonators For Bass Absorption
However, if you want to tackle those really low bass frequencies, a resonant absorber could be the solution.
The idea behind this type of absorber is to build a structure that vibrates at the desired frequency and this way transforms the sound energy into kinetic energy.
They rely on the mass-spring system and the two most common designs of this type of absorber are plate resonators and Helmholtz resonators.
The plate resonator is mounted, so it can vibrate freely. The mass of the plate involving the size and density of the material as well as its distance to the wall must be calculated carefully to achieve the desired effect.
In the case of the Helmholtz resonator, which is a container with a pipe opening, the volume of the box as well as the size and length of the pipe are crucial.
You can already tell that the construction of a resonant absorber is more tricky and involves quite a bit of math. But as a result, you get a very narrow band of frequency absorption, reducing precisely the bass frequency that is problematic in your studio.
To broaden the frequency band in which the resonator is effective, fill the space between the plate and the wall or inside the Helmholtz resonator with a porous material.
Where To Put Bass Traps (Depending On Their Type)
Before we get into the detailed positioning of your bass traps, one more note:
Like with many things in audio, make sure you know which goals you want to achieve by installing bass absorbers prior to jumping right into it potentially wasting your time and money.
Bass traps are powerful and useful, but are not a silver bullet to all your treatment woes.
That being said, taming the low end in a control room or studio is arguably the most difficult but most important room acoustical treatment out there, especially if you make bass-heavy music.
So, once you have measured your studio, located room modes and problematic bass frequencies, and chosen the types and sizes of absorbers you want to use to reduce them, you are ready to set them up.
Corner Bass Traps Or Porous Absorbers
Let’s start with the obvious: corner bass traps go in the corners of your room. Porous absorbers work fantastically close to a wall, and treating more walls is even better.
Ideally, you install bass traps like this in the corners behind your speakers. The reasons lie a bit deeper in control room design: Since you don’t only absorb bass frequencies but everything through the mids and highs, the impact on your room acoustics goes further.
Generally speaking, fewer reflections in the area of the speakers and the half of the room in which you are working are desired.
That’s why corner bass traps can also help by giving you more direct sound and fewer reflections where you are sitting.
However, there are reasons to choose the corners behind you. One would be, that the corners behind your speakers are not available because of the way your room is laid out.
That’s totally fine: The bass frequencies will still be reduced with your corner bass traps on the opposite side of the room.
The symmetry aspect also applies to resonant absorbers. Since you are most likely targeting low room modes and bass frequencies are less directional, the effect shouldn’t be underestimated.
If you are working in a smaller studio you don’t have room for many acoustic elements, so in the best case scenario, your resonant absorbers (e.g. plate resonators) are set up to have some high-frequency absorption properties. Then their positioning becomes more crucial.
One trick to finding good places to position your absorbers is the mirror technique. Move a mirror along the walls (and ceiling) of your studio.
If you can see the speakers from where you are sitting, the place of the mirror is a good position for an absorber. It will take away undesired early reflections and give you a more transparent sound image.
Do bass traps need to go to the ceiling?
Sure, if you can. Regular corners are great, 3-way corners are even stronger. But the absorption still works if they don’t go all the way to the ceiling.
Should I put bass traps behind my speakers?
Yes. Again, if that place is available for you. It’s a good idea since you will be dealing with two issues at the same time: bass frequencies and transparency.
How many bass traps are needed?
This totally depends on the issues in your room and the types of bass absorbers you chose. You might want to have different resonant absorbers for different frequencies.
However, especially if you choose broadband absorbers, remember that symmetry is crucial for your stereo image. So an even number of absorbers evenly distributed is beneficial.
How do bass traps stop the bass build up in corners?
Depending on the type of absorber you choose, the bass energy will either be transformed into heat through friction or into kinetic energy through vibrating plates or boxes.
How thick do bass traps need to be?
This depends on the type of absorber and how low the frequency lies that you want to target.
For porous absorbers to be effective low in the frequency spectrum, they have to be quite thick (1/4 of the wavelength of the lowest frequency). Consider a resonant absorber for the really low bass frequencies.
What are some tips for mixing bass?
We’ve got a whole article on this topic just for you: Mixing Low End (8 Tips For Face-meltingly Large Bass).
Looking to improve your home studio setup? Check out 7 Best Monitor Isolation Pads (All Budgets)