Unfortunately, over 90% of home studios do not have bass traps. Yet, they are part of one of the essential elements of studio acoustic treatment.
I say “unfortunately” because practically every room is subject to problems in the low frequencies.
Do you sometimes struggle to get the bass right in your mixes?
Do you feel that, from one note to another, some low frequencies vibrate super loud — while others seem almost inaudible?
Yes indeed: these are two signs that your room needs bass traps.
Indeed, the smaller it is (which is often the case in home studios), the more problems you will see in the lower spectrum of frequencies.
There are always compromises to be found.
That said, if you have a bit of space to add bass traps, your mixes will only improve — but you need to choose “the right” bass traps and, above all, place them correctly.
Through this article, we will therefore address the following topics:
- What are bass traps for?
- Types of bass traps
- Where to place bass traps?
- Which model to choose?
- How to make your own bass traps
Note: this article mainly mentions the use of bass traps in home studios, but the principles are exactly the same if you want to treat the acoustics of an audiophile room.
What are bass traps for?
Let’s start by understanding what a bass trap is and what problem it is supposed to solve.
Note: in this article, we will discuss some somewhat complex notions of acoustics. Some diagrams may therefore be a bit difficult to read at first, but I will do my best to simplify them in the text.
The problems of modal resonances
In any improperly treated room, there are bass problems.
To realize this, I invite you to do the following experiment:
- Play music with a good amount of bass on your speakers (here’s a suggestion);
- Stand in the center of the room;
- Gradually move towards a corner of the room, until you are as close as possible to the wall;
- If necessary, continue to move around the rest of the room.
By doing this, you will notice that the bass you hear changes: in certain spots, some frequencies stand out extremely strongly, while in other spots, they seem to literally disappear.
This phenomenon is very easy to notice, especially since it is common to have dips of -30 dB or more!
At your listening point, that is to say, the chair on which you mix your tracks, there will also be bass problems — or more precisely problems of modal resonances.
In fact, these resonances depend on the dimensions of the room you are in and occur particularly when you have two parallel surfaces (two walls): the sound waves emitted will bounce off the walls, and you will have interferences between the waves emitted by your speakers and the reflected waves.
If you’re interested in the subject, I discuss it in a bit more detail in my file on acoustic treatment.
The usefulness of bass traps
This is where bass traps come into play.
Indeed, by installing this type of acoustic treatment, you will be able to control the energy of low frequencies and reduce the modal resonances specific to your room by transforming this energy into heat through friction.
Warning: this is not about reducing the bass globally, but rather about absorbing the reflections corresponding to this type of frequencies.
In other words, unlike absorbing acoustic panels that can make a room “dead” when used in excess, you can theoretically place as many bass traps as you want — it won’t be a problem.
Let’s take a look at the graph below, where you can see the frequencies on the x-axis and the amount of absorption on the y-axis:
These are of course theoretical values, but it is clear that our bass traps, due to their construction and positioning, will primarily target the lows — but absorb very little of the mids and highs.
You can therefore place many in your room (and generally, a certain quantity is needed!), without turning your studio into a dead room…
Types of bass traps
There are two main types of bass traps:
- porous bass traps, which will work to absorb the lows in general;
- resonant bass traps, which will target specific frequencies.
Porous bass traps
Porous bass traps generally take the form of panels or massive triangular blocks containing a material capable of resisting the passage of air molecules in the interstitial spaces it contains.
In other words, capable of absorbing the lows.
Most of the time, they consist of mineral wool with very specific physical properties: fiberglass, rock wool… — but there are also bass traps designed from acoustic foams or, for example, Caruso IsoBond.
All of this is usually covered with fabric and sometimes held together by a wooden or metal structure.
The thickness can vary, but what is certain is that the more material there is in the bass trap, the more effectively it will be able to absorb low frequencies.
In some cases, a sheet of paper or aluminum is integrated into the device to ensure that it reflects sound waves above 400 or 500 Hz, in order to avoid making the room “dead” through over-absorption of mids and highs.
Finally, compared to resonant bass traps, porous bass traps have two main advantages:
- they are cheaper, especially because they are simpler to manufacture;
- and above all, they can absorb sound energy over a fairly wide frequency range.
Resonant bass traps
Resonant bass traps, on the other hand, are tuned to absorb specific frequencies through sympathetic vibration.
This means that if you have a problem exactly at 80 Hz in your room, a resonant bass trap can target it precisely and correct or attenuate it.
However, this obviously requires knowing the exact frequency to correct, using modal resonance calculators and on-site acoustic measurements.
There are two main types of resonant bass traps:
- Helmholtz resonators;
- membrane bass traps.
Helmholtz resonators correspond to hollow structures with one or more openings specifically measured to absorb a very narrow band of frequencies.
Often empty, they can nonetheless be filled with an absorbent material to slightly widen the frequency band.
An effective analogy to understand how they work is the water bottle at the edge of which one blows to make it resonate: depending on the size of the opening and the available volume, the resonance frequency will change.
Well, it’s a similar principle that governs Helmholtz resonators.
The membrane bass traps, on the other hand, are very different: they contain a soft or rigid membrane that vibrates when subjected to a specific frequency.
In front of this membrane (imagine a plywood sheet simply held at the corners), a plate of absorbent material like mineral wool is positioned, with a small gap between the two (Xavier Collet explains how this system works very well in the SawUp training on acoustic treatment).
And it is this entire structure that is capable of properly absorbing the frequencies for which it was designed.
Necessarily, compared to porous bass traps, resonant bass traps are much more complex to build: they require performing a number of mathematical calculations and taking into account construction parameters such as tuning changes depending on humidity.
However, they have the advantage of being able to reach much lower bass frequencies, which can be useful for correcting, for example, a modal resonance at 70 Hz.
In fact, they are generally devices dedicated to high-end studios with a significant budget — in a home studio, I strongly advise you to stick to porous bass traps.
Where to place your bass traps?
Properly positioning your bass traps is essential to ensure they are effective: if placed incorrectly, they could even harm the acoustics of your studio or audiophile room.
In general, bass traps are always placed in the corners of the room, as this is where one can influence the maximum number of modal resonances.
Let me explain:
Modal resonances are created by waves bouncing between two parallel walls (at least, for axial modes — as there are also other types of modes that are more difficult to control, and generally not taken into account in home studios).
When you place a bass trap in a corner of your room, you are thus intervening on the modal resonances of the three directions:
- length (we talk about mode 1 0 0);
- width (0 1 0);
- and height (0 0 1).
One can indeed suspect this if we look at this type of 2D modeling, which represents the pressure variations in a rectangular room:
That said, “placing your bass traps in the corners” is not precise enough to know how to install them properly.
Indeed, while corners are ideal positions for the reason mentioned above, a resonant bass trap and a porous bass trap are not placed in the same way.
How to position a resonant bass trap
Resonant bass traps, such as Helmholtz resonators, work in pressure — meaning they function when the air is at maximum pressure.
Keeping this information in mind, let’s look at the following diagram that illustrates the amplitude of pressure variation (red curve) associated with a standing wave, that is, a wave generating modal resonances:
As can be seen, at the wall positioned on the right of the diagram, the pressure variations are maximal: the red pressure curve oscillates from one maximum value to another.
This is exactly where you will need to place your resonant absorbers for them to be effective.
If you move them away from the wall, they will lose all usefulness.
How to position a porous bass trap
Now, for porous absorbers, it’s a different matter.
Indeed, they do not work on pressure, but on velocity. When air molecules pass through a porous bass trap, they rub against the walls of the pores in the material and thus lose energy: thanks to this friction, their movement is transformed into heat.
So, what happens if you position your bass trap right against the wall?
Well, it will be (almost) useless.
Indeed, at the moment a sound wave strikes a wall, its pressure is at its maximum but the particle movement speed is zero!
To convince ourselves, let’s take another look at the mathematical simulation from earlier, but this time paying attention to the black curve:
This time, at the wall on the right, we can clearly see that the black curve remains at zero: this is normal, at this point, the particle movement speed is zero.
However, as soon as you move a little away from the wall (thus going towards the left of the diagram), you can see that variations in speed occur.
Instead of sticking porous bass traps to the walls, it is much more useful to leave an empty space by positioning them at the point of maximum particle movement speed, which is precisely located at 1/4 of the wavelength you wish to absorb.
To be precise and/or for those who like mathematics, the formula is as follows:
where the number 343 corresponds to the average speed of sound at 20 °C.
In practical terms, this translates into two design options for your porous bass traps:
- either you leave a space to properly absorb low frequencies;
- or you set up very large bass traps.
Typically, in the first case, we will use panels that we will position diagonally in the corners, but with an air gap at the back.
In the second case, however, we will use “superchunks” type bass traps, massive, which will often have a triangular shape allowing them to be easily positioned in the corners of the room.
In any case, if you treat each corner of the room (which is often more of an ideal target than an achievable target in a home studio context, since there is always a door or a piece of furniture that gets in the way…), your acoustic treatment will take up a significant amount of space…
Which bass trap model to choose?
Out of curiosity, while preparing this article, I went to see if there were cheap bass traps on Aliexpress.
And yes, they exist: for a few euros, you can get about ten bass traps.
The good deal for home studios with a limited budget?
“The question is quickly answered,” as the saying goes: avoid this type of low-cost treatment as much as possible, which can also be found on Amazon or similar.
On one hand, the dimensions have nothing to do with real bass traps: 12×12 centimeters will absorb absolutely nothing.
This means that it cannot even be a temporary or entry-level solution: it will have no effect on the acoustics of the room.
Moreover, nothing guarantees that the fire safety standards recommended to follow in studios (after all, quite a bit of electronic equipment is used) are respected.
And then, if you can avoid acoustic foams for your bass traps, that would be just as good: they are not the ideal materials for this kind of applications.
However, the American brand Auralex still offers products that can be interesting for a home studio through its “LENRD” bass traps, which you can check the lab test report by following this link.
► See the current price on: Thomann Woodbrass
Sure, it’s not perfect, but it can still do the job — at least above 100 Hz.
However, there are other options for bass traps to put in your home studio.
Thus, HOFA is a German brand that I greatly appreciate.
They manufacture (in Germany!) good quality bass traps, available in two different series:
- the classic series, simply named HOFA Basstrap, contains mineral wool;
- the HOFA Basstrap Natural series contains a material made from sheep wool — thus more ecological and potentially better for health.
► See the current price on: Thomann
Finally, another option is to turn to the very serious brand GIK Acoustics, whose products are easily accessible in France, and which produces different types of bass traps that are more or less advanced in terms of design.
Among these different series, the “Bass Trap 244” will be the model to look at first if you are looking for something simple and unadorned, but particularly effective.
► See the current price directly on the (French-speaking) site of Gik Acoustics
Making Your Own Bass Traps
If you have some DIY skills, you also have the option to make your own bass traps in DIY mode.
Sure, building a Helmholtz or membrane bass trap will undoubtedly be a big project, with a significant risk of error and especially the need to perform various mathematical calculations to define the perfect dimensions.
On the other hand, porous bass traps are much simpler to make, whether you go for massive superchunk type models or for panels to place across the corners of your room.
In absolute terms, you just need to create a wooden structure in which you will pack a material with good low-frequency absorption properties, such as certain mineral wools (be careful, not all are suitable).
Of course, dimensions and thickness also matter.
I will write an article on the subject in some time, to explain how to make your own bass traps… 😉
In Conclusion
There you go, you now know what a bass trap is, what it is used for, and how to position this type of treatment in your room.
To go further, I recommend reading my guide on acoustic treatment, which contains plenty of other information to improve the acoustics of your studio, home studio, or audiophile room.