What is a bass trap?

A bass trap is an acoustic treatment product specifically designed to treat low frequencies, i.e. the bass. These frequencies can be difficult to treat in certain rooms and are between 20 Hz and 400 Hz.

More concretely, their purpose is to decrease the amplitude of the sound wave, evaluated by the RT 60 (reverberation time) on these frequencies.

Low frequencies

As a reminder, we have 3 types of frequencies:

1. The low frequencies (or bass frequencies): from 20 Hz and 400 Hz
2. The mid frequencies: from 400 to 2000 Hz
3. The high frequencies (or acute frequencies): from 2000 and 20000 Hz

These frequencies correspond to a number of oscillations per second and characterize a sound. In concrete terms, some sounds will be strident (high frequencies: noise of clashing glass for example), others will be low (contralto, for example). It should be noted that some frequencies are audible or not depending on age. Also, some animals do not hear quite the same frequencies as humans. Cats can hear up to 40,000 Hz.

Uses of a bass trap: recording studio, hifi and home

A bass trap can be used in several settings. It can be used for :

• A recording studio
• A dedicated or non-dedicated hifi room

Bass trap for recording studio

The bass traps can have a great utility in the recording studios.

It will be appropriate to use specific bass traps for this use. We will come back to this type of bass trap later.

Home, auditorium and hifi bass traps

As explained, a bass trap can also be used for a dedicated or non-dedicated hifi room. It can therefore be used for a professional auditorium or in a room of your house dedicated to hifi.

The problem of a bass trap in a non-dedicated room will be its volume. It is, indeed, not discrete and takes up space.

Types of bass traps: tuned or not

There are different types of bass traps. They can be differentiated by their shape, for aesthetic reasons, but this will not affect their efficiency. However, they differ mainly on the process and materials used. We can distinguish 2 types of bass traps:

1. The non-tuned bass traps: they are not designed to absorb only the low frequencies. They are the result of a standardized industrialization process.
2. The tuned bass traps: They are specifically designed to absorb low frequencies according to a room.

The different bass traps: standard, Helmholtz resonator, membrane and electronic

We can divide bass traps into 4 categories:

1. The bass traps known as “standard”
2. The bass traps with Helmholtz resonator
3. The bass traps with membrane
4. The electronic bass traps

The standard bass traps will be untuned bass traps. Whereas the 3 other types of bass traps will be able to be tuned and thus be more effective by adapting to the needs of your room.

The “Standard” bass traps

These bass traps are made of mineral wool or acoustic foam. They are very easily found in stores but are not designed to specifically absorb low frequencies. Their interest is therefore limited.

Moreover, a large surface with mineral wools and acoustic foams is required to absorb low frequencies. It is therefore necessary to buy a large quantity of materials and this is particularly cumbersome in a room.

The helmholtz resonator bass traps

These bass traps are based on mathematical calculations. When they are custom made, they are very efficient. Concretely, they work like a bottle. Indeed, when we blow into a bottle, the body of the bottle will resonate and return the energy of the breath. It will come out a small shrill sound. This bass trap works the same way with sound energy (because it is not a breath that is sent back). The goal is to make a body and an opening that will correspond to the frequencies that we want to absorb in the room. To be effective, the walls of the resonator must vibrate as little as possible. Also, it must be of a certain thickness (unlike an acoustic panel). However, a Helmholtz resonator does not need to be as thick as an acoustic foam to work.

The resonator is only designed to absorb one frequency. However, we want to absorb a range of frequencies, not just one frequency.

This is why an absorbing material is added to the whole. We can thus treat the frequencies, for example, from 150 to 200 hz, and not only the frequency of 200 hz.

They have the advantage of being thinner than mineral wool or foam bass traps. Moreover, they are designed to absorb only the low frequencies. They will not “touch” the midrange and high frequencies. Furthermore, these bass traps are those which offer the best ratio quality/price. Finally, the performance of these bass traps lasts over time and they are relatively aesthetic.

In conclusion:

1. Not very thick
2. Absorb only the low frequencies
3. Best quality/price ratio
4. Last over time thanks to their solidity
5. More aesthetic than other types of bass traps

We can distinguish 3 types of bass traps with Helmholtz resonator :

1. Perforated panel
2. Laminar
3. Cylindrical

The helmholtz resonator with perforated panel

These bass traps will be made from a wooden frame (or box), with absorbent inside, and holes will be made at different places.

They will work on 4 principles:

1. The deeper the box, and thus thicker, the more it will be able to absorb the low frequencies
2. The smaller the sum of all the surfaces of the holes, the more efficient the resonator will be in the low frequencies
3. The more evenly the holes are distributed, the more efficient the resonator will be
4. The dimensions and the number of holes will determine the absorption frequencies

The helmholtz resonator with laminar panel

This type of resonator works by positioning blades side by side. The holes will be replaced by the space between the blades.

The cylindrical Helmholtz resonator

They can be adjustable or not. They are cylindrical in shape and are few on the market.

The membrane bass traps

These bass traps work on the principle of mass / spring. Concretely, a spring, i.e. an absorbent material, is added inside one of the faces. In visual, it gives the impression of a box with absorbent inside. Often, these bass traps are made of wood.

They can also be integrated into walls or corners.

The membrane can be flexible or rigid. Its flexibility or rigidity will determine, among other things, its absorption frequency. The membrane can be painted or covered with a fabric.

The electronic bass traps

These bass traps use a still experimental technology. They work in an electronic way, by diffusing sound to cover the frequencies that we wish to absorb.

The principles of operation of a bass trap

The need for porous material and open cells

The bass traps will work with porous materials. They can be made of mineral wools, wadding or absorbent cotton.

To function, a bass trap must be made of materials with a multitude of “cells” or “pores” that are interconnected.

In concrete terms, a bass trap seeks to slow down the speed of sound with the help of an absorbent material. It will then transform it into heat thanks to the friction of the molecules against the walls of the bass trap cavities. Thus, the more interconnected cavities there are and the greater the compressibility of the material, the more effective the absorption. To optimize the efficiency of a bass trap and to target only the bass, the size of the cells will play a role. Thus, the larger they are, the more effective the absorption will be in this frequency range.

The shapes: cube traps or rounded traps

As explained previously, the shape will not influence the efficiency of the bass trap.

You can find 3 main shapes of bass traps:

1. The cube bass traps
2. The rounded bass traps
3. The triangular bass traps for the corners

Where to place them: standard bass traps in the corners?

A standard bass trap, to have the best possible efficiency, will be placed in the corners of a room and on all its height. It is therefore often necessary to stack several of them on top of each other. The ideal is to place them in each corner of the room.

In an audiophile context, it is better to place them behind the speakers.

One of the advantages of bass traps is that they can be easily moved. This allows you to proceed by trial and error and to make purchases empirically. It is important to keep in mind that the acoustic recommendations drawn from curve analyses are not always in phase with our listening experience. Thus, this empirical approach is a good way to get the best compromise for you.

It is advisable to position them well or you risk a comb filter effect.

The materials of bass traps: glass wool, rock wool and acoustic foam

Acoustic foam bass trap

These bass traps, generally of low thickness, are made of open cell foam in polyurethane or melamine and sometimes in polyamide. They are, very often, not very effective because they do not absorb the low frequencies. Also, this type of product can degrade much faster than other materials. They are, however, very common on the market.

For them to be effective, the material would have to be close to a meter thick to start absorbing around 100hz. The problem is that they will also absorb the mid and high frequencies.

Acoustic foams can be :

1. With open cell: they must be open cell to absorb.
2. With closed cell: they will be used to diffuse. Generally, they are “melted” polystyrene cut with a hot wire… Their interest is very limited because inertia is needed for a diffuser to act as such.

Bass trap in glass wool

Glass or rock wools are widely used as absorbing materials in the manufacture of tuned bass traps. For reasons of cost and availability of the product in particular.

The problem of these materials lies in their very polluting manufacture which destroys the ecosystems. Many associations are therefore opposed to the establishment of factories in their cities. This phenomenon occurs everywhere in the world.

Moreover, because of the micro particles they release, these materials could be carcinogenic. They have been classified as such in the past by the IARC, a cancer research agency of the WHO. The long-term effects of these materials are not well known, but in view of their manufacturing process, their irritability and their controversy by experts, it is appropriate to be wary of them. Thus, some recommend not using them, others recommend covering them with a waterproof coating.

Absorbent cotton bass trap

Absorbent cotton is a very interesting alternative as absorption material for a custom-made bass trap. In fact, it will perform the function just as effectively.

In addition, it is a more stable material, which does not compress over time (like glass or rock wool) and does not disintegrate like polyurethane foam.

The efficiency of a bass trap

The most efficient bass traps will be those made to measure. Indeed, each room will have its own frequency problems.

To know these problems, it is advisable either :

1. To carry out a modal calculation according to the rigid walls (thus without taking into account the plasterboard)
2. To carry out measurements

Bass traps, if they are made by conscientious professionals, are a much more efficient solution than a traditional bass trap. In fact, they will fit into the building, respond to a problem identified in the room and in consideration of a placement and a type of speakers.

NO, they do not have to be opposed, certainly they are less powerful, more cumbersome, less adapted… but you can also add some, remove some; which is practical if you change system regularly or if you wish a modularity in the listening.

This raises the question of the interest of ready-made bass traps. Are they good to throw away?

The helmholtz resonator with perforated panel

The criteria to look at

The following criteria should be considered to ensure the quality of a bass trap:

1. The density
2. The size
3. The tortuosity and quantity of cavities
4. The flexibility, viscosity, porosity and permeability of the fibers
5. The compressibility of the material and its structure
6. The resistivity to the passage of air (notably expressed by the AFR: Air Flow Resistivity)

The most important factor is the air flow resistivity. It is calculated by the following formula: PA.s/m². The most important thing to remember is that a material with a lower AFR will be more effective in absorbing low frequencies. Thus, an AFR 5 is more efficient than an AFR 7.

Making and building a bass trap: why call in a professional?

As explained before, each piece is unique. It is therefore important to understand it well before considering a treatment. Just like a doctor, it is advisable to make a diagnosis before recommending or not, medication. It is the same thing for acoustics, especially when it comes to the low frequency range where the materials in the room will interact because of the vibrations.

The real problems are that it requires expertise and equipment:

• Measurement equipment
• Knowledge to understand the measurement, for the calculation of bass traps and for the realization of the bass traps which requires an even higher technicality than the measurement

To pass by an acoustician thus makes it possible to save time and to ensure the final result.

The realization of a custom bass trap costs between 2x and 10x more expensive than a standard bass trap.

What is a cheap bass trap?

As with many products, you need to look at the durability of the product. As mentioned, acoustic foams are not known to be very durable. Similarly, you should look at the performance versus the price.

At PYT Audio, we can assist you in the creation of your specific bass traps with an acoustic study and measurements.

Do not hesitate to contact us so that we can give you a quote.

However the starting price is often around 2000$.

This will include the service of the acoustician (modal calculation) and the manufacture of a full height bass trap.