Bass traps: a complete guide

Modes explained

A common problem is the presence of standing waves. These low frequency problems cause modes which result in an uneven frequency response. Due to these modes, the listener hears certain frequencies booming in the room when others are absent. This makes mixing frustrating and become even harder, and the music won’t sound as it should while rehearsing.

Standing wavelength equals room length (the wavelength of modes is equal or a multiplication of the path length it takes for the modes to return to its origin). When sound reflects of a wall, it returns in antiphase of the original wave. Therefore the reflected wave and the original wave are added up and create nodes and antinodes. Depending on where you stand in the room it seems this frequency is not present (antinode) in the room or is really loud (node).

Standing waves which move in one direction are called axial modes. They only go back and forth between two walls. When a wave moves in two dimensions, it reflects of four walls and is called a tangential mode. When they occur in three dimensions, they’re called oblique modes.

To minimize modal problems, before acoustic treatment, one can pick a good listening spot. Remember the guide to create a good sweet spot? Place your desk at 38% of the room to avoid most of the axial modes on the longitudinal axis of the room.

Luckily these modal problems only occur in the low end of the frequency spectrum. At higher frequencies the modes in the room are dense and therefore start to even each other out. The starting frequency of this transition area is also known as the Schroeder frequency. However this still leaves us with the challenging task to flatten the low-end in the room.
 

One way of tackling this problem is by absorbing the reflected waves. Most of the time foam absorbers, also known as velocity absorbers, are installed in rooms. These absorbers transfer sound energy into frictional heat. To transfer this energy with maximum efficiency, the speed of sound must be at full velocity, which is at ¼ of the wavelength. So to successfully absorb down to 40Hz, the absorber has to be more than ± 2 meters thick, which means a tremendous loss of space.

A more effective way of handling this problem is to install pressure absorbers such as membrane or Helmholtz systems, commonly known as bass traps. With a membrane absorber, the membrane resonates at certain frequencies and reduces the energy of the reflected waves. These specific absorbers target smaller regions in the frequency spectrum so additional velocity absorbers are required to reach the desirable reverb response for every octave band.

As pressure is at its peak near walls, this solution requires less volume to absorb the lower frequencies. However bass traps are location sensitive since maximal and minimal pressure zones at the low end are located further away from each other, so location is key. A good starting point is by placing the bass traps in the corners as pressure builds up where walls come together. This causes an efficiency boost of the bass traps. Fewer elements are required to get a good result.

Bass traps are active in the two lowest octave bands. To be effective and cover the whole low-end, Artnovion subdivided the low end in 3 regions. The 125Hz octave band are treated by wall bass traps. The lowest octave band absorbers are divided in two modules, the corner trap (60Hz to 80Hz) and the sub trap (40 to 60Hz). By subdividing the lowest octave band in two modules, standing waves are reduced with maximum efficiency.

In conclusion bass traps have a double purpose in rooms, flatting the frequency response and reducing reverb time.

Artnovion developed an innovative technology, which they later patented. Their bass traps use the TPDA (tuneable pistonic diaphragmatic absorber) technology, allowing the panels to be tuned for better modal control. Membrane absorbers (the TPDA bass trap), when excited, resonate on its natural frequency, also known as eigenfrequency. Due to the vibration, they start absorbing the incident sound and therefore shorten reverb time and attenuate the peaks and throughs of the standing waves.

The eigenfrequency of the membrane depends on a couple of characteristics, one of them being the mass. Our Portuguese supplier cleverly used this characteristic to make bass traps tuneable. They mounted metal weights on the front of the panel which could be added or removed and thus could change the mass of the membrane.

In the past, changing the weight of the whole membrane absorber would also change other characteristics, making them harder to tune. Due to Artnovion’s separately mounted weights this is in the past. They integrated a porous velocity absorber (VABT at the back of the panel) to improve performance even more.

The Sub trap uses the same principle as the corner trap does, but besides absorbing even lower frequencies, this model is equipped with 3 separate TPDA absorbers to enhance efficiency. The wall trap has one non-tuneable membrane with the VABT absorber on top.