It is well documented that the most difficult frequencies to control are in the deep low bass region. This is due to the long wavelengths that require a lot of energy to produce, and then tremendous effort to control. Once a room is overly excited with bass energy, room modes develop whereby sound bouncing off the walls will react with the direct sound from the loudspeaker and either cancel each other out – if out of phase – creating a lull or combine together – if in phase – to create a peak. These cause frequency bumps (modes) in the room that cause the sound to change depending on where you are positioned.
The solution is to integrate a bass trap into the room. Because low frequencies below 300Hz are considered to be omnidirectional, a bass trap can be positioned anywhere in the room to have effect. However, because sound travels within the confines of the walls, ceiling and floor, it tends to migrate into corners. This makes the corner the most efficient location for capturing sound energy.
There are several types of bass traps including Helmholtz resonators, diaphragmatic resonators, large masses of foam, and rigid panels with air cavities. They all work… albeit some are more effective than others. In all cases, their task is to reduce bass energy so that their effect on the direct sound is minimized and room modes are lessened.
These are very effective. They employ an air cavity that is tuned to a specific frequency – like blowing into a Coke bottle. For the ultra-ambitious, one could measure the room modes and then build a bunch of these to pull out the problem frequencies. Helmholtz resonators are rarely used due to the complexity and high cost.
These generally come in two forms – hard and limp. Hard resonators work like the Helmholtz device and are made to target very specific frequencies. Limp mass resonators like the Primacoustic MaxTraps vibrate as they automatically center where the room energy is greatest. These are very effective, but more expensive than foam or air cavity type bass traps.
Foam Bass Traps
These absorb bass by allowing bass energy to penetrate the foam and cayuse the soft membranes to vibrate. Using what is known as the ¼ wavelength calculation, one can estimate the low frequency cut-off. As a rule, the deeper the foam, the lower the bass they will absorb. A 12″ x 36″ (30.5cm x 91.4cm) foam trap will absorb energy down to about 125Hz – although this will vary significantly depending on the density of the foam and whether large areas have been cut out to create an anechoic wedge appearance. The higher the density, the better the bass absorption. Foam is losing popularity due to dusting, health concerns and lack of fire ratings.
Panel With Air Cavity
Primacoustic London Bass Traps employ a glass wool that is as much as 12 times more dense than the typical foam used in bass traps. The higher density creates a barrier that not only traps energy as it enters, but also does the same when the energy reflects into the air cavity behind and tries to escape. The 17″ air cavity dramatically intensifies the low frequency absorption point down to 100Hz when using the same ¼ wave calculations.
The other benefit of using large panels like the London Bass Traps is that they not only absorb bass, but absorb energy at all frequencies above the lowest point. This means that they also help tame flutter echo, standing waves and primary and or secondary reflections.