The Science: How The Fulltrap Was Developed

Graph C shows the result of introducing the
FullTrap. Modal distortion is reduced.

Standing Waves

The physical size of the room imposes limits where frequencies will resonate as they echo off the walls and ceilings. These are commonly known as standing waves. A typical room that is 14 feet long, 12 feet wide and has an 8 foot ceiling (4.2m x 3.6m x 2.4m) will resonate at 80Hz, 94Hz and 141Hz respectively as sound echoes between the parallel surfaces.

12ft. x 14ft. x 8ft. room and its resonant frequencies.

As detailed above, these low frequencies will either amplify each other or cancel each other out depending on where you are sitting within the room. This is precisely why studio owners often complain that the mix may sound great at the listening position, but when they move to the back of the room, the bass can either be thin (lacking) or is so intense; it completely messes up the mix. This one of the main reasons why bass trapping is so important in small rooms.

You can predict the fundamental room modes in your studio by simply dividing the speed of sound (1130/ft. per sec.) by the room dimensions as shown. For instance, in a room that is 12 feet wide by 14 feet long and 8 feet high we can predict modal frequencies at 94Hz, 80Hz and 141Hz.

The Power In Bass

Let's take this one step further by understanding the 'power' that bass unleashes versus high frequencies. You only need to stand outside a night club to realize how much more energy is contained in bass versus high frequencies. If you listen, chances are you will only hear the bass frequencies of the music as it travels through the clubs concrete walls to the outside. To illustrate, try stopping a bicycle. Now try stopping a train. Once a locomotive starts moving it is almost impossible to stop quickly. The inertia of a slow moving train is monumental compared to that of a bicycle.

In figure-A below, the 'energy' of a 50Hz sine wave is illustrated in the red 'area'. Figure-B shows 200Hz at the same amplitude. Notice the red area is so much smaller? Figure-C takes this a step further by illustrating the comparative energy at 800Hz. Bass contains enormous amounts of energy compared to high frequencies.

This is why the bass speakers in a PA system are usually supplied with as much as 10 to 20 times more amplifier power than the high frequency horns. And once that bass gets moving, it is practically impossible to stop. In fact, controlling low frequencies is probably the most difficult and challenging aspect of acoustic sound control because of the disportional amount of energy bass contains.

Absorptive Panels… What they can and cannot do

One can 'predict' the performance of an absorptive panel by using the quarter-wavelength calculation. This will provide a relatively accurate cut-off point to where the panel will provide 100% absorption. (Remember, high frequencies have very little energy compared to lows. So they are easy to absorb and control using absorptive acoustic panels. The thickness and density of the acoustic panel will determine the actual performance. Controlling bass is where the real challenge lies.

The math is easy: divide the speed of sound (1130 feet/second) by the frequency to calculate the wavelength. Now divide the wavelength by four for the quarter-wave and then again by two to compensate for the angle of incidence. So if you want to absorb 100Hz, you would take 1130/100=11.3 feet wavelength. Convert feet to inches by multiplying 11.3 x 12 = 135". Divide by 4 and then by 2. You will need about 17" of bass trapping material to absorb 100Hz.

This is in fact how products like the Primacoustic Australis and the corner traps used in the London 12A room kit are measured. They will do a good job down in this low region. But where they fall short is in the deep low bass, below 100Hz. If you were to try to absorb 50Hz using the same math, you would need a bass trap that would be 34" deep!

In fact, this is exactly how anechoic chambers are calculated. They often have huge sound trapping wedges that will extend anywhere from 4 feet to 6 feet in depth (1.8m) depending on the size of the room and lowest frequency that they must absorb. This of course is clearly impractical for small room acoustics. Another solution is required.

The MaxTrap & FullTrap - A real solution for bass

Both the FullTrap and the MaxTrap were conceived from the ground up to control troublesome bass, while providing effective absorption throughout the audio listening range. In fact you can think of these devices like a 3-way speaker working in reverse. With a speaker; the woofer produces bass, the mid range driver delivers the mids and the tweeter delivers the high-end. The Primacoustic MaxTrap and Full trap follow a similar approach whereby it combines three different sound absorbing techniques in a single device to provide full bandwidth control.

Both the MaxTrap and Full Trap employ a 3" thick 24" x 48" face panel that is made of 6lb per-cubic-foot high-density fiberglass. This is the same material used to control the acoustics in professional recording and broadcast studios around the world. This panel has been laboratory tested and will absorb frequencies starting from 125Hz to well above the threshold of human hearing. The front face panels works in conjunction with other absorptive panels in the room to control flutter echo, room chatter, resonance and helps eliminate strong primary and secondary reflections.

To increase the low frequency performance, a deep air cavity behind the face panel extends back to the corner or wall to absorb low frequencies below 100Hz. By employing quarter-wavelength calculations, one can predict 100% absorption to 100Hz. The third piece is an internal diaphragmatic resonator. This magical device does the 'heavy lifting' to help reduce low frequency modes and control the standing waves that tend to clutter the sound field. This is how it works…

The Diaphragmatic Resonator

Photo of the diaphragmatic resonator inside the MaxTrap.

To understand how a resonator or membrane works, one simply needs to look at a microphone. Sound waves cause the microphone diaphragm to vibrate which in turn causes an electric impulse. If a given frequency is louder, for instance the low bass from a kick drum, the microphone will resonate at that frequency and deliver bass to the audio system. The microphone is not selective. It is reactive. It simply picks up whatever is in the room and renders it to the best of its ability.

A diaphragmatic resonator works very much the same way. The one inside the Max Trap and Full Trap is basically a huge microphone diaphragm that is suspended so that it can float or vibrate. But unlike a microphone that is made from a very light material so that it is sensitive to all frequencies, the heavy barium-loaded vinyl inside is so heavy it will only vibrate at low frequencies where there is sufficient energy in the wave to cause it to move. Herein lays the magic: The diaphragm will sympathetically vibrate at the loudest frequencies in the room and remove them by thermo-dynamic transfer… or in layman's terms, by converting sound energy into heat as the membrane vibrates.

How efficient is it? To put it bluntly, it completely blew away the techs at Riverbank Labs. It was literally off the chart. They had never encountered such an amazing device. (A word of advice: If a company claims the performance of their bass traps is better than quarter-wavelength calculations, ask them for a lab test from an independent facility. If they claim to do the tests themselves, be afraid. Bass testing is extremely difficult and unless subjected to real scientific scrutiny, the tests will likely be unsubstantiated.)

To fully understand how the MaxTrap and FullTrap work, we need to first appreciate the differences between bass trapping options they employ.

The absorption of typical 3" polyurethane acoustic foam vs. 3" Broadway fiberglass panels.

Acoustic Panels

As described earlier, acoustic panels will absorb bass but their performance is tied directly to the thickness and density of the absorption material that is used to build the panel. If the density it too high, then high frequencies will reflect. If the density is too low (like foam) then bass absorption will be minimal. Because the performance is tied to the thickness, they are limited in how low they will work. For the most part, even with an air cavity behind, they will typically only absorb 50% of the energy down to around 75Hz.

Curve comparing a hard-membrain resonator to the MaxTrap's diaphagmatic membrain.

Hard Membranes

These are typically wood panels that are suspended with some form of spring which is connected to an absorbent material. Because the panel is rigid, it will have a very specific resonant frequency based on the size, thickness and density. Hard membranes are narrow band by nature. This means that they will vibrate at some frequencies and effectively absorb energy in this narrow band but do nothing at other frequencies. So unless your room has a problem at that specific frequency, they will not provide an effective solution.

Curve comparing a Helmholtz resonator tuned at 125Hz to the Maxtrap.

Helmholtz Resonator

A Helmholtz resonator is best described as a big bottle. And just as you can create a sound by blowing across the Coke bottle like a flute, a Helmholtz resonator can be designed to suck out a certain frequency by building one at a very specific frequency. Helmholtz resonators have same problem as hard membranes: they are narrow band and only work so long as you identify the problem and then create a very specific solution. But since every room has slightly different dimensions, a 'one size fits all' solution is ineffective.

Curve comparing the MaxTrap to Real Traps.

Diaphragmatic Resonator

The membrane or diaphragmatic resonator used inside the MaxTrap and FullTrap is different. It is a limp mass. In other words, it does not have a specific resonant frequency - but will vibrate based on the loudest or most powerful energy in the room. So no matter what the resonant frequency, it will self adjust to the problem and suck it out. The problem here is that the design is more complex and the heavy loaded barium impregnated vinyl is expensive. So these cost can be slightly higher. Ah the price of perfection!

A complete solution!

By combining the MaxTrap or FullTrap with standard Broadway panels, you will notice that bass will immediately tighten up, chatter echo will be reduced and these will help eliminate troublesome standing waves that cause nulls and lobes. With less interference, the listening sweet spot will expand making your room more comfortable to work in. With a properly treated studio, your mixes will become more accurate and translate better as you listen to the results in different rooms, in your car and on different sound systems.