Sub Woofer Cabinets
Our sub woofers are cabinets that contain a lot of energy for their respective size. They also produce a lot of energy for their size. It is like putting a large horsepower engine in a small car. The power to weight ratio is a large number. I used to have a cobra kit car with a 427 cu.in engine that produced 500 horse power in a 2,300 pound car. Lets see, that’s about 5 horse power per pound of car. That is about the same power to weight ratio as a sub woofer.
Same Old Drivers
Sub woofer cabinets have gotten smaller but driver size has not. We still see 12″, 15″, and 18″ diameter drivers in our sub woofer cabinets. Back 10 years ago, if we had a sealed box for a sub woofer and an 18″ driver inside, we had a box that was huge. I remember the Hi-Fi, Velodyne, 18″, sub woofer cabinets that were 40″ long and at least 30″ tall. They had that electronic servo monitor system that provided vibrational energy data to the amplifier when the driver was at its performance limits to avoid driver meltdown. Cabinet construction was a single layer of material.
Today, Smaller Cabinets
Today, we see large drivers in much smaller cabinets. Technology has allowed sub woofer designers to shrink the cabinet size and still realize low frequency output in the 20 to 30 cycle range. That smaller cabinet, however, does come with more vibrational issues. If we make the cabinet smaller, we have more pressure inside our cabinet. More pressure means more cabinet vibrational energy that the enclosure must deal with. Smaller is OK, if it is stronger.
Past Cabinet Materials
Today’s cabinet materials are different from cabinet construction materials of the past. In the past,to reduce cabinet vibrations, we had to use mass and a process known as constrained layer, mass damping. Multiple layers of materials with different densities were sandwiched together. Mass was the key ingredient in the formula. There was no substitute for cubic inches.
Layers, layers, Layers
Each layer of material was separated from the other by a substance known as a viseo-elastic damping compound. These multiple layers of materials with the damping compound reduced the amplitude of cabinet born vibrations. The ultimate goal was to reduce cabinet resonances, so they were below the audible sound of the driver in the cabinet.
Today, we have new materials to choose from. We have composites that are made of new polymers that have more strength per square inch than our multiple layer approach of the past. These compounds can be formed into different shapes that are more conducive to speaker internal pressure reduction by having the ability to be splayed or canted to minimize parallel surface resonances inside the cabinet. With the same size drivers as we had in the past, mated with smaller cabinets, something must give and cabinet construction materials have been selected.
With a sub woofer, we have cabinet movement even with the new cabinet compounds. A cabinet if moving enough and that movement gets laced with the driver sound, we have now to contend with room sound. The driver moves back and forth to produce a wave of energy. This process produces vibrations that are transmitted through the cabinet. Together, the driver wave output and the cabinet “sound” produce the sound energy that will be sent into our rooms. Sub woofer, sound energy is radiated from all sides, top, and bottom of our cabinet in a 360 degree array.
Room Is Speaker Cabinet
Our room is another cabinet with the same issues that a sub woofer cabinet has. Our room must deal with sound pressure injected into it from our sub woofer. The room then acts its size not its age. Room dimensions determine the room resonances we will have inside our room “cabinet”. The room resonances and the sub woofer produced energy, all work together to produce the final sound we hear in the room. To improve our sub woofer cabinet and driver sound, we need to reduce both vibrational energy from the cabinet and also absorb the low frequency energy produced by the driver.
To accomplish both of these tasks, we turn to an old technology and blend it with a new twist. Diaphragmatic absorption is a time tested and proven method for low frequency absorption. If is combined with vibration draining cabinet fill, one can achieve both cabinet vibrational level reductions along with sound pressure absorption.
Build Diaphragmatic Absorber
If we calculate the proper cabinet depth and make sure we have the required cabinet material densities in place, we can calculate the approximate cabinet resonant frequency. Frequencies above that frequency will be absorbed and frequencies below the resonant frequency will not. If we add activated carbon or charcoal to our diaphragmatic absorber cabinet, we lower the cabinet’s Q value and achieve both a sound pressure absorber and as a platform for our sub woofer to sit upon.
Close To The Source
Since our sub woofer cabinet produces energy through all its walls, top, and bottom, we can place our diaphragmatic absorber under the sub woofer and gather a lot of this energy. This procedure works very well if the sub woofer is down firing. Since our diaphragmatic absorber works on sound pressure, we now have it positioned as close to the sound pressure producing source.
Charcoal / Activated Carbon
The activated carbon cabinet fill is also a vibrational reducing material. Activated carbon is a fancy name for charcoal and each piece of carbon or charcoal has numerous pores that assist us in a process to minimize and absorb vibrations. The activated carbon now reduces vibrations and each carbon granule has numerous holes or pores that create a high pore to material ratio thus increasing overall absorption levels and rates with the diaphragmatic absorber.
If we build a diaphragmatic absorber that has the correct material cabinet density, the proper cabinet depth (about 12″) to achieve our targeted overall cabinet resonant frequency and then place the appropriate amount of charcoal within our diaphragmatic absorber cabinet, we have the correct formula for both sound energy absorption and vibrational control. In order to achieve the best results, we must place our sub woofer cabinet on our 12″ high diaphragmatic absorber. Elevating the sub woofer off the floor will also improve our overall room response.
Clean And Tight Bass
With all of this in place, we will have a tight, clean, and more dynamic sounding sub woofer. Attack and decay of each bass note is more clearly defined. With an activated carbon absorber close to the speaker, we have an absorber that is pressure reactive right next to the highest pressure source. Our just built absorber is functioning at its maximum ability.
Less Vibrations / More Sound
With the vibrational controlling properties of the activated carbon inside our diaphragmatic absorber, we have less cabinet vibrations to compete with the wanted sound from the sub woofer driver. Less cabinet vibrations produces less energy that the drive unit has to compete with. Less sonic competition amongst the components produces a tighter, cleaner bass response.