In order to attenuate the low frequency resonances in the room the application of acoustic panels are required. The so called "Bass Traps" panels are designed to attenuate the lower frequency resonances in the room. There are many different types available such as the active and passive. I chose the passive design that is easy to build and very effectively work in the low frequencies ranges.
The used two 2" thick rock wool slabs placed into an open frame design, which is open on the sides and works more effectively than the closed frame design. The soft wood stiffened in the corners with L-shape metal brackets. The finished panels are covered with densely woven sheet material in order to keep the rock wool particles inside the panel.
Soft wood timber 1" x 2"
L-shape metal brackets
Rockwool 2" thick
Screws for wood
The receipt of a passive bass trap is pretty simple. Get a rockwool slab and cover it with a sheet. In my case I wanted to stick two 2" rockwool slab together in order to increase the thickness of the absorption panel. A wood frame around the rockwool slabs that stick them together is a usual solution, but I didn't want to cover full of the sides of the rockwool panels. Therefore the design that I came up leaves the sides open, thus the panels able to attenuate the incoming sound waves that arrive from the sides.
As you see the picture below, the rockwool panels fit tightly into the wooden frame, thus not necessary to add any additional material that keeps the panels inside the frame. I've cut the black sheet material and laid down before I placed the rockwool slabs into the frame to avoid spilling the rockwool particles and pieces around the room. I kept the vacuum cleaner close just in case... After the rockwool was in place I fixed the top of the frame then stapled the sheet material to the frame.
The finished bass trap is light and looks great. Good enough to compete with other commercially distributed bass traps that cost 10x more. I've prepared four of these traps and placed them in the front corners in 45 degrees. As one panel height is about 125 cm, the two bass traps placed top of each other almost cover the entire corner from the top to the bottom, which is one of the most effective bass trap arrangement in a rectangular room.
After a few measurements, it became apparent that the panels are very efficiently attenuating the low frequency resonances. As we can see on the first measurement, the resonances occurring in the room quite easily separable in the graph by spotting the peaks and the long tails on the first graph.
After the bass trapping the second measurement results look much better. The graph below clearly shows that above 100 Hz the resonances well attenuated by the bass traps. Also the lower frequency resonances occur at 80 Hz and 70 Hz decreased, but not that effectively like in the higher frequencies. Also the 60 Hz resonance attenuated a bit, but these type of panels due to their physical properties are not that effective on the sub frequency ranges.
The panels worth their price and the spent working hours, because they work very efficiently. Unfortunately really difficult to attenuate the first room modes, because those modes have lots of energy and their wavelengths are very long. For example a 50 Hz wave is about 6.8 meters long. In order to attenuate this frequency efficiently we need to built 1/4 wavelength deep bass traps which would be 1,72 meters deep. However active bass traps like resonators work very well on these frequencies, but their built require more time and experimentation therefore their price higher. The further built of active panels that focus on lower frequencies such as 60 Hz would be adequate.