Project Studios / Full Studios
I have been in many project studios and full studios lately. Some project studios have one room that does everything. It does vocals, guitars, and even drums on some occasions. Larger studios have separate rooms for drums and vocals. None of the rooms I have been in have had any low frequency absorption technology. None of the engineers or owners like the low end of their rooms. It does not surprise me.
Bass Absorption – 101
I am amazed today that we have dropped the ball when it comes to educating ourselves about low frequency absorption. Most of the individuals I speak with, still believe that foam or rock wool will stop bass absorption. They build or purchase panels filled with rock wool which is a building insulation material. It is designed to have a thermal value for heat retention and also for a barrier to heat. It was never designed for acoustical treatment use, especially in the area of low frequency absorption.
Rock wool or mineral fiber panels have a characteristic sound to them. They absorb a wide range of middle and high frequencies and do it so well. They do not go below 100 cycles and if they do, they don’t absorb very much if anything at that claimed frequency. They drain the life out of your room by absorbing too much energy at middle and high frequencies without absorbing any bass energy at all.
Foam Will Not Go Low Enough
Fibrous materials and acoustic foam will not absorb enough energy at the correct frequencies to assist us in any form with low frequency absorption. Acoustic foam would have to be 11′ thick to absorb 100 cycles completely. Rock wool or mineral fiber like materials will need to be similar thicknesses. It just can not physically happen with foam or other materials when it comes to low frequency absorption.
Low Frequency Pressure
I am also amazed that individuals do not relate low frequency issues in their room as a sound pressure issue formed from large waves and not a ray issue that concerns middle and high frequencies. We are dealing with waves that can be 40 – 50′ long and full of energy. Picture yourself as that 40 cycle wave that is 30′ long and now put yourself in a room less than 15′ in any direction. You would not be happy either and you would voice your disapproval through resonance creation.
Room size is a critical factor in dealing with low frequency resonances. The volume of the room determines how the room will react to sound pressure inserted into it. Rooms with volumes less than 1000 cu.ft. which would be a room that measures 10’x 10′ x 10′ are not large enough to handle middle and high frequency issues, let alone any bass energy build up. Rooms that have volumes exceeding 4,500 cu.ft. are a good starting point to work from when addressing low frequency issues. Run the resonance numbers for the room size you are considering. They may tell you to make the room even smaller.
Speaker / Room Interaction
I also see a lot of large drivers for low frequency “reproduction” used in small rooms. The size or diameter of the driver must be matched to the room size. I was in a studio that had 4 – 18″ sub woofers in one room. Granted the room was the correct size for one 18″ driver and great care had gone into the construction and size selection of the room. However, 4 – 18″ drivers produced enough energy to reach acoustic pain thresholds. I guess some people like a dose of sonic pain with their music. Do not put large drivers in small rooms if your goal is clean and accurate sound reproduction in the low end or any end for that matter.
One must use two different types of low frequency absorption: diaphragmatic absorption and resonating absorbers such as a Helmholtz resonator to deal with the pressure created within our rooms by low frequency energy. A diaphragmatic absorber is a sealed box that is constructed in a certain manner. It has a front wall that vibrates in response to sound energy. This moving front wall slows the low frequency energy down, so that it now can enter the cabinet inside where more sound absorbing material exists. The sealed unit has a depth and mass to it that is used to calculate the resonant frequency of the unit. Frequencies above the resonant frequency are absorbed and frequencies below the resonant frequency are not.
A Helmholtz resonator is another technology that can be used to go after low frequency energy. An example that is commonly used is that of a coke bottle. If one blows across the top of a bottle, the tone produced is the natural frequency of resonance for that bottle size. The length, width, and circumference of the bottle all contribute to its resonant frequency. If one changes the volume of the air cavity, length or diameter of the neck, the resonant frequency is changed. Sound is absorbed at the frequency of resonance and also at nearby frequencies.
Absorber Placement Positions
Proper placement within the room of any chosen method of low frequency absorption, must be considered. Low frequency pressure build ups occur in the corners of our room where all room modes end at. Low frequency pressure areas are also found at the intersections of our walls to floor and our walls to ceiling areas. There is also a high pressure area that builds up behind our speakers between the front wall and the speakers or monitors. Other areas within the room can be sources of low frequency pressure build up but addressing the aforementioned locations with the proper type and amount of absorption can minimize these. One can even build low frequency absorption technology into the existing walls.
Low frequency pressure issues must be addressed in any studio scenario. Small room create big low frequency issues. One must choose the size of the room carefully to minimize these issues from the beginning. Maybe, we need to make the room smaller to avoid resonances at certain frequencies. Choose the proper low frequency absorption technology to deal with your low frequency issues. Make sure you place the low frequency absorption technology in the high pressure areas such as corners and room boundary intersections. Don’t forget to treat the area behind the speakers and front wall.