When Calculating Soundproofing, for our example we can see that the control room within the residential property requires at least 20db of soundproofing. However when calculating the amount of soundproofing you will need for your recording studio design, you cannot use a linear scale to simply calculate the amount of soundproofing required. Calculating Soundproofing must be done with the logarithmic scale, basically a sound source that is 10db louder than another source is in actually TWICE as loud. Similarly a sound source that is 15db louder is in fact THREE times as loud.
So using the above, you will see that you cannot simply add 20db of soundproofing to the walls of your control room if the current walls are providing 45db of soundproofing. This is where you use the sound reduction index (SRI) and the clever use of air gaps come into play and also as important where your studios size is reduced to accommodate these air gaps.
When calculating Soundproofing, the sound reduction index is a guide that states that a certain mass per meter squared will reduce the sound passing through it within a certain frequency range (100z to 3khz). For example a single sheet of plasterboard has a mass per meter squared of around 7kg which will give it a sound reduction level of 21db, however two sheets of plasterboard with a total mass of 14kg only have a reduction level of 28db which is an increase of 7db even though the mass has doubled. I won’t go into to much detail here, but as the mass increases the gap between the level of reduction drastically decreased. For example, the difference between between 30kgm2 and 60kgm2 is only 3db. This is where air gaps become extremely useful.
I would at this point like to point out that the use of different materials with different densities is also important, like using MDF mixed with plasterboard. The reason this is important is that the different properties within the materials effect different frequencies.
Air gaps are basically free insulation if used correctly, however these are at the same time big wastes of space. So it’s a fine balance between soundproofing and usable space especially when space is a premium in built up city locations. Air gaps are basically physical gaps between two isolated structures (between peaces of glass or between walls or doors). The more effective the air gap, the more effective the sound reduction. However in order to create an effective air gap, the two structures need to be decoupled from each other physically to prevent the sound from transferring. For example, if you build a room within a room type soundproof structure but fix both the inner and outer walls to the same floor, the sound frequencies with the most energy (mid and low frequencies) can travel from the inner room walls to the floor and back up the outer room walls effectively reducing the efficiency of the isolation between the two walls even though you have an air gap between the two walls. However if you build the inner room on a floating floor, the inner room becomes decoupled from the floor that the outer walls are connected to, reducing the amount of frequencies that can then get through the outer wall. In general the smallest air gap you should consider between walls is around 100mm, basically this is because more noticeable frequencies are able to jump across gaps smaller than 100mm. If you want to find out why, check out some calculations that shows you the actual length of a given frequency in millimetres.