In order to deal with recording studio sound isolation one should understand how sound travels. When considering sound isolation there are really two aspects. Keeping sound out of the listening environment, and retaining sound within the listening environment.

You would think that if you had one you would necessarily have the other, but that's not always the case. First consider the level of noise and the source of noise that might get into the listening environment. Is there a busy road outside? Is there a hard surface floor above the room where people walking will be heard?

There are four essential factors that determine sound isolation: distance, mass, rigidity, and damping.

Our goal is not to cause noise to our neighbors and to prevent any noise from outside to enter our recording studio. If we can get far enough away from noise sources and our neighbors, then we will have solved our sound isolation problems. The distance from noise sources has to be taken into account when we are choosing location of our studio. When we have the location of the studio fixed, we can only invest in other measures for sound isolation.

The characteristics of mass, rigidity and damping are a little more complex. One of the basic principles is that to move a large mass you need more energy than for a small one. And because of inertia a large mass will reflect back more energy if subjected to a sound wave. A greater tendency to impede the path of the wave means greater acoustic impedance. If the mass is resonating, its surfaces will be in movement and will act as a source of acoustic energy. Every mass has its own resonant frequency. Resonance means absorbing and re-radiating energy. The outer wall surface would selectively re-radiate the sound which is striking the inner surface. Isolation is therefore dependent on the degree of the freedom of the wall to resonate.

If the mass is rigid then it can not move and consequently can not resonate. If a sealed studio were made from an infinitely rigid then would be sound proof because it could not vibrate. Unfortunately, infinitely rigid materials do not exist. The solution for high degree of recording studio sound isolation over short distances is by the use of highly rigid, massive structures.

Damping can have a great influence on the ability of any element to provide sound isolation. Damping is the degree to which a propagating wave within a material or structure is internally absorbed. Normally, absorption converts the vibrational energy into heat. The damping of a material or structure can also be achieved to some degree by the addition of a damping material to its surface. An acoustically highly damped (lossy), massive structure can in many cases, for the same degree of isolation, be less rigid, because the passage of the vibrational waves through the structure is severely attenuated before the waves can re-radiate from another surface.

Limpness, to some degree, achieves the same end as rigidity - the inability of the structure or material to vibrate sympathetically - but unsupported limp materials are incapable of forming a structure - they have inertia, but no stiffness. This leads us to partitions that are essentially controlled by the mass law, which roughly states that when the inertia of a panel, rather than its stiffness, is the dominant principle for sound transmission loss, that loss increases by 6 dB for each doubling of the mass per unit area, and by 6 dB for each doubling of frequency. At least, that is, for plane waves at a given angle, this is why the mass law is only an approximation to normal circumstances.

In practice, the means by which isolation is usually achieved is by mechanically decoupling the inner structure of a room from the main structure of the building. Isolation by pure absorption is very unwieldy - rooms made from 10 m thick mineral wool walls and ceilings are not an option. As no lightweight super-rigid structures are readily available, then neither is rigidity alone a practicable solution. To some degree, mass is used, but if it were to be used alone, then it too can become unrealistic in its application. For example, let us presume that a concrete block wall of 20 cm thickness and 40 dB isolation were to be augmented, by mass alone, to achieve 60 dB of isolation at low frequencies. The mass law would add at the most around 6 dB of isolation for each doubling of the mass per unit area, though the increasing rigidity of the more massive structure could tend to raise the isolation to above the 6 dB mark. Under ideal circumstances, which may not be realized in practice, doubling the thickness to 40 cm would yield 46 dB. Doubling this to 80 cm would result in 52 dB, and doubling this yet again to 1 m 60 cm would still only provide 58 dB of isolation. We would therefore require walls of around 2 m thickness to achieve 60 dB of low frequency isolation if we were to rely on increasing the mass alone, and even this may be compromised by internal resonances within the structure.

The practical answer to the isolation problem lies in decoupling the inner and outer structures. This can be achieved by many means, such as by steel springs, rubber or neoprene blocks, fibrous mats, polyurethane foams and other means. There have even been cases of rooms floated on shredded car tires, and even tennis balls, but the problem with tennis balls is that the air will gradually leak out over time. Re-inflatable air bags use the same principle, and this is another practical solution sometimes used.

If the first mass is set in motion, the force exerted on the spring will be resisted by the inertia of the second mass, and above the resonant frequency of the system the spring will be heated by the vibrational energy. This converts the acoustic energy into thermal energy. Such isolation systems work down to about 1.4 times the resonant frequency of the system, below which the decoupling ceases to become effective. The resonant frequency is a function of the mass that is sprung and stiffness of the springs. Increasing the mass and decreasing the spring stiffness both tend to reduce the resonant frequency. The effect of this decoupling of the masses is to render the two systems (floated and structural) acoustically independent.

Therefore, a structure of 20 cm sand-filled concrete blocks, with 40 dB of low frequency isolation, would only need an internal floated structure with 20 dB of isolation in order to achieve 60 dB of total isolation. This may be achieved by internal floated walls of 10 cm thickness, such as of sand-filled, concrete blocks, spring isolated from the floor, with a 5 cm mineral-wool lined air space between the two walls. Such a system would achieve in a total thickness of 35 cm what could only be achieved by two meters thickness of blocks that were mechanically connected.

This works because the internal wall attenuates the sound from inside the room by 20 dB, which is the resultant level on the outside of the inner wall. The air between the walls acts as a spring and it is not capable of efficiently pushing or pulling the much greater mass of the inert outer wall. If the walls were in contact, the masses would be reasonably comparable, so the vibration in the inner layers would have relatively little trouble progressing through the structure. In the isolated wall system, it is only the sound pressure which has already been attenuated by 20 dB which impinges on the outer wall, which then attenuates the sound by another 40 dB before reaching the outside of the building, thus achieving the 60 dB of isolation.

Generally, noise is isolated by using heavy materials to block the noise along with decoupling devices to reduce structural vibrations. Sound absorbing materials may also be used within wall cavities to reduce unwanted structural resonance. The quality of soundproofing is determined by the correct choice of materials and their combined physical characteristics. Regardless of whether you are building an isolation booth in a professional recording studio, or trying to prevent sound from leaking into or out of your home studio, sound isolation is key.