How to apply STC rating of the material when soundproofing a DIY Recording Studio

The Sound Transmission Class (STC) is a single-number rating of a material’s or an assembly’s ability to resist airborne sound transfer at the frequencies 125-4000 Hz. In general, a barrier with higher STC rating blocks more noise from transmitting through a partition. The STC ratings allow accurate ‘apple to apple’ comparison of materials for soundproofing.

First of all knowing STC rating of a barrier helps to measure your expectation of what can be achieved when using various types of soundproofing materials.

The STC rating is based on performance with frequencies from 125 to 4000 Hertz (the speech frequencies). The rating provides no evaluation of the barrier’s ability to block low frequency noise, such as the bass in music or the noise of some mechanical equipment.

STC What can be heard

25 Normal speech can be understood quite easily and distinctly through wall

30 Loud speech can be understood fairly well, normal speech heard but not understood

35 Loud speech audible but not intelligible

40 Onset of “privacy”

42 Loud speech audible as a murmur

45 Loud speech not audible; 90% of statistical population not annoyed

50 Very loud sounds such as musical instruments or a stereo can be faintly heard; 99% of population not annoyed.

60+ Superior soundproofing; most sounds inaudible

Why the STC Rating Numbers are so close in Value? STC rating measurement uses a special scale like the Richter Scale for earthquakes. An increase from 30 to 40 means 90% of the noise is reduced. A change from 30 to 45 represents a noise reduction of over 95%. Small increases can mean a lot of extra noise reduction. The increase from 40 to 45 does not sound like much (90% versus 95+% in reductions) – but going from 90% to over 95% means that 60% of the remaining noise was removed; every number represents a large amount of noise.

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STC rating of some building materials:

STC Partition type

33 Single layer of 1/2″ drywall on each side, wood studs, no insulation (typical interior wall)

39 Single layer of 1/2″ drywall on each side, wood studs, fiberglass insulation

44 4″ Hollow CMU (Concrete Masonry Unit)

45 Double layer of 1/2″ drywall on each side, wood studs, batt insulation in wall

46 Single layer of 1/2″ drywall, glued to 6″ lightweight concrete block wall, painted both sides

50 10″ Hollow CMU (Concrete Masonry Unit)

54 Single layer of 1/2″ drywall, glued to 8″ dense concrete block wall, painted both sides

55 Double layer of 1/2″ drywall on each side, on staggered wood stud wall, batt insulation in wall

59 Double layer of 1/2″ drywall on each side, on wood stud wall, resilient channels on one side, batt insulation

63 Double layer of 1/2″ drywall on each side, on double wood/metal stud walls (spaced 1″ apart), double batt insulation

72 8″ concrete block wall, painted, with 1/2″ drywall on independent steel stud walls, each side, insulation in cavities

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Materials which can improve STCs in walls include mass-loaded vinyl (MLV), standard drywall, “soundproof” drywall , such as QuietRock, Supress, SoundBreak, or ComfortGuard, and damping compounds such as Green Glue.

Due to their high mass, concrete and concrete block walls have good TL values (STCs in the 40s and 50s for 4-8″ thickness) but their weight and added complexity of construction limit their use for DIY Vocal Booths.

Can STC of the walls be improved?

STC is highly dependent on the construction of the partition. A partition’s STC can be increased by:

Adding Mass
The density of the material (mass and thickness) the partition is made of is the major factor in its ability to block sound. For example, a thick concrete wall will block more sound than a thin gypsum wall. When the mass of a barrier is doubled, the isolation quality (or STC rating) increases by approximately 5 dB, which is clearly noticeable.

NOTE: Doubling the mass of a partition does not double the STC. Doubling the mass (going from two total sheets of drywall to four, for instance) typically adds 5-6 points to the STC. Breaking the vibration paths by decoupling the panels from each other will increase transmission loss much more effectively than simply adding more and more mass to a monolithic wall/floor/ceiling assembly.

Increasing or Adding Air Space
An air space within a partition can also help to increase sound isolation. This, in effect creates two independent walls. However, the STC will be much less than the sum of the STC for the individual walls. The airspace can be increased or added to an existing partition. A common way to add an airspace is with resilient channels and a layer of gypsum. An airspace of 1 ½” will improve the STC by approximately 3 dB. An air space of 3″ will improve the STC by approximately 6 dB. An airspace of 6″ will improve the STC by approximately 8 dB.

NOTE: Unless the walls are separated by a considerable distance, the cushion of air between the walls couples energy from one wall to the other, reducing the isolation. But double wall structure will invariably perform significantly better than a single-layer barrier of similar mass, even if the air gap is only a few inches wide.

Adding Absorptive Material in the Partition
Sound absorptive material can be installed inside of a partition’s air space to further increase its STC rating. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4-6 dB, which is clearly noticeable. It is important to note that often times, specialty insulations do not perform any better than standard batt insulation.

NOTE: Insulation materials must have sound absorbing properties. Thermal insulation products not always have sound absorption qualities. Sound absorption product have to have fibrous matter that causes loss of sound energy (absorption) by vibrating and also by friction of vibrating fibers.

Changes in STC/Changes in Apparent Loudness:

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Things to remember:

ü When building Sound isolation enclosure remember that a wall must extend to the structural ceiling in order to achieve optimal isolation. Walls extending only to a dropped ceiling will result in inadequate isolation. For best results – consider building the complete sound booth with the ceiling and floor.

ü Even with a high STC rating, any penetration, air-gap, or “flanking” path can seriously degrade the isolation quality of a wall. Sound can flank over, under, or around a wall. Sound can also travel through.

ü Sound will travel through the weakest structural elements, common ductwork, plumbing or corridors, doors, windows or electrical outlets There is no reason to spend money or effort to improve the walls until all the weak points are controlled.

ü Metal studs perform better than wood studs. Staggering the studs or using dual studs can provide a substantial increase in isolation.