When it comes to flooring, the more muffled the sound, the better, especially in a multi-story building, house, apartments, or condominiums. Sound is transmitted when hard objects, like hard-soled shoes, furniture, dropped objects, or even dog claws impact a hard surface, like tile or stone. Without anything to absorb it, sound waves can spread, multiply, and echo off the walls, amplifying even a small sound to a distracting crescendo with the right acoustics. Impact sounds, such as those created by footsteps, the dropping of an object or the moving of furniture, can be a source of great annoyance in residential buildings.

Flexco/Roppe AcoustiCORK CorkPLUS 250 (commercially warranted under most resilient flooring systems) http://
www.flexcofloors.com/documents/acousticork/AcoustiCORK,%20CorkPLUS%20250,%20Quick%20Spec.pdf

Sound Ratings
Sound transmission is rated using three different standards, each defining a different way in which sound is transmitted. Sound absorption of floor coverings may be measured on two of those scales. The third covers room-to-room transmission, more appropriate for walls. The higher the number, the better the sound isolation.

IIC (impact insulation class)

IIC measures a floor/ceiling assembly’s resistance to the transmission of structure-borne or impact noise. IIC is measured in terms of sound impact, or how well sound vibration travels through a floor to the room below. Generally, an IIC rating of 50 will satisfy the building code requirements. The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies (including hotels, motels, apartments, condominiums, monasteries and convents).

NRC (noise reduction coefficient)

NRC measures the amount of noise that is absorbed by the material and not reflected. A room with a high NRC rating would eliminate background noise and echoes to help clarify speech. Theaters, for example, must be constructed to have a high NRC rating. Without it, sound waves would bounce off the walls and crash into each other, creating a cacophony of continuous sound, making individual words impossible to distinguish. Carpet, with an NRC rating of .40–.50 is the most efficient absorptive material. Vinyl, cork, and rubber have a fairly high NRC rating, with hardwood, bamboo, tile, and stone at the bottom of the scale for sound absorption.

STC (Sound Transmission Class)

STC is a laboratory measurement of the ability of a specific construction assembly to reduce airborne sounds including voice, television and alarm clocks. Basically, rates a material’s ability to block airborne sound.

The higher the value of any of the Sound Classes listed above, the greater the airborne or impact isolation provided by the assembly. With resilient flooring, the IIC rating is generally specified, and occasionally an STC rating. This is classified through ASTM E492.

I receive many calls from dealers asking for products that meet a specific IIC rating. Before giving them an answer, there are many subfloor/ceiling factors to consider that can affect the IIC rating. Many sound reducing membrane manufacturers will publish an IIC rating, but will not specify what the tested subfloor/ceiling system is, which can make quite a difference in the IIC rating.

For instance, is the subfloor;
Concrete
6 inches thick or 8 inches thick (the thicker the concrete, the better the IIC)
Is there a gypsum topping on the concrete (gypsum has very good sound deadening properties)
Is there a gypsum/suspended ceiling or not

Increasing the mass of the concrete, thickness of the gypsum ceiling board, or the amount of sound absorbing material all increase the IIC rating.

Joist Systems
Solid wood, I-Joist, Steel joist, or wood trusses
Height of the joist system
Does it have a gypsum topping on top of the subfloor
Is there insulation
Is there a gypsum/suspended ceiling or not

In such simple joist floors, the most important factor influencing the impact sound values is the total mass of the subfloor and the ceiling layers. The thicker the system, the better the IIC rating.

As you can see, the choice of subfloor/ceiling system, combined with the type of flooring structure, has a major affect on the value of the impact sound. So before asking for a product with an IIC rating, we need to know what the subfloor/ceiling system is. Different systems, different numbers.

Under the Floor
Sound transmission is affected by more than just the top layer of flooring. Other factors include the underlayment and subfloor, plus adhesives and sealants used during installation. If used on a higher floor in a multi-story structure, insulation between the floor of one room and the ceiling of the room below plays an important part in deadening sound. Any kind of insulation packed between the joists will help to trap and dissipate sound.

Fortunately today, there are several good flooring choices with effective sound dampening properties.

Please note, Johnsonite/Tarkett does not have an acoustical underlayment, but they do have products with an acoustical barrier built into the product to assist with sound deadening.
iQ Optima Acoustiflor 3.7mm thick (Homogeneous)
Acczent Wood Acoustiflor 3.35mm thick (Heterogeneous)
Vento Acoustiflor 3.8mm thick (Linoleum)

It is important that the finished flooring not directly contact the perimeter walls or vertical partitions of the entire floor area, including any openings or protrusions such as electrical boxes, heating ducts, cold air returns, columns or pipes in the subfloor installation. Installing a perimeter isolation barrier prevents the transmission of vibration in the subfloor to the walls, where it could bypass the flooring structure and transmit the sound.

Nailing through a sound reducing membrane negates the sound deadening properties. The nails will transmit the sound through the sound deadening membrane. Example, a 3¼″ wide engineered hardwood stapled every 6 inches in a 10 foot by 10 foot room will require 738 staples.

With glue down Hardwood flooring today, other than cork underlayments, the use of “All In One
Adhesives” also offers sound reduction. These products include;
Anderson enSurance 3X Triple Options
Armstrong Summit Adhesive
Bostik MVP in use with Bostik’s Best, BST, EFA and TKO Adhesives
Bostik Ultra-Set SingleStep
DriTac 1001, DriTac SMC, and the new DriTac 7700 Easy Clean
Mapei Ultrabond ECO 985 & 995

Thanks to JJ Haines for the info!