Acoustical Properties of Wood Floors

Preventing Indoor Noise Pollution

According to the Structural Building Components Association, proper sound mitigation in buildings occur at the design phase which must be taken into consideration by the building designer and/or engineer. Noise pollution can be very distracting, and disruptive. Noise can come from the outdoors, an adjacent apartment, the floor below or the floor above; from people speaking, the radio or television, footsteps, a barking dog, and such. Impact noise and airborne noise are the two main source points. Impact noise is also known as footfall noise. Airborne noise is sound that does not originate from direct physical contact with the floor. Below is a chart that shows what decibel level each activity is heard at.

decibel levels

Structural Building Components Association. 2016. Research Report: Sound Transmission in Wood Floor and Roof Trusses. [accessed 2017 March 6]. https://www.sbcindustry.com/system/files/sbca/research-report/node/1457/srr1601-03soundtransmissioninwoodfloorandrooftrusses.pdf

Wood Floor Underlayments

When installing hardwood floors the use of an underlayment can help to absorb sounds that cause noise issues. Some popular options are: foam, felt, cork, and rubber. Underlayments serve other purposes, as well: moisture barrier, reducing subfloor imperfections, and insulation.

  • Cork is a natural material taken from the bark of a quercus surbos tree (cork oak). This underlayment is versatile and can be used with floating wood floors, glue-down floors, and staple/nailed wood floors.
  • 15 lb black felt paper is used under stapled/nailed down wood flooring, solid or engineered, as a moisture barrier from beneath to prevent moisture issues.
  • Felt underlayment is one of the most popularly used, available in thickness of 3-6mm. Felt has decent moisture barrier but certain humid areas may require a plastic moisture barrier attached to the felt.
  • Foam underlayment is used under engineered wood floors and made up of recycled fibers and rubber, with excellent density, moisture barrier, durability and sound absorption. It can be recycled at the end of its life and resists mold.
  • Rubber underlayment has excellent moisture and sound barrier properties, as well as, acts as an insulator. The thickness is anywhere from 2-9mm.
  • 4-6 mil Plastic moisture retarders are attached to underlayment pads in situations where the subfloor is concrete. In an area with higher humidity, moisture barriers are recommended. However, in general, wood should be allowed to breath and moisture barriers are not recommended.

Solid wood floors have greater acoustic resistance than engineered wood floors. Installing floating wood floors, instead of attaching it by gluing it down or stapling/nailing it, can reduce the impact noise. The nails and staples can increase sound transmission.

When deciding what type of underlayment to choose for your project, please allow a professional to assess the site. Some sites may require extra vapor barrier, and others may not require an underlayment.

Sound Absorption vs. Sound Blocking

Sound absorption and sound blocking are two separate things. Sound absorption is when energy is received by a material. Sound blocking is when sound is not allowed to pass through a certain area. Sound absorption is measured by Sound Transmission Class. The Impact Isolation Class (IIC) measures the sound impact from floor to ceiling in a structure.

 

Sound Absorption is measured by the noise reduction coefficient (NRC). A material is measured by the amount of sound it can absorb. For example, if 30% of the sound is absorbed, the coefficient is 0.30.

 

Sound Blocking is measured by Sound Transmission Class (STC), transmission loss (TL) curve, weighted sound reduction index value. STC is used to measure a material’s ability to absorb sound. The higher the STC value a material has, the greater its ability to reduce the unwanted noise. Below is a chart that showcases what each rate level means in terms of how sound is absorbed.

stc rating

Structural Building Components Association. 2016. Research Report: Sound Transmission in Wood Floor and Roof Trusses. [accessed 2017 March 6]. https://www.sbcindustry.com/system/files/sbca/research-report/node/1457/srr1601-03soundtransmissioninwoodfloorandrooftrusses.pdf

 

Flooring sound transmission class is calculated through ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.

Flooring impact insulation class is calculated through ASTM E492: Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.

The illustration below shows the different ways sound impacts building material barriers. In the top scenario, sound transmission class is shown to travel from room-to-room. The scenario below shows the impact insulation class that sound travels from floor-to-ceiling.

 STC and IIC
http://mcgrawimages.buildingmedia.com/CE/CE_images/2016/April-CertainTeed-3.jpg

 

Disclaimer

To the best of our knowledge, the information contained herein is accurate and reliable as of the date of publication; however, we do not assume any liability whatsoever for the accuracy and completeness of the above information.