Guest Blog – The Role of Precast Concrete in Better Building Acoustics

Guest Blog by PCI-MA Producer – John M. Jones, P.E., Vice President of Business Development, Nitterhouse Concrete Products

Noise from people in adjoining spaces whether in a hotel room, dormitory, healthcare setting, or others can hinder sleep, healing, learning, and productivity. Precast concrete floors and walls can limit noise intrusion while providing other construction advantages such as all-weather, year-round construction and robust, durable structures that deliver multi-hazard protection. This article will explain some basic acoustic terminology and demonstrate how NiCore™ Planks easily rise to the challenge of minimizing disruptive sounds.

Precast, prestressed hollow core slabs have been reliable and frequently chosen construction components for over fifty years. Designers, contractors, and owners appreciate their high-performance benefits making them a preferred choice for floor and roof construction in hotels, apartments, schools, dormitories, and condominiums. Machine casting of hollow core slabs provides high quality, mass production, and value pricing.

An often-overlooked feature of NiCore™ Planks from Nitterhouse Concrete Products, Inc. (NCP) is their excellent airborne sound attenuation, a very distinct advantage over structures constructed with wood framing where sounds from surrounding occupied spaces can be rather disturbing. Hotels, apartments, and dormitories are often constructed with exposed ceilings and minimum floor finishes, so it is challenging to achieve reasonable sound attenuation. The purpose of this paper is to explain some basic acoustic terminology and demonstrate how NiCore™ Planks easily rise to the challenge of minimizing disruptive sounds. Two common metrics will be considered as described below.

The ability of a separation to reduce the intensity of airborne sound is commonly designated by the Sound Transmission Class (STC) and is a function of the weight of the units, which is why NiCore™ Planks are the solution of choice. The STC integer rating very roughly reflects the decibel reduction in noise that a separation can provide. The degree of isolation for airborne noise transmission depends not only on the building construction but also on the type of source, the level of the noise, and on the background noise in the receiving space. The STC is unable to convey differences of soundproofing effectiveness at all frequencies because it disregards those outside its defined range of 125-4000 Hz. It is, however, still a useful metric for designers even though many of the noises that commonly annoy people are < 125 Hz. Such noises include traffic, home theater speakers, and construction equipment. For example, loud speech can only be faintly heard when 40 ≤ STC < 50, but it can barely be heard at all when 50 ≤ STC < 60. Acceptance criteria can be variable among different agencies, but the Department of Housing and Urban Development (HUD) sets a minimum STC of 45 between living units and 50 between living units and public space. The table below shows STC ratings for various thicknesses of NiCore™ Plank, both as stand-alone units and including a 2” cast-in-place concrete topping.

The STC rating is approximated by the following equation:

STC = 0.1304W + 43.48 Statistical Tolerance = ± 2.5

Where W = Weight in psf

Note: STC calculated ratings typically represent independently published data used for design purposes, regardless of the lower and upper boundaries.

Footsteps, dragged chairs, dropped objects, slammed doors, and plumbing generate impact noise. In general, thickness or unit weight of concrete does not greatly affect the transmission of impact sounds. Structural concrete floors in combination with resilient materials effectively control impact sound, and the metric used is the Impact Insulation Class (IIC), sometimes known as Impact Isolation Class.

Somewhat like STC ratings, IIC ratings operate within their own defined range of 100-3,150 Hz. Because “real world" footstep noise is also generated at frequencies below 100 Hz the IIC value may not accurately describe the complete noise attenuation profile of a floor. Also, just as with STC ratings, acceptable criteria can be variable among different agencies, but the Department of Housing and Urban Development (HUD) sets a minimum IIC of 45 between living units and 50 between living units and public space. The IIC for a pad and carpet is about 45, which combined with a robust floor system such as NiCore™ Plank yields considerable resistance to impact noises as shown in the table below.

Human measurements of sensitivity to sound, vibration, light, etc. are by their very nature subjective, so these metrics of STC and IIC represent acceptable values for an average sampling of people. It might be more desirable in luxury accommodations to achieve STC and IIC ratings approaching 70 and should be researched by the project architect for the specific combination of building materials.

Learn more about how precast concrete delivers exceptional indoor environmental quality for occupants as well as other design and construction features. Case studies, webinars, and other resources provide proof positive that precast concrete buildings are designed for aesthetic versatility, energy efficiency, and for 100-year service life with minimal upkeep.

References

For additional information regarding STC and IIC ratings of precast, prestressed concrete systems:

• PCI Designer’s Notebooks: Acoustics (free PDF download)
• PCI Manual for the Design of Hollow Core Slabs and Walls (free PDF download)
• Additional information can also be found in Architectural and Engineering Calculations Manual by Robert Brown Butler as published by McGraw Hill (Note: The data is quite variable). 

Nitterhouse Concrete Products, Inc. in Chambersburg, PA, is a family-owned company serving the construction industry since 1923. You can reach them at 717-267-4505 or visit their website at www.nitterhouse.com for information on more quality precast, prestressed products to meet your design and construction needs.