Sound Control – Part 1 – STC and NRC

Introduction

Today we are going to discuss the technical factors to consider when thinking about sound control. When thinking about acoustics, we want to focus on two major ways that sound moves – how sound travels within a space, and how sound travels from space to space. Once we understand how sound travels we can design systems to control the impact that sound has on the space.

In trying to determine the ways that acoustics can be controlled, architects looks at a number of factors. The most common ways to think about sound control is with understanding the Sound Transmission Class (STC) of a material and the Noise Reduction Coeffecient (NRC) of a material. There are other ratings of materials and assemblies that give the designer information about sound control, such as the Impact Isolation Class (IIC), and your designer will have to consider such information in order to design the best system. However, in most cases the greatest impact of noise control will come from understanding the STC and NRC, and when each one is important given the situation.

Sound Transmission Class (STC)

STC (Sound Transmission Class) is a rating that represents a material or product’s ability to block sound from travelling through assemblies (wall, ceiling, floor or other building assembly). In other words, how well a product or materials stops airborne noise transferring from place to place. 

The higher the STC rating, the better a material’s ability to block sound.

A limitation of STC, however, is that it only measures certain frequencies. This means it doesn’t accurately capture how well a material blocks low frequency sounds such as common noises that come from airplane engines, large trucks, and heavy equipment.

Noise Reduction Coefficient

NRC (Noise Reduction Coefficient) is a rating that measures the amount of energy absorbed when it strikes a particular surface. It’s how well a product or material reduces the echo into the space’

An NRC rating of 0 indicates complete reflection, meaning a material bounces 100% of the sound back into the room. An NRC rating of 1.0 indicates perfect absorption, meaning a material soaks up 100% of the sound and no echo is put back into the room.


While STC and NRC differ, both deal with airborne sound, however, vibrational sound must be taken into account as well, and that is where Impact Isolation Class (IIC) is examined.

Impact Isolation Class (IIC)

IIC (Impact Isolation Class) is a rating that measures impact noises. Impact noises are erratic sounds caused by footsteps, dropped objects, equipment vibrations, etc. The impact causes vibrations in construction assemblies and those radiate sounds to other locations.

The IIC rating is represented with whole number. A larger number means more impact sound is being blocked.

Blocking and absorbing airborne and impact sounds are key to designing sound control systems.

Which One is More Important?

Well, which rating, NRC or STC, is more important for controlling sound. Unfortunately, the answer is – it depends. Remember, STC measures how well a product keeps sound from escaping the room to an adjacent space. NRC is a measurement of how well a product makes the room you are in quieter.

If you are in a noisy room, like a daycare classroom or school gymnasium, you want products with a higher NRC on the walls or ceiling to absorb the sound, making the room less noisy.  If you are in a office or medical clinic where you are talking about sensitive topics that you don’t want people in adjacent rooms to hear, then you should focus on a product on the wall with a high STC. In certain cases, such as hotels and multi-family units housing, both NRC and STC will play a big role in determining the design and product selection.

Similarly, in Sound Control – Part 2 we will discuss and examine examples to maximize acoustical design for sound control.

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