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Episode 4: Understanding the ratings behind acoustical wall, ceiling and roof assemblies

Apr 9, 2025

Familiarize yourself with STC, IIC, CAC and OITC rating classes and testing

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Noise pollution can have harmful effects on human health, ranging from simple annoyance and distraction to increased blood pressure and stress.* Effectively reducing the distance sound can travel is critical to creating pleasant spaces where people live, work and play. The first step toward designing quality acoustical assemblies is understanding the differences between Sound Transmission Class (STC), Impact Insulation Class (IIC), Ceiling Attenuation Class (CAC) and Outdoor-Indoor Transmission Class (OITC).


Listeners will learn:

  • Why noise reduction is important
  • What the four acoustic ratings — STC, IIC, CAC and OITC — measure and how they are tested
  • Real-world examples of noise challenges gypsum board acoustical assemblies solve
  • The role National Gypsum’s Fire & Sound Assembly Selector can play in selecting assemblies that meet both acoustical and fire ratings


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The information provided in this podcast is for general information purposes only. For specific building or design questions, please consult with the 1-800-NATIONAL® Construction Services team or your regional National Gypsum construction design manager. Always use products according to the manufacturer’s recommendations.

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