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What is IIC and STC?

Published in Acoustics 2 mins read

Impact Insulation Class (IIC) and Sound Transmission Class (STC) are both single-number ratings used to describe the acoustical performance of building elements, but they measure different aspects of sound transmission. STC focuses on airborne sound, while IIC focuses on impact sound.

Sound Transmission Class (STC)

STC is a single-number rating that measures a building material's ability to reduce airborne sound transmission. A higher STC rating indicates better sound isolation, meaning less sound is transmitted through the material. This is crucial for reducing noise from speech, music, or traffic from traveling through walls, floors, and ceilings.

  • What it measures: Airborne sound reduction (e.g., speech, music, traffic).
  • How it's measured: Decibels (dB). Higher STC = greater sound reduction.
  • Typical applications: Walls, floors, ceilings, doors, and windows in residential and commercial buildings.
  • Example: A wall with an STC of 50 will reduce airborne sound transmission more effectively than a wall with an STC of 30. An STC of 50 might mean that loud speech is barely audible through the wall, whereas an STC of 30 would mean that speech is easily understood.
  • Importance: Enhances privacy and reduces noise pollution in buildings.

Impact Insulation Class (IIC)

IIC is a single-number rating that measures a floor-ceiling assembly's ability to resist impact sound transmission. Impact sound is generated by direct physical impact on a surface, such as footsteps, dropped objects, or furniture moving. A higher IIC rating indicates better impact sound isolation.

  • What it measures: Impact sound reduction (e.g., footsteps, dropped objects).
  • How it's measured: Decibels (dB). Higher IIC = greater impact sound reduction.
  • Typical applications: Floor-ceiling assemblies in multi-family dwellings, apartments, and condominiums.
  • Example: A floor with an IIC of 55 will reduce impact noise, such as footsteps, better than a floor with an IIC of 45. An IIC of 55 means the footsteps in the apartment above are less audible in the apartment below.
  • Importance: Reduces disturbance and improves comfort in buildings, particularly those with multiple stories.

Key Differences Summarized

Feature Sound Transmission Class (STC) Impact Insulation Class (IIC)
Sound Type Airborne Impact
Source Speech, music, traffic Footsteps, dropped objects
What it Measures Airborne sound reduction Impact sound reduction
Desired Outcome Reduce noise pollution Reduce disturbance
Typical Application Walls, doors, windows, ceilings Floor-ceiling assemblies

In conclusion, STC and IIC are essential metrics for evaluating and improving the acoustical performance of buildings. They focus on different types of sound and require different solutions for effective sound control.