Understanding Impact Insulation Class (IIC) and How It Affects Soundproofing

Why Impact Sound Matters

You’re relaxing after a long day when suddenly … thud. A heavy footstep from the apartment above breaks the silence. Next comes the drag of a chair, the sound of a dog padding back and forth from room to room, or the sudden drop of the remote. 

If it just happens every once in a while, these noises can be somewhat bearable, but over time they start to add up — and really begin to wear on your nerves!

These are all examples of impact sound: vibrations caused by physical contact with the floor materials above, traveling through the building and into your space.

In multi-level buildings, like apartments, hotels, or commercial office spaces, impact sound transmission can be a major source of discomfort and frustration. Enter Impact Insulation Class (IIC), the standard metric used in acoustical testing to evaluate how well a floor-ceiling assembly reduces these levels of sound.

Understanding IIC Ratings

Impact Insulation Class is a single-number rating that quantifies how well a building system blocks the sound transmission of impact noise. Unlike airborne sounds (like voices or music), impact noise is a result of some sort of physical contact, such as footsteps, dropped objects, or furniture movement.

The IIC rating is determined through acoustic noise testing using a standardized tapping machine. This device drops small hammers in a rhythmic pattern on the floor surface, while microphones in the room below capture the sound transmitted through the assembly.

Once the testing is completed, the data is analyzed across ⅓ octave frequency bands. The result is a single IIC number, and the higher the rating, the better the structure isolates impact noise.

IIC vs HIIC: What’s the Difference?

In recent years, High-Frequency Impact Insulation Class (HIIC) ratings have emerged as a complementary testing solution to IIC. While standard IIC testing focuses on low-frequency impact noise (things like heel strikes or bassy thuds), HIIC instead hones in on high-frequency ranges, such as clicking heels or tapping noises.

This distinction in ratings matters most in buildings looking to install lightweight flooring solutions such as laminate or luxury vinyl tile (LVT), both of which often transmit higher-frequency sounds more easily. HIIC ratings can help builders understand how a material will perform under a variety of noise conditions. 

It’s very important to note that HIIC does not account for low-frequency data, so it’s not a replacement for standard IIC. Instead, HIIC testing offers an additional data point for certain applications where high-frequency noise is an added concern. 

What IIC Measures (and What It Doesn’t)

IIC testing evaluates the entire floor-ceiling assembly, not just the flooring itself. This includes the subfloor, underlayment, joists, insulation, and ceiling structure. 

To optimize noise-dampening performance, you often need to use a variety of different material combinations through professional acoustics testing to identify the best system for a space.

It's also important to note that IIC testing doesn’t measure airborne noise. Sounds such as voices, televisions, and music fall under a different rating: Sound Transmission Class (STC). While STC testing evaluates airborne sound, IIC is solely focused on impact sound, such as the vibrations caused by contact.

How the Tapping Machine Test Works

To conduct sound transmission class testing for IIC, a tapping machine is placed on the floor in the upper room. This machine has five small steel hammers that drop in a fixed pattern to simulate human footfall.

In the room below, sensitive microphones record how much sound is transmitted through the assembly. The captured sound is analyzed across a range of frequencies, and the data is then averaged into a single-number IIC rating. 

Remember, this test shows how sound moves through the entire flooring-ceiling system, not just the top layer, which offers valuable insight into real-world applications.

Manufacturers, architects, and consultants use IIC testing to evaluate how different materials and assemblies perform together, ensuring buildings meet both code and performance expectations and create a comfortable, quiet environment for end users. 

What’s a Good IIC Rating?

There are general benchmarks for sound ratings to improve building performance and occupant satisfaction, depending on the type of building or application. So what’s a “good” rating?

• IIC 50 is the minimum code in many jurisdictions for multi-family buildings to reduce noise complaints between occupants
  
  
• IIC 60+ is considered a good rating
  
  
• IIC 70+ is considered an excellent rating and is often used in luxury or premium construction projects
  
  
• HIIC ratings can vary by flooring category and are typically used for product comparisons rather than code compliance

Remember, surface materials alone (such as carpet or LVT) won’t guarantee a high IIC because performance depends on the design of the entire floor-ceiling system. Using materials like resilient underlayments, floating floors, or decoupled ceilings can dramatically boost IIC ratings, but only when tested together in a complete assembly. 

Why IIC Ratings Matter in the Real World

Both acoustic noise testing and sound transmission class testing are often used during the design process to avoid costly retrofits down the road. By prioritizing IIC performance upfront, developers can prevent tenant dissatisfaction and reduce post-occupancy noise issues before the building is even completed.

Similarly, commercial office spaces prioritize minimizing disruptions between floors, and hotels are focused on total guest comfort and privacy while occupied. Understanding and optimizing IIC is a crucial consideration for designing code-compliant, comfortable, and acoustically sound buildings.

For developers, architects, and engineers, it's important to take a holistic approach, evaluating not just the visible flooring but also how all structural elements work together to manage sound transmission.

The Importance of Building with Sound in Mind

Comprehensive acoustic testing gives you a real-world look at how sound actually behaves in a space — not just how it should behave on paper. It takes into account things like room size, shape, and use, and digs into how sound performs across different frequencies.

With proactive acoustic testing in place, you can fine-tune your setup to meet specific needs and meet (or beat) industry standards. Plus, it helps you find that sweet spot where great design and great sound come together, making the space not just functional but also enjoyable to be in.

If you’re working on a project that demands reliable impact sound performance, we can help. Our team at NGC Testing Services offers expert acoustic noise testing and sound transmission class testing to ensure your assemblies meet the required standards and provide real-world comfort.

Contact us today to schedule acoustical testing or consult on the best path forward for your soundproofing needs.