Acoustics, Architectural Theory

Acoustic Rating Systems – NRC, STC, and IIC Explained

Choosing the right acoustic material can get tricky at times. Each acoustic material has its own unique property it can contribute to your building acoustics. And the materials are often classified on the basis of their soundproofing performance. And honestly, to someone who has no idea on what the rating systems are, those numbers will all be just jargon.

The soundproofing performance is determined on the basis of three types of acoustic rating systems. These are namely:

  1. Noise Reduction Coefficient (NRC) Rating
  2. Sound Transmission Class (STC) Rating
  3. Impact Insulation Class (IIC) Rating

The NRC rating helps determine the sound absorption of the materials in a room. STC rating helps us measure how much sound is transmitted between rooms. And IIC rating helps determine the soundproofing between different floors of the building. 

All three rating systems are very important to creating a well-soundproofed building. And prior consideration of these three values before deciding on the acoustic materials to use can prove to be extremely useful. 

  1. Noise Reduction Coefficient (NRC Rating)

The Noise Reduction Coefficient rating represents the average sound absorption coefficient of materials. It indicates how much sound the absorber can absorb. 

NRC is measured on a scale from 0 to 1. 0 means that the material absorbs no sound and 1 means the material has the ability to absorb 100% of the sound.

How NRC rating is measured:

NRC can be measured for practically any material you want to put inside a room, be it furniture, decor, and so on. 

The sound-absorbing coefficients of material are taken and the arithmetic mean of it is taken, which is then rounded off to the value the nearest 0.05 to measure the NRC rating of a material. 

The range of sound-absorbing coefficients taken into consideration is – 250, 500, 1000, and 2000 Hz. 

Acoustic Material to Increase NRC Rating: 

  • Drywall
  • Soundproof fiberglass
  • Acoustic panels
  • Ceiling tiles
  • Carpet  
  • Mineral wool
  1. Sound Transmission Class (STC Rating)

STC helps in measuring how effective a material is in reducing the sound transmission between rooms. 

The higher the STC rating, the more effective the material is at reducing sound transmission, and the better the insulation. 

STC is measured in decibels. And the STC rating of 38-42 dB is said to be good soundproofing. STC above 50 is said to be very good and is often achieved for commercial projects. 

How STC rating is measured:

STC Rating measurement is quite simple. Add all the frequency measurements together and divide the value by the total number of frequencies to get the STC value. 

Acoustic Material to Increase STC Rating:

  • Acoustical thermal panels
  • Floor underlayment
  • Soundproof membrane
  • Soundproof drywall
  1. IIC Rating

Impact Insulation Class (IIC) measures the sound insulation and transmission of impact noise. This noise includes footsteps, vibrations, and so on, on floors and ceilings. 

It is basically the amount of sound that travels between floors. IIC is measured in Decibels where the higher the number is, the quieter the floor will be. 

According to the International Building Code(IBC), an IIC rating of 50 is required for residences to be ideally soundproofed. 

How IIC rating is measured:

IIC rating of a floor can be determined with the help of the tapping machine on an existing floor assembly. This helps determine the amount of sound traveling through to the lower floors. 

Acoustic Materials to Increase IIC Rating:

  • Soft floor coverings
  • Cushioned backings on floors
  • Sound mat
  • Resilient ceiling system
  • Wood subfloor
  • Gypsum boards

For careful consideration to find the right acoustic materials for your buildings, it is best to understand their rating systems. These rating systems will give you a good idea of how much soundproofing you will achieve with the material you choose.

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