What are Noise Reduction Coefficient (NRC) and Sound Transmission Class (STC) in SoundProofing?

Noise Reduction Coefficient

NRC and STC are two rating systems that are used to measure the sound absorption and sound isolation capabilities of the material, respectively. But what do they actually mean?

According to the report of the Noise Control Engineering Division of the National Institute of Environmental Health Sciences (NIEHS), the NRC is “a single-number rating system describing the overall sound-absorbing performance of a material.” 

Because the NRC is a single-number rating, it doesn’t give you any information about how the material performs at different frequencies. But STC, on the other hand, is a frequency-dependent rating system that tells you how well a material can block sound.

That’s why you need to completely read this article. Because in this article, I have shared what are NRC and STC in soundproofing. After reading this article, you will be able to understand their meanings and differences. 

Also, I have covered what is impact insulation class. If you are not familiar with it, then you will get to learn about it too. 

So, Let’s get started.

What is Noise Reduction Coefficient (NRC)?

In sound engineering, while we talk about the soundproofing of any area you definitely heard about sound absorption.

Whenever any sound waves insert into your private space, due to multiple reflections from hard surfaces like walls, floors, or ceilings cause amplification and create an echo in the room.

This echo can be minimized using sound-absorbing materials, and the ability of any material to absorb that sound frequency is rate based on the Noise reduction coefficient.

Noise reduction coefficient in short term also known as NRC is a logarithmic average attenuation or decay rate (dB/s) of sound frequencies in the range of 250,500,1000 and 2000 Hz by any sound-absorbing object in an area compared to the area where no sound-absorbing material available.

In simple term value of NRC specify the ability of any material to absorb Sound waves in the frequency range of 100 Hz to 5000 Hz.

The higher the NRC of the object or material the more ability it has to absorb the Sound waves and less reflection. The material with 0 NRC will reflect all the sound waves and no absorption.

The highest number given to any material is around 1 which has the ability to absorb 100% sound waves but the material tested in the laboratory has more NRC as compared to the real world.

All NRC are rated based on the face dimension of the object however the edges and mounting of the object also absorb more sound waves than what is rated based on the object’s face dimension.

Find below the reference NRC of different materials: If the NRC given as 0.1 means the material has the ability to absorb 10% and 90% of the sound waves will be reflected back.

  • Brick – 0.00-0.05 NRC
  • Glass – 0.05 NRC
  • Plywood – 0.10-0.15 NRC
  • Carpet over concrete – 0.20-0.30 NRC
  • Carpet with foam pad – 0.30-0.50 NRC

The NRC will be calculated based on the frequency range of human speech such as 100 Hz to 5000 Hz. So frequencies higher than this range may not be efficiently absorbed by these materials or judged by NRC.

Well, you got an idea of what this Noise reduction coefficient or NRC stands for and now let’s discuss the Sound transmission class or STC.

What is the Sound Transmission Class (STC)?

I hope you knew the differences between sound and noise the basic phenomenon both have is the transmission of the waves.

Sound transmits by vibrating the air particles coming in its way and this vibration cause displacement of air particles and continues unless it found any barrier in its way.

The stronger the barrier is, the more its ability to block the noise and less pass through the material hence the ability of any material to block the sound waves is rated based on the STC or sound transmission class score.

STC stands for the Sound transmission class, which is an integer rating given to any soundproof material to identify its potential to attenuate airborne sound.

If the material is having a higher STC score and sound waves come from the other side of the material and try to pass through than most of its energy got dissipated while passing through the material.

It is derived from the sound attenuation values tested on the frequency range of 125 Hz to 4000 Hz and it measured the deduction rate of the sounds in the dB scale.

If a sound coming from the other side of the wall is 30 dB but after passing through the isolation or wall it reduces to 10 dB that means the STC rating of the wall is 20.

If we talk about soundproofing than the partition material should have an STC score of above 50 and the STC rating given on any products is based on laboratory testing.

This means it is tested in an ideal environment but in the real world, you have lot many factors to consider.

So make sure to purchase a product that has more than a 55-60 STC score to meet your soundproofing baseline requirements.

Here are a few examples that explain how any material STC score can impact the transmission of the sound.

  • 25 – Normal Speech Clearly Audible
  • 30-  Normal Speech is hard to audible but loud speech clearly audible
  • 35 – Loud Speech audible but not clearly
  • 60 – Very loud music is barely audible
  • 70- Power tools are faintly heard
  • 75+ Most sounds are completely inaudible

From the above STC ratings, you got an idea the higher the number the more its ability to block the sound waves. Blocking sound waves is the prime need for soundproofing.

To improve the STC score in soundproofing two techniques are mainly used.

  • Isolation – By creating a cavity wall using a resilient channel for two-layer drywall.
  • Adding Mass – By setting up extra mass on the existing wall, which can be added by using the Mass loaded vinyl (MLV).

These two will be the perfect way of blocking the sound waves and improving the STC of the partition wall.

The difference between NRC vs STC?

The noise reduction coefficient (NRC) is mainly focused on absorbing the ability of the material and it is mostly rated on a scale of up to 1.

However, the sound transmission class (STC) is focused on the blocking capacity of any material specially referred to in the case of isolating material used as a partition and rated on a scale of 100.

If you are struggling with the noise from the neighbor and looking for overcoming noise than while opting any soundproof material more focused on STC score instead of NRC.

However, if looking to improve the sound quality in the house or room than better to focus on minimizing echoes by absorbing sound waves than more focus on the NRC instead of STC.

Let’s take an example, NRC is mainly needed for materials that are used in the Gymnasium on the ceiling, walls, and every single space to absorb the much noise as possible such as carpet, Mat, or acoustic foam.

However, the STC score will matter for those spaces where you want to maintain privacy from the other side of the private space, so no other side person can hear your secret chat.

A few examples are like insulating materials and Drywall or Green glue used for making partitions.

Well till this point you got a good idea of what is noise reflection coefficient and Sound transmission class and their major differences but now let me give you an idea of one more factor that we call Impact insulation class.

What is Impact Insulation Class?

The generated noise could be airborne noise which is a noise generated due to transmission by air such as people chattering, traffic, television noise, etc.

But there are few noises generated due to the impact on the other objects such as footfalls or dragging of chairs or dropping of any material to the floor.

In such cases, the noise generated is due to the impacts on the ground and which is different than airborne noise. To minimize this you won’t need to focus on STC or NRC.

You will need to focus on the impact insulation class (IIC), this will define the ability of any material to minimize the impact noise.

IIC is also rated based on the standard frequency range of 100 to 3150 Hz, wherein real-world footsteps generate a noise frequency range of 100 Hz.

It is widely needed to be focused on those materials used for the soundproofing of the floor such as Carpet, Mat, Subfloor plywood, or underlay.

Wrap Up on NRC vs STC

Sound absorbing is a component of soundproofing, which means to fully soundproof any space you also need to focus on sound-absorbing materials.

Better to understand both STC and NRC is, that STC should be focused more if you are looking for ways to avoid sound transmission to another side of your area however NRC will help you to improve the sound quality by absorbing noise.

Almost every soundproof material will have these two factors on the products, so better to check their ratings and based on need make sure to place a buy order for the same.

How was your experience with the NRC rating in your noise area better to share with our readers so they will be confident about the standard and real rating of the NRC and STC.

FAQ: What Is NRC And STC

There are many questions in people’s minds regarding NRC and STC. But here I am answering some important questions.

Q1. How do you calculate the NRC rating of a product?

Ans: The NRC (Noise Reduction Coefficient) is a single-number rating system used to measure a material’s ability to absorb sound. A product with an NRC of 1.0 absorbs all sound that strikes it, while a product with an NRC of 0.0 absorbs no sound.

Q2. Why is an STC important for a product?

Ans: STC is important for a product because it provides a measure of how much sound the product can block. The higher the STC, the better the product is at blocking sound.

Q3. How can you figure out how effective NRC and STC will be in your own home?

Ans: By looking at the NRC and STC ratings of different materials, you can get a good idea of how effective they will be in your own home. The ratings can be found on the websites of manufacturers or on the websites of retailers that sell soundproofing materials.

Hey, I am Kirtish Vyas a YouTuber and believer in making life peaceful, a couple of years back I started soundproofing my house, bedroom, studio, and Car to reduce the unwanted noise, and the same experience I am sharing on SoundProofidea. Read More