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

Whenever you plan to buy an acoustic or soundproofing materials than you probably have seen two numbers on most of the products one is Noise reduction coefficient (NRC) and other is Sound Transmission Class (STC).

Have you ever thought about what does these two mean in sound engineering and why these two numbers labeled on every product?

May be yes or no but if you are reading this guide that means you find relevant to understand both of these concepts and there is one more we call Impact insulation class (IIC), will also discuss the same.Noise reduction coefficinet vs sound transmission class

While I was doing research for soundproofing, I have gone through various parameters and everywhere to judge the soundproofing quality of material the very best numbers which matter a lot were NRC and STC.

So let go through each one of them and get an idea of what do STC and NRC stand for.

What is Noise Reduction Coefficient (NRC)?

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

Whenever any sound waves insert to your private space, due to multiple reflections from the hard surfaces like wall, floor or ceiling cause amplify and create an echo in the room.

This echo can be minimized using the 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 the 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 it rated based on object face dimension.

Find below the reference NRC of different materials: If the NRC given as 0.1 means 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 judge by NRC.

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

What is the Sound Transmission Class (STC)?

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

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

If the material is having a higher STC score and sound waves coming 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 a 30 dB but after passing through 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 tested in an ideal environment but in the real world, you have lot many factors to consider.

So make sure to purchase a product which has more than 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 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 improve 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, Sound transmission class (STC) is focused on blocking capacity of any material specially referred in 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 house or room than better to focus on minimizing echos 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 more 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 on one more factor that we call Impact insulation class.

What is Impact Insulation Class?

The generated noise could be an 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 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.

Conclusion 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, 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 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.