NRC Ratings in Acoustic Design

Acoustic Slat Panelling

In the intricate world of architectural and acoustic design, where the harmony of spaces meets the precision of science, one pivotal metric stands out: the Noise Reduction Coefficient (NRC). This metric serves as a guide for designers and architects in crafting and shaping environments conducive to focus, learning, or entertainment.

What is NRC?

The Noise Reduction Coefficient (NRC) is a measure used to quantify the effectiveness of a material or surface in reducing the reverberation or echo within a space. It's commonly used in architectural and acoustic design to assess how well a particular material absorbs sound.

NRC values range from 0 to 1, where 0 indicates perfect reflection (no absorption) and 1 indicates perfect absorption (no reflection). Materials with higher NRC values are more effective at absorbing sound and reducing reverberation.

NRC is typically determined through standardised testing procedures, where the absorption properties of a material are measured at different frequencies. These measurements are then averaged to calculate the material's overall NRC.

Knowing the NRC of materials helps architects and designers select appropriate materials for specific spaces based on their acoustic requirements, such as reducing noise levels in offices, classrooms, auditoriums, or recording studios.

How is the NRC Rating Determined for Building Materials

The testing of Noise Reduction Coefficient (NRC) involves standardised procedures to accurately measure the sound absorption properties of materials. Here's an overview of the typical testing process:

Test Setup:

The material sample to be tested is mounted on a rigid backing in a large test chamber called a reverberation room. This room is designed to minimise sound reflections from surfaces other than the sample being tested.

Sound Source:

A sound source, usually a loudspeaker, is placed at a specific distance from the material sample. This source emits sound at various frequencies across the audible spectrum.

Measurement Microphones:

One or more microphones are strategically placed in the reverberation room to measure the sound levels. These microphones capture both the direct sound from the source and the sound reflected from the material sample.

Frequency Sweep:

The sound source emits sound at different frequencies, typically ranging from 125 Hz to 4000 Hz or higher. Each frequency is tested individually.

Absorption Coefficient Calculation:

For each frequency, the sound absorption coefficient of the material sample is calculated based on the difference between the incident sound energy (before hitting the sample) and the reflected sound energy (after hitting the sample). This calculation accounts for the sound absorbed by the material.


The absorption coefficients are typically measured at several frequencies within the audible range. These values are then averaged to obtain the overall NRC value for the material.


The NRC value, ranging from 0 to 1, is reported along with the frequency range over which it was measured. This information helps designers understand how well the material absorbs sound across different frequencies.


The testing procedures and equipment used to determine NRC values are standardised to ensure consistency and reliability across different laboratories and manufacturers. Common standards for NRC testing include ASTM C423 (for building materials) and ISO 354 (for acoustics).

By following these standardised testing procedures, manufacturers and acoustic engineers can provide accurate NRC values for various materials, allowing architects and designers to make informed decisions about acoustic treatments in their projects.

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Lab Conditions vs Real-World Environment

While NRC testing provides valuable insights into the sound absorption properties of materials under controlled laboratory conditions, there can be differences between these results and the performance of materials in real-world environments. Here are some factors that can contribute to these differences:

Room Configuration:

The reverberation room used for NRC testing is designed to minimise reflections from surfaces other than the material being tested. In real environments, such as offices, classrooms, or concert halls, the room configuration, layout, and presence of furniture, equipment, and people can affect sound absorption and reflection differently.

Installation Method:

The way materials are installed in a real environment can impact their performance. Imperfect installation, gaps, or inconsistencies in mounting can affect the overall sound absorption properties of the material.

Surrounding Surfaces:

In NRC testing, materials are typically tested in isolation. However, in real environments, materials interact with surrounding surfaces, which can affect sound reflection, absorption, and diffusion. For example, sound-absorbing panels installed on a wall may interact differently with adjacent ceilings, floors, and other walls.

Environmental Conditions:

NRC testing is conducted under controlled environmental conditions, such as temperature, humidity, and air pressure. In real environments, variations in environmental conditions can influence sound propagation and absorption.

Occupancy and Usage:

The presence of people, furniture, and equipment in a real environment can affect sound absorption and reflection. Occupied spaces tend to have different acoustic characteristics compared to empty spaces.

Background Noise:

NRC testing is typically conducted in quiet environments to isolate the effects of the material being tested. In real environments, background noise levels can vary, affecting the perception of sound and the effectiveness of acoustic treatments.

Frequency Response:

While NRC provides an overall measure of sound absorption across different frequencies, the acoustic performance of materials may vary depending on the frequency range and specific sound sources present in the environment.

Considering these factors, it's important for designers and acoustic engineers to not solely rely on NRC values but also consider the specific characteristics of the intended environment and how materials will interact within that space. Field testing and simulations can provide additional insights into the acoustic performance of materials in real-world conditions.


NRC ratings guide the composition of spaces where sound meets structure. Yet, as with any product ratings, the transition from lab to life introduces complexities that cannot be fully captured in standardised tests. Acknowledging the nuances of real-world environments, with contextual understanding, crafting spaces where form, function, and sound seamlessly converge.

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