Understanding NRC values.
NRC values or Noise Reduction Coefficient values is single number value ranging from 0 to 1. This number describes the average absorption value of an acoustic material and in no way does it relate to noise control. The naming is ambiguous and is more accurately described as a Sound Absorption Average (SAA).
The NRC values are derived from the mean of the absorption values at 250Hz, 500Hz, 1000Hz and 2000Hz which are measured in a reverberation chamber as specified in ASTM C423 to ISO 354.
The intent with NRC values were to provide a single number. However its application is limiting due to insufficient data to make a meaningful judgement on 2 different materials. One material might have a higher absorption in the lower frequencies and another material at higher frequencies…..!!
There are also a number of factors that affect NRC values.
The way the test sample is mounted in the test chamber is often not specified. An example would be to introduce an air gap behind the sample and in this way the absorption can be increased and NRC values are higher.
The size of the sample tested in the reverberation chamber is not specified. Larger samples will typically have a higher NRC value.
Sealing the edge of the materials being tested also affects the total absorption value.
What improves NRC values?
Material Thickness
Thicker materials generally have more material to increase absorption values, especially in the lower frequencies.
Open-Cell versus Closed-Cell.
Open cell foam materials generally have higher NRC values compared to closed-cell materials. This is because open cell structures allow sound waves to penetrate and get absorbed while closed cell structures reflect sound to a greater extent. An important consideration in acoustic foams is with time dust penetrates the open cells and the products efficiency at absorbing sound deteriorates. Most acoustic foams have a life span of 5 years.
Combining Acoustic Materials.
By strategically combining materials with different NRC values better sound control can be achieved and tailored to specific requirements. See Acoustic Worx Product Range for Multi Density Acoustic Board.
Are Curtains an effective acoustic absorber?
Curtains can provide some level of sound absorption, but their effectiveness as acoustic absorbers depends on various factors. Here are a few considerations regarding the effectiveness of curtains as acoustic absorbers:
- Material and construction: The material composition and construction of the curtains play a significant role in determining their sound absorption capabilities. Heavy and dense materials, such as velvet or thick woven fabrics, tend to be more effective at absorbing sound compared to lightweight materials like sheer curtains or thin fabrics.
- Thickness and density: Thicker and denser curtains generally offer better sound absorption. Multiple layers of fabric or curtains with added backing materials, such as sound-absorbing foam or mass-loaded vinyl, can enhance their ability to absorb sound.
- Coverage and placement: To maximize sound absorption, curtains should cover a significant portion of the reflective surfaces in a room. Hanging curtains from ceiling to floor and ensuring that they extend beyond the window or wall edges can help reduce sound reflections and improve absorption.
- Frequency range: Curtains typically provide better absorption in the mid to high-frequency range, where human speech and many common environmental sounds exist. However, they may be less effective at absorbing low-frequency sounds, such as heavy bass or rumbling noises.
- Room characteristics: The effectiveness of curtains as acoustic absorbers also depends on the specific room characteristics. Factors like room size, shape, and overall acoustic design can influence the performance of curtains in absorbing sound. In larger spaces or rooms with significant sound reflections, curtains alone may not provide sufficient absorption.
While curtains can contribute to reducing sound reflections and improving the overall acoustics of a room to some extent, their performance may not match that of purpose-built acoustic panels or specialized sound-absorbing materials. If significant sound control is required, especially for critical listening environments or spaces with specific acoustic needs, it is advisable to consult with an acoustic professional who can recommend appropriate solutions tailored to the specific requirements of the room.
Are Carpets an effective acoustic absorber?
Carpets are not the most effective acoustic absorbers as they are typically not dense enough to effectively absorb higher-frequency sound waves. Carpets might help with reducing some mid-range frequencies, but do not have the ability to absorb a wide range of frequencies like specialized acoustic products.
More information on Testing of Carpet and what improves performance.
