Acoustical Blog

NRC – Noise Reduction Coefficients

The Noise Reduction Coefficient (NRC) is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. In particular, it is the average of four sound absorption coefficients of the particular surface at frequencies of 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. These frequencies encompass the fundamental frequencies and first few overtones of typical human speech, and, therefore, the NRC provides a decent and simple quantification of how well the particular surface will absorb the human voice. A more broad frequency range should be considered for applications such as music or controlling mechanical noise.

Specifications for materials used in sound absorption commonly include an NRC for simplicity, in addition to more detailed frequency vs amplitude charts.

Acoustical materials manufacturers often report NRC values higher than 1.0 due to the way the number is calculated in a laboratory. A test material’s area does not include the sides of the panel (which are exposed to the test chamber) which vary due to its thickness. A certain percentage of the sound will be absorbed by the side of the panel due to diffraction effects.

STC – Sound Transmission Class

Sound Transmission Class (STC) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used. The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time (that is the differences in test method from the 1970’s to today are vast).

The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce. Alternatively, Outdoor-Indoor Transmission Class (OITC) is a standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.

SAA – Sound Absorption Average

The single number rating obtained from ASTM C423 is the Sound Absorption Average (SAA). This is the average of the absorption coefficients for the twelve one-third octave bands from 200 to 2500 Hz.  The SAA supersedes the Noise Reduction Coefficient (NRC), which is the average of the sound absorption coefficients of a test specimen for 250, 500, 1000 and 2000 Hz rounded to the nearest multiple of 0.05. The higher the SAA or the NRC value, the better the material absorbs sound.

The Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA) values are both single number ratings that indicate the level of sound absorption provided by the product being tested. The NRC value is the average of the sound absorption coefficients at four 1/3 octave frequencies (250, 500, 1000 and 2000 hertz). The SAA value is the average of the sound absorption coefficients at twelve 1/3 octave frequencies ranging from 200 to 2500 hertz. The NRC value is rounded off the nearest 0.05 increment. The SAA value is rounded off the nearest 0.01 increment. The NRC and SAA values normally range from 0.00 to 1.00, with 1.00 indicating 100% sound absorption per square foot of material. These values can exceed 1.00 when thick specimens or specimens with large air spaces are being tested.

Contrary to popular belief, sound treatment and soundproofing are not the same process. Sound travels through large spaces as well as very small spaces and travels through walls, floors and other materials you would think would stop sound waves. When sound waves hit solid surfaces that reflect the majority of those sound waves, you have what is called reverberation. Reverberation is basically all of the echoes in a room combined, and in small areas, those echoes usually do not sound like an echo you would hear in a long hallway or large space like a gymnasium but instead a collision of sound waves making the sound unclear and often louder than the original source. To get a better idea of how sound and acoustics works, visit “Basic Behaviors of Acoustics”.

Sound travels through the air by vibrating the particles in the air. When sound hits a surface, that surface can either absorb some of the sound, reflect some of the sound or both. Every surface or object has different acoustical properties such as water transmits sound waves very well, wood vibrates and sends the sound waves through rooms and so on.

Sound Treatment

In the process of sound treatment, one would place sound absorption products on floors, walls, and ceilings in order to reduce the reverberation in a room and to reduce transfer of sound waves to other rooms. The keyword in the last sentence is reduce. All sound treatment products do not “soundproof” rooms, they only reduce sound travel where some products perform better than others. Fiberglass has been a preferred method of treating sound by absorbing as much of the sound waves as possible and reducing reverberation considerably. Recently, polyester products are becoming a standard to treat sound and although polyester products perform well, they are not as effective in sound absorption as fiberglass but this gap in deficiency is a narrow one. However, the reason polyester is becoming so popular is because it is easily made from recycled materials (post-consumer and post-industrial) as well as the end product being recyclable itself. Polyester is also much cleaner and easier to handle during manufacturing and installation and is much lighter in weight.

Soundproofing

Soundproofing is a difficult task and involves skilled engineers and/or plenty of trial and error. To sound proof a room, you are basically blocking every avenue that sound has the ability to travel. This includes walls, ceilings, floors, door jambs, windows, window jambs, A/C duct, and more. This even includes electrical outlets. The materials used to absorb sound are helpful but to truly soundproof a room, you will need to use sound barriers that prevent sound travel or any vibrations caused by sound.