Common Acoustic Definitions Explained


Noise Reduction Coefficient (NRC)

NRC is a single-number rating representing and overview of how much sound is absorbed by a material. Example: ½” gypsum board (“drywall”) on 2×4 studs has an NRC of 0.05.

Soft materials like acoustic foam, fiberglass, fabric, carpeting, etc. will have high NRCs; harder materials like brick, tile and drywall will have lower NRCs. A material’s NRC is an average of its absorption coefficients at 250, 500, 1000 and 2000 Hz. In general, the higher the number, the better the absorption. NRC is useful for a general comparison of materials. However, for materials with very similar NRCs, it is more important to compare absorption coefficients.

Sound Absorption Coefficient (a)

The actual absorption coefficients of a material are frequency dependent and represent how well sound is absorbed in a particular octave or one-third octave band. Example: ½” drywall on 2×4 studs has an absorption coefficient at 125 Hz of 0.29.

Comparing the absorption of materials should involve a comparison of their respective absorption coefficients in the different bands. Provided the materials are tested in a similar fashion, the material with a higher absorption coefficient in a particular band will absorb more sound in that band when you use it in your room. Be careful though: Materials are tested using different mounting methods. For example, if one material is tested by laying the materials out on a predetermined area of the floor – called A mounting – and another tests their materials by spacing them off the floor by several inches, then the comparisons are “apples and oranges.” To truly compare, find numbers derived from tests that used the same layout of materials in the test chamber. Also, there are three main standard methods used to test materials for absorption. Two of them are reverberation chamber methods – ASTM C423 in the U.S.A. and ISO 354 in Europe. These two methods are quite similar, but the ISO method – in general – will produce slightly lower overall numbers than the ASTM method. The other method is the impedance tube method, or ASTM C384. This method places a small sample of the material under test at the end of a tube and measures the absorption. Again, the numbers from this test are usually lower since a different method of calculation is used. They are also not as representative of real-world applications of materials relative to the reverberation chamber methods.

Sound Transmission Class (STC)

STC is a single-number rating of how effective a material or partition is at isolating sound. Example: ½” drywall has an STC of 28.

Hard materials like rubberized sound barriers, concrete, brick and drywall will have high STCs. Softer materials like mineral fiber, acoustic foam and carpet will have much lower STCs. Virtually every material filters out some of the sound that travels through it, but dense materials are much better at this than are porous or fibrous materials. Like NRC, STC is useful to get an overview-type comparison of one material or partition to another. However, to truly compare performance, the transmission loss numbers should be reviewed.

Sound Transmission Loss (STL or TL)

STL represents the amount of sound, in decibels (dB), that is isolated by a material or partition in a particular octave or one-third octave frequency band. Example: ½” drywall has an STL at 125 Hz of 15 dB.

Comparing material or partition performances should involve comparing the STLs of each in the different bands. If both materials or partitions are measured in accordance with the STL/STC standard, ASTM E90, then the comparisons being made will be “apples to apples.” It should be noted that real-world performance is not going to provide the same level of STL that is achievable in the laboratory. However, the relative performance of one material or partition versus another typically holds true in real-world construction. I.e., if the lab measures one partition better than another, it should hold true for a real partition built in your studio. Even though an actual field test of a concrete wall might reveal a field STC (FSTC) that is 5 points lower than the lab test, it is still better – relatively speaking – than a simple, single-leaf, uninsulated drywall partition in the same configuration.


This is the concept of detaching partitions from each other, or physically detaching layers in a partition in order to improve sound isolation.

The most common methods of decoupling are:

• Air gaps or air spaces between two partitions.

• Using resilient channels (RC8 from Auralex) between layers and structural framing members for walls and ceilings.

• “Floating” a floor using springs, rubber isolators (such as U-Boats from Auralex), or other decoupling layers.

Room Modes

A room mode is a low frequency standing wave in a room.

Normally, this is a small room phenomenon, though large rooms have (very, very low) modes as well. A mode is basically a “bump” or “dip” in a room’s frequency response that is facilitated by the room’s dimensions and the way those dimensions cause sound waves to interact with each other. There are three types of room modes

• Axial modes: Standing waves between two parallel surfaces.

• Tangential modes: Standing waves between four surfaces.

• Oblique modes: Standing waves between six surfaces. (Oblique modes are more complex, higher in frequency and decay faster. Therefore, they are not typically a big problem.)

For a complete treatment of modes, there are ample discussions in acoustic reference books. There are intricate formulas in these texts that can help you determine your room’s modes. There is also software that can do the same. We have developed our own proprietary software and would be glad to work with you or your salesperson in figuring your room’s modes to help steer you in the direction of the proper acoustical treatments. (Note that rectangular rooms are the easiest to predict. Our software is based on rectangular rooms. For non-rectangular spaces, we can assist to a degree, but the software required to actually predict the exact modes – which Auralex does not use – is much more complex.)