Acoustical Term Index

Below are a few definitions of some acoustical terms that you might run into on this site. This is only a brief list so if you have questions about anything else on our site, please Contact Us and we would be glad to help. In addition to the definitions provided, here is an additional source recommended for further study of acoustical term definitions:


Noise Reduction Coefficient (NRC)

NRC is a single-number rating representing an overview of how much sound is absorbed by a material. Example: 1⁄2” 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.

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 1/3 octave band. Example: 1⁄2” drywall on 2″x4″ 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: 1⁄2” 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.

Music/Machinery Transmission Class (MTC)

MTC or Music/Machinery Transmission Class is a newer calculation based on STC with more emphasis on accuracy for the lowest two frequency bands.


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 (like our RC-8) between layers and structural framing members for walls and ceilings.
  • “Floating” a floor using springs, rubber isolators (such as U-Boats), 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 also utilize commercially-available 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.