Within the building trades, acoustics is the science of controlling sound within a building. This can take many forms including: directing sound to the back rows of a theater, stopping sound from passing from one residential unit to another, preventing sound from escaping a loud musical practice room, and isolating vibrations from mechanical equipment. The ABCs of acoustics cover the three main types of addressing sound, which are described below.
Acoustics: Absorb, Block, or Cover
There are three methods of dealing with sound, commonly referred to as the ABCs. Sound can be absorbed, it can be blocked or redirected, and it can be covered. The method chosen depends on the final goal of the project.
Sound should be absorbed when the goal is to keep sounds within the room from reverberating or echoing. If a room has hard surfaces that reflect sound, it will take a long time for the sound level to diminish to a point that it is inaudible. This is most noticeable in an empty room with hard walls, floors, and ceilings: you can hear the sounds bouncing around the room. Installing sound absorbing materials, such as carpet, acoustic ceiling tiles, or fabric covered panels, will reduce the reverberation by absorbing the sound. Noise Reduction Coefficient, which is defined below, is used for rating a material's ability to absorb sound.
If the goal is to keep sound within a space, for privacy or other reasons, the sound should be blocked. This can be combined with absorption so the reverberation in the space is reduced. Blocking sound is generally accomplished by reducing short circuits between two spaces. Some common short circuits include HVAC ducts or grilles, back-to-back outlets, pipes that pass through a space, partitions that stop above the ceiling but do not extend to the structural deck above, and windows or other openings. After eliminating the short circuits, adding mass to the walls will help block the sound. For instance, a concrete wall blocks sound better than a stud wall. Additional layers of gypsum wall board and insulation in the wall cavity will also help block sound. Sound Transmission Class, which is discussed below, is the measurement of a material's ability to block sound.
Sound can also be covered to help maintain speech privacy. This is most noticeable in an office environment when the forced air system is turned off - you can immediately hear more conversations from a greater distance. Sound is generally covered with white or pink noise, either from the HVAC system or an electronic system. The idea is to provide just enough background noise to make nearby conversations unintelligible; the conversation is still there, but the quiet hum of the white noise masks it.
We have an article dedicated to acceptable room sound levels that will help you select the appropriate noise levels for different types of spaces.
Sound Transmission Class (STC)
Sound Transmission Class, abbreviated STC, is the measurement of a material's ability to block airborne sound within the frequency range of human speech. The STC number is the decibel (dB) reduction across a material or assembly. For instance, if a sound in a room is 60dB and that same sound in the next room is 20db, then the wall between the rooms has an STC rating of 40.
Most building codes require walls between dwelling units to have an STC rating of 50. However, very loud speech can still be heard through such a partition, so an STC rating of 55 to 60 is generally used in higher-end housing. It is important to note that STC ratings are given to partitions by rating agencies after testing in a laboratory environment. Partitions installed in the field can have an effective rating of 5dB lower and if significant short circuits in the construction are present (cracks, air gaps, back-to-back electrical boxes, ducts, etc.), the partitions can be ineffective at blocking sound. The table below provides an understanding of STC levels; however, sound levels are subjective and vary by individual.
| STC | AUDIBLE THROUGH PARTITION |
|---|---|
| 25 | Normal speech is clearly audible and understandable |
| 30 | Normal speech is hard to understand, loud speech is clearly audible |
| 35 | Loud speech is audible, but not clear |
| 40 | Loud speech can be heard, but is not understandable |
| 45 | Loud speech is barely audible |
| 50 | Very loud sounds, like musical instruments, are audible |
| 60 | Very Loud music is barely audible, power tools are audible |
| 70 | Power tools are faintly heard |
| 75+ | Most sounds are completely inaudible |
Outside-Inside Transmission Class (OITC)
Outside-Inside Transmission Class, abbreviated OITC, is the measure of an exterior wall's ability to block sound from transmitting into the building. It is similar to STC; however while STC measures transmission in the frequency range of human speech, OITC measures frequencies in the range of cars, planes, mechanical equipment, and other low frequencies that may be experienced outside a building. Understanding the OITC of an exterior wall is critical for buildings in loud environments, such as airports, hospitals, or loud urban environments.
Noise Reduction Coefficient (NRC)
Noise Reduction Coefficient, abbreviated NRC, is a measure of a material's ability to absorb sound within the frequency range of speech. A material with an NRC of 0 will reflect all sound that hits it. A material with an NRC of 1.0 will theoretically absorb all sound that hits it. Some materials have a listed NRC rating above 1.0 due to the complexity in testing a material within a laboratory. Although a material is tested based on its face dimensions, the material has an inherent thickness. Due to diffraction and since the edges absorb sound, the average sound absorption of the material as a whole is greater than that of its face.
It is important to reiterate that NRC is a measurement of noise reduction within the human speech range, so a material that has a high NRC rating, may actually be poor at absorbing music, mechanical equipment noise, or other very low or very high frequency sounds. In addition, NRC ratings are often provided based on a given assembly. For instance, a carpet manufacturer may indicate an NRC rating of .50, but this rating may be for the carpet, pad, and sub-floor -- it is not necessarily just for the carpet.
The table below gives some NRC ratings for common building materials.
| MATERIAL | NRC RATING |
|---|---|
| Brick | .00 - .05 |
| Carpet over concrete | .20 - .30 |
| Carpet with foam pad | .30 - .50 |
| Concrete (smooth) | .00 - .20 |
| Glass | .05 |
| Gypsum Wall Board | .05 |
| Plywood | .10 - .15 |
| Polyurethane Foam (1" thick) | .30 |
Ceiling Attenuation Class (CAC)
Ceiling Attenuation Class, abbreviated CAC, is effectively the STC of a ceiling tile. CAC is generally used to measure sound transmission between two spaces when the wall between them stops at or just above the ceiling. Since the wall does not extend to the underside of the structural deck above, the ceiling is the only barrier preventing sound transmission between the spaces.
