Whole System Acoustical Treatments
Sound Transmission Class
With the acceptable background noise levels thus established, the building designers need to focus on achieving them by restricting unwanted sound from entering the spaces. This means creating wall, floor, and roof components or assemblies that first effectively block the amount of airborne sound transmitted through them. The measurement for this effectiveness is determined by a Sound Transmission Class (STC) rating. A higher STC rating means that more airborne sound is blocked by the component or assembly. Lower STC ratings mean that more sound passes through the components or assemblies adding to the background noise level in the space, degrading the ability to hear and understand speech.
It should be noted that, contrary to the popular notion that sound passes through a structure, such is not the case. Sound generated on one side of a wall will energize the wall structure and set it in motion, much like a diaphragm. The wall itself becomes the transmitter of the sound energy which can be heard on the opposite side of the wall by the listener. Hence, the ASTM test methods used to determine STC ratings have focused on this direct transmission process, although this testing has changed over the years meaning that STC results posted before 1999 may not produce the same results today. Currently, the STC number is derived from sound attenuation values tested at 16 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 Transmission Loss curve (TL) to determine a final 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. As a supplement to STC ratings, 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. At least one significant research study considered students at a school in the regular flight path of an airport. After taking into account variables such as socioeconomic status, students in that school performed as much as 20% lower on a reading test than students in another nearby school (G. W. Evans & Maxwell, 1997).
In educational settings under ANSI Standard S12.60-2002, single or composite walls, floor/ceiling and roof/ceiling assemblies should provide specific sound transmission class (STC) ratings whenever separating a core learning space (e.g. a classroom) from other specific adjacent spaces as follows:
- STC-45 if the adjacent space is a corridor, staircase, office or conference room.
- STC-50 if the adjacent space is another core learning space, speech clinic, health care room or outdoors.
- STC-53 if the adjacent space is a restroom.
- STC-60 if the adjacent space is a music room, mechanical equipment room, cafeteria, gymnasium or indoor swimming pool.
- Classroom doors should be rated as STC-30 or more, and music room doors as STC-40 or more. Commonly, entry doors located across a corridor are staggered to minimize noise transmission from one room to another across the hallway.
It should be noted that open-plan classroom designs will not meet the requirements of this standard since there is nothing to impede the sound transmission from one space to another. In addition, STC ratings ranging from 45 to 60 are also outlined in the ANSI Standard for assemblies separating non-classroom spaces from adjacent spaces.
Ambient or Background Noise Level
Is the totality of all sounds within the room when the room is unoccupied.
In an unoccupied space, sounds can be heard from a variety of sources. Careful scrutiny of the room can lead to identifying the intrusive sources. The diagram illustrates a few of the most common sources of noise.
Illustration courtesy of Acoustical Surfaces, Inc.
The Sound Transmission Class is a rating of the effectiveness of a material or construction assembly to retard the transmission of airborne sound. The sound transmission loss between the source and receiving rooms are plotted on a graph by frequency and sound level in decibels. Once the appropriate contour has been selected the STC is determined by the decibel value of the vertical scale at 500 Hz. The STC is expressed as a single STC number (e.g. STC 32).
Chart and photo courtesy of Acoustical Surfaces, Inc.
Impact Insulation Class
Beyond airborne sound, multi-story building designs need to address the resistance of structure borne sound, usually created by people walking or creating other impacts onto the floor/ceiling above the classroom space. Similar to STC ratings which address airborne sound, floor/ceiling assemblies can be tested or calculated based on Impact Insulation Class (IIC) ratings. These IIC ratings reveal the ability of a floor-ceiling assembly to absorb or deflect impact/structure borne noise and keep it from being transmitted to the space below. A floor/ceiling assembly with a low IIC rating will allow distracting noise to be transmitted into the room below leading to the associated problems of distraction and hampered communication. As such, Standard S12.60-2002 identifies specific ratings and recommendations for classroom learning spaces including the following:
- IIC ratings for floor-ceiling assemblies above core learning spaces should be at least IIC-45 and preferably IIC-50 as measured on floors without carpeting.
- In new construction, a gymnasium, dance studio, or other spaces with high floor impact activities shall not be located above core learning spaces.
- In existing facilities IIC-65-70 (depending on the volume of the space below) is recommended if gymnasia, dance studios or other spaces with high floor impact activities are located above core learning spaces.