A common acoustic issue in virtually any space is sound transmission. Sound transmission can be both airborne and/or structure borne vibration. (Structure borne vibration is assessed by a different standard, Impact Insulation Class - IIC, and is not addressed in this text). Airborne sound travels through the air and can transmit through a material, assembly or partition. Sound can also pass under doorways, through ventilation, over, under, around, and through obstructions. When sound reaches a room where it is unwanted, it becomes noise. Noise such as that from automobiles, trains and airplanes can transmit through the exterior structure of a building. In the same way, noise from mechanical equipment or speech can transmit from one room within a building to an adjacent space.
Sound transmission can cause noise control, confidentiality, and privacy issues. Sound from a noisy environment such as a mechanical equipment room or an area with loud activities or music can transmit through a partition into a quieter space. This will cause unwanted noise within the quieter space. This is not only an annoyance; in several cases it can cause the quieter space to become unusable for its intended purpose. Several spaces require confidentiality. Offices of counselors, lawyers, or human resource departments cannot function in a space where sound will transmit through the surrounding walls and into an adjacent space. In most other office situations if confidentiality is not an issue, privacy is. If sound transmission is not properly controlled, the space or environment will not provide privacy for its users.
Transmission Loss is a measurement of a partition's ability to block sound at a given frequency, or the number of decibels that sound of a given frequency is reduced in passing through a partition. Measuring Transmission Loss over a range of 16 different frequencies between 125-4000 Hz, is the basis for determining a partitions Sound Transmission Class.
The Sound Transmission Class (STC) is a single-number rating of a material's or an assembly's ability to resist airborne sound transfer at the frequencies 125-4000 Hz. In general, a higher STC rating blocks more noise from transmitting through a partition.
STC is highly dependant on the construction of the partition. A partition's STC can be increased by:
- Adding mass
Increasing or adding air space
- The weight or thickness of a partition is the major factor in its ability to block sound. For example, a thick concrete wall will block more sound than a thin gypsum/2x4 wall. Mass is commonly added to existing walls by adding additional layers of gypsum. When the mass of a barrier is doubled, the isolation quality (or STC rating) increases by approximately 5 dB, which is clearly noticeable.
Adding absorptive material within the partition
- An air space within a partition can also help to increase sound isolation. This, in effect creates two independent walls. However, the STC will be much less than the sum of the STC for the individual walls. The airspace can be increased or added to an existing partition. A common way to add an airspace is with resilient channels and a layer of gypsum. An airspace of 1 ½" will improve the STC by approximately 3 dB. An air space of 3" will improve the STC by approximately 6 dB. An airspace of 6" will improve the STC by approximately 8 dB.
- Sound absorptive material can be installed inside of a partition's air space to further increase its STC rating. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4-6 dB, which is clearly noticeable. It is important to note that often times, specialty insulations do not perform any better than standard batt insulation.
A partition is given an STC rating by measuring its Transmission Loss over a range of 16 different frequencies between 125-4000 Hz. 125-4000 Hz is consistent with the frequency range of speech. The STC rating does not assess the low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than speech, such as mechanical equipment or music.
Even with a high STC rating, any penetration, air-gap, or "flanking" path can seriously degrade the isolation quality of a wall. Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors. Noise will travel between spaces at the weakest points. There is no reason to spend money or effort to improve the walls until all the weak points are controlled.