Tuesday, November 12, 2019

Understanding STC & FSTC Ratings

Here at SoundAway, we're in the business of sound. More precisely, we deal with sound isolation and changing sound quality in specific spaces to best suit your needs. That space may be a large auditorium or concert hall where sounds need to be contoured to reach each individual section of the space, or a small piano practice room where sounds need to be attenuated to a volume low enough so as not to disturb individuals practicing in adjacent areas.

Sound Vibrations
Sound vibrations decrease in volume as they travel through the air, as is demonstrated by yelling to someone standing a block away from you and someone else 10 feet away. The closer to the sound source, the louder it seems.

In addition to volume reduction caused by increased distance, you can also affect sound loudness by introducing a physical barrier to block the sound. This barrier could be a wall, a door, a window or some type of panel. Each of these barriers will have a different effect on the sound it’s blocking, depending on the sound-blocking ability.

Human sensitivity to changes in sound intensity levels is also worth considering. Measurement of sensitivity to differences in the rate of change of auditory signal parameters is complex and must take into account duration, extent of change, and velocity of the changing sound. Here is what you can typically expect:
1 dB / generally not perceptible
3 dB / just perceptible
5 dB / clearly noticeable
10 dB / twice as loud (or half as loud if it describes drop in sound level)
20 dB four times as loud

There are Two Measurement Units of Airborne Noise:
  1. Sound Transmission Class (STC) – associated with tests carried out within a controlled laboratory environment. Laboratory tests results are generally higher than those performed in the field.
  2. Field Transmission class (FSTC) – associated with tests carried out in the field. This allows for more of a real-life measurement that factors in things such as background noise levels, surface areas, environmental factors, etc.
Requirements of the National Building Code allow for a minimum acoustic rating of FSTC 50. At this level, the soundproofing is acceptable but you may still be able to hear a neighbor who snores loudly. FSTC 55 is a bit more comfortable and the desired acoustic comfort is around FSTC 60.

Classification / STC / FSTC
Minimum code / 50 / 45
Minimum quality / 55 / 50
Medium quality / 60 / 55
High quality / 65 / 60

STC Ratings
In the soundproofing industry, the ability of a barrier such as a wall or a door to reduce sound volume from one side of the barrier to the other is measured by a standard called STC, which stands for sound transmission class. This is a standard used primarily in the U.S. to rate partitions in a building as to their sound-attenuating capability. These partitions include interior and exterior walls, doors, windows, floors and ceilings. STC ratings provide you with an approximate idea of how much airborne sound a particular barrier such as an interior wall could stop. Simply stated, sound volume ratings in decibels (dB), measuring how loud a sound is, are recorded on both sides of the barrier being tested.

If the noise you're measuring is 90 dB, which is the volume of a typical police car siren, and the barrier decreases the sound to 70 dB on the other side of the barrier, you're said to have a 20 dB transmission loss. Note that if the test tone used in the above example is altered in pitch (frequency or Hz), the resulting transmission loss figure may be totally different.

STC ratings, introduced in 1961 as a means for comparing various types of barriers (wall, window, door, floor and ceiling assemblies), are often used to compare different products from competing manufacturers. STC is determined by taking transmission loss figures at 16 standard frequencies falling between 125 Hz and 4000 Hz. The results are plotted on a graph to form a curve, and this curve is compared with standard STC curves the industry has adopted.

STC Examples - Expected Field Results
45 / Normal voice not audible, raised voice plainly audible
50 / Loud voice understandable, raised voice not audible
55 / Shouting voice understandable, loud voice plainly audible
60 / Shouting playing audible, loud voice audible
65 / Shouting audible, loud voice not audible
70 / Very loud music understandable
75 / Very loud music plainly audible
80 / Very loud music audible

Note: The International Building Code requires a minimum of STC 50 for floors, ceilings and walls in new construction.

Shortcomings of STC Ratings
Typically, STC ratings numbers run between 27 and 72. The higher the number, the better the soundproofing capability of the barrier tested. It is important to note that STC Ratings are measured on a logarithmic scale: the values are not linear. Therefore, sound measuring at 50 decibels is not twice as loud as a 25-decibel sound. Here are some useful examples:

STC 27 – Single-pane window glass
STC 46 – 1/2” drywall glued to a 6” concrete block and painted on both sides
STC 72 – 8” painted concrete block wall with 1/2” drywall on both sides, installed on independent steel studs, with insulation in all cavities

One of the problems that exists with accurate STC ratings is that frequencies below 125 dB are not considered. Unfortunately, many sound-isolation problems come from noise sources that are below this 125 dB threshold. This includes most sounds emanating from home theaters; noise from heavy equipment, airplanes and trucks; musical instruments such as guitars, bass guitars and drums; and certain industrial equipment. For this reason, it's a mistake to rely solely on STC ratings to determine sound isolation capabilities, especially where low-frequency sounds are involved.

Monday, November 11, 2019

Understanding IIC Ratings

Impact insulation class (IIC) ratings represent a standard used in the building trade to designate the capability of a floor/ceiling assembly in a multistory building to attenuate sounds transmitting from one level to the next level below. Whenever something that produces sound occurs on an upper floor, whether that be a footfall hitting the floor, an object dropping or furniture being moved about, the amount of noise that's transmitted to the room below, called impact sound transmission, can be measured on a sound scale developed for this purpose.

The IIC was developed by ATSM International (formerly the American Society for Testing and Materials) using ATSM Method E989, incorporating a five-hammer tapping machine that's specified in ATSM Method E492. The five hammers on the tapping machine are placed in a line, and during testing, each hammer is lifted and then dropped in sequence, causing a total of 20 impacts per second hitting the floor. A sound level meter is placed in the room below to measure the difference between the initial sound, the frequency and force of which is known, and the resulting sound below. An industry standard is used to determine the sound reduction, or transmission loss, in dB (decibels).

Impact Sound Transmission
Unwanted sound vibrations transmitted through floor/ceiling assemblies are annoying, and assemblies that can reduce or eliminate these noise levels are highly desirable. These impact sound transmissions are attenuated, or lessened, by a variety of different flooring materials and floor coverings. Consider the difference in noise transmission to the room below if you have a bare hardwood floor or a floor covered in a thick carpet on top of a quality pad. The difference is significant.

Each of these floors and floor covering types have an IIC rating, typically falling somewhere between 25 and 85+. Higher numbers signify better noise reduction. IIC 50 would be considered the least amount of acceptable impact sound transmission between occupied floors and would be unsatisfactory to many occupying the lower floor. IIC 60 is considered medium noise reduction, and IIC 65 would likely be an acceptable noise reduction level for most lower-level occupants.

Determining IIC
In testing a sample floor/ceiling assembly in the laboratory using a five-hammer tapping machine, sounds are generated at 16 standard frequencies between 100 Hz and 3150 Hz. Results from each tap get plotted on a graph, each point depending on the amount of lost impact sound from each tap. The resulting graph is then compared to a standard reference graph to determine the IIC rating.
This testing, done in a laboratory on a single sample section of a floor/ceiling assembly, is the least accurate of two methods for determining a specific IIC rating. A more accurate and realistic rating is obtained through a testing method occurring in an actual building after installation of the floors. When testing is conducted under these real-world conditions, a total IIC value is obtained:
  • Floor covering
  • Subfloor
  • Underlayment
  • Floor joists
  • Lower room ceiling
  • Sealants and adhesives used for installation
  • Sound-deadening materials such as insulation and resilient channels

This method of testing is known as the field impact insulation class (FIIC).

Cushioning the Blow
Quite simply, one of the easiest and most effective ways of reducing impact sound transmission is by “cushioning the blow.” Adding a thick carpet with a high-quality carpet pad is extremely effective in lessening impact sound transmission and improving a floor's IIC rating.

Floors made from resilient materials such as rubber, cork or vinyl can also provide a slight improvement in sound reduction. Floating floors, such as a wood finished floor sitting on a resilient underlayment can also provide a higher IIC rating value. On the other hand, a concrete floor covered directly with a non-resilient material such as wood, tile or stone has a low IIC due to the fact that there is no “give” in this type of flooring system.

Additionally, since the current IIC rating system only tests sounds within a range of 125 Hz to 3150 Hz, which is approximately the sound range of the human voice, noises existing below 125 Hz, such as those that may be heard when someone is walking above on a floor with a lightweight joist system, may be audible. Noises made by people walking on a floor with a loose joist construction are also not accurately accounted for by standard IIC testing.