Definitions:

When discussing the attributes of underlayment certain terms are commonly used.  The following is a list of the most commonly used underlayment terminology:

Decibel (dB)
A decibel is a logarithmic unit of measure of sound pressure.  Zero dB corresponds to the threshold of audibility while 130 dB corresponds to the threshold of pain.

Source of Sound	Intensity Level
Threshold of Hearing (TOH)	0 dB
Rustling Leaves	10 dB
Whisper	20 dB
Raindrops	40 dB
Normal Conversation	60 dB
Busy Street Traffic	70 dB
Vacuum Cleaner	80 dB
Large Orchestra	98 dB
Walkman at Maximum Level	100 dB
Front Rows of Rock Concert	110 dB
Threshold of Pain	130 dB
Military Jet Takeoff	140 dB
Instant Perforation of Eardrum	160 dB

Sound Transmission Class (STC):
STC ratings are a measure of the effectiveness of a given partition construction in reducing airborne sound transmission.  Because of the frequency range covered (125-4000 Hz), STC ratings are best used to evaluate speech privacy for partition and floors/ceilings separating adjacent offices, rooms in one and two family dwellings, hospital patient rooms, classrooms, dormitories, apartments, courtrooms, small conference rooms, etc.  The higher the STC value of a floor/wall, the better its ability to control sound transmission.

STC Rating	Speech Patterns
25	Speech heard through walls or floors
30	Loud speech fairly well understood
35	Loud speech heard; not understood
45	Some loud speech barely heard
50	Loud speech not heard

Impact Isolation Class (IIC)
A measure or specification of isolation effectiveness of building structures from impact noises such as slammed doors, dropped objects, footfalls, shuffled furniture, etc. The higher the IIC rating is, the better such isolation. Impact noises can be transmitted through walls, floors, and ceilings throughout a building and re-radiated at distant locations. Careful design and special construction materials (floating floors, isolation pads, resilient channels, spring rails, flexible connectors and hangers, for example) can help improve IIC ratings, which may be thought of as the structure-borne equivalent of the airborne noise ratings addressed by Sound Transmission Class (STC).

The test is performed by placing a tapping machine in a source room and measuring the noise levels in a receiving room, which is located on the opposite side of the partition being tested and is the one most directly affected by the noise. Multiple measurements are conducted for each test. When the tapping machine is activated, the machine taps the floor with five small steel hammers at the rate of ten taps per second. Several measurements are then made in the receiving room to determine the average sound level. The noise levels measured in the receiving room are divided into sixteen one-third octave bands ranging from 100Hz to 3150Hz inclusive. The tapping machine is then moved to a different location where another set of noise measurements are taken. This procedure is repeated two more times for a total of four locations and four sets of measurement data.

Noise Reduction Coefficient (NRC)
The Noise Reduction Coefficient of an acoustical material is defined as the arithmetic average, to the nearest multiple of 0.05 of its sound absorption coefficients at the four one-third octave bands with center frequencies of 250, 500, 1000, and 2000 hertz

Outdoor-Indoor Transmission Class (OITC)
A standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure. It is based on the ASTM E-1332 Standard Classification for the Determination of Outdoor-Indoor Transmission Class. An alternative similar standard for determining the rate of acoustic isolation of a separation between spaces is STC. While STC is based on a noise spectrum targeting speech sounds, OITC utilizes a source noise spectrum that considers frequencies down to 80 Hz (Aircraft/Rail/Truck traffic) and is weighted more to lower frequencies.

Noise reduction between rooms
The Noise Reduction (NR), in decibels, between two rooms is the amount by which the mean-square sound pressure level averaged throughout the source room exceeds the sound pressure level averaged through-out the receiving room.

Continued: Learn about the Physics of Sound

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