Absorption
The amount of sound energy incident upon a surface which is absorbed, changed to heat energy, and not reflected from that surface as sound.

Acoustic impedance
The ratio of sound pressure at the surface to the volumetric velocity through it.

Attenuation
The reduction of sound intensity, measured in decibels, that is caused by any factor such as distance or the presence of absorbing materials.

Decibel (dB)
A unit of sound intensity. It is the logarithmic ratio, to the base 10, of the intensity of sound to be reported compared to a standard reference sound intensity. Practically, a change in intensity of 2 dB is not noticeable to the ear, 5 dB is noticeable and 10 dB is twice as loud (or soft). The decibel is a commonly reported as dB(A), where the A reference conforms to the response of the ear.

Dew point
The temperature at which a sample of air, with no change in pressure or water vapour content, becomes completely saturated, i.e., attains a relative humidity of 100%.

Emissivity
A rating of the ability of a material to give off heat as radiant energy. It is expressed as the percentage of energy given off by a perfect black body radiator.

Flanking transmission
The transmission of sound between two points by an indirect path.

Frequency
The number of vibrations per second. The unit is Hertz (Hz) which represents the number of vibrations per second.

Heat transmission coefficient (U)
A unit expressing heat passage through a complete building section, including air films. Technically, heat transmission is measured in Watts per square metre, per degree C of temperature difference from air to air for a composite building section. It is used as a basis for determining transmitted heat loss or gain.

Insertion loss
The sound attenuation achieved by insertion of a sound absorbent material.

Intensity
Sound intensity is proportional to the sound power or energy.
Latent heat: The heat required at constant temperature to cause a change of state in matter.

Mass law
The attenuation or sound transmission loss due solely to the surface density or weight per square metre of a material. In general, a doubling of mass will improve attenuation by 4 to 5 dB for a homogeneous materials. A non-homogeneous structure such as a partition wall can give much greater attenuation due to stiffness, vibration and reverberation control. For example, use of infilling in a light-weight partition can improve attenuation by 3 to 6 dB, with little increase in mass of the structure.

Noise criteria (NC) curves
Specify the maximum noise levels permitted in each octave band for a specified single rated NC number. Normally used in architectural specifications.

Noise reduction coefficient (NRC)
A single-number sound absorption coefficient used as a guide to the average acoustic performance of a material. It is the arithmetic average of sound absorption coefficient at 250, 500, 1000 and 2000 Hz (to the nearest 0.05).

Octave
Two sound frequencies are an octave apart when one frequency is twice the other.

ODP (Ozone Depleting Potential)
Glasswool, rockwool, polywool, polyethylene foams, loaded vinyl, foils and tapes are free from any ozone depleting potential.

R-Value
See “Thermal Resistance”

Reflectivity
A measure of the ability of a material to reflect heat moving by radiation through air. It is expressed as a ratio of radiant heat reflected by a material to the radiant heat reflected by a perfect mirror under similar conditions.

Resonance
A build-up in amplitude of vibrations of a structure, which occurs when the frequency of the sound impinging on the structure corresponds to the natural frequency of the structure.

Reverberation
The persistence of sound in a room after the sound source has stopped. The greater the absorption in the room, the lower the reverberation.

Sensible heat
The heat required to change the temperature of a substance without change of state.

Sound absorption coefficient (SAC)
The proportion of incident sound energy which is not reflected.

Sound breakout
The residual sound energy transferring through the walls of a duct system from a generated sound source within the system.

Sound transmission class (STC)
A single number rating displayed on standardised curves which represents the dB transmission loss performance over all frequencies. The higher the STC, the more efficient the structure.

Surface coefficient
Refers to the thermal conductance of an air film immediately adjacent to an exposed surface of a material. Commonly called the ‘f’ factor.

Thermal conductance
A unit specifying the amount of heat, in Watts, that passes through a square metre of material which has a given thickness and one degree Celsius of temperature difference between its surfaces. Used to compare insulating efficiencies of material with varying but stated thickness or materials composed of two or more basic materials.

Thermal conductivity (k)
A unit expressing the amount of heat, in Watts, that passes through a square metre of material which is exactly one metre thick and has one degree Celsius of temperature difference between its surfaces. Used for comparing the insulating efficiencies of homogeneous materials.

Thermal resistance (R-value)
The reciprocal of thermal conductance. Measure of a products ability to resist the flow of heat. Units of m2K/W.

Thermal resistivity (r or 1/k)
The reciprocal of thermal conductivity.

Transmission loss
The reduction in sound intensity across a wall. It is the ratio of incident sound to sound transmitted through the wall.

Vapour barrier
A material that does not readily permit the passage of water vapour. Normally a material is rated at one perm or less. (Perm = a vapour transmission rate of 1 grain of water vapour per square foot, per hour, per inch of mercury pressure difference).

Vapour permeability
A rating of material giving the amount of water vapour that passes through a precise thickness of the material.

VOC (Volatile Organic Compounds)
Organic chemicals that have a high vapour pressure at normal room-temperature conditions. Many VOCs are dangerous to human health or cause harm to the environment.

Watt
A unit of rate of heat loss or gain from a material, measured as joules per second.

Weighted sound reduction index (Rw)
A single figure index that gives an indication of the sound isolation of a partition, measured in decibels. This is commonly nominated by the Australian Building Code as a standard for habitable (Rw30) and non-habitable (Rw45) rooms.