Acoustic Noise: any unwanted sound.

Acoustic Frequency: a measure of the pitch of a sound, expressed in Hertz (abbreviated Hz) or cycles per second (cps).

Acoustic Pressure: rapid fluctuations in the atmospheric air pressure at a point in space, expressed in microPascals (N/m2).

Air-borne Sound: Sound that travels from one space to another via the impact of air molecules on one another. This includes sound transmission through walls, windows, doors, ceilings and floors, provided that the source of the sound is in the air.

A-weighting: A commonly used frequency weighting that closely approximates the frequency response of the human ear. It should be noted that the human ear does not perceive sounds of equal sound pressure level as being equally loud if the frequencies are different. The ear is less sensitive at low frequencies. For example, a sound of 50 dB at 1000 Hz will sound twice as loud as a sound of 50 dB at 125 Hz, and four times as loud as a sound of 50 dB at 75 Hz.

A-weighted Sound Pressure Level: the overall sound pressure level of a sound (including all frequencies) after it has been frequency weighted with the A-weighting filter, abbreviated dBA.

A-weighted Sound Power Level: the overall sound power level of a sound after it has been frequency weighted with the A-weighting filter, abbreviated LwA.

Decibel: a numerical scale, which is proportional to the common logarithm of the ratio of the quantity of interest to some reference value, abbreviated dB. An increase or decrease of 10 dB represents an order of magnitude (10X) increase or decrease in the value of the quantity of interest. An increase or decrease of 20 dB represents two orders of magnitude (20X) increase or decrease in the value of the quantity of interest.

Sound Pressure Level: a measure of the strength or intensity of a sound, expressed in decibels (abbreviated dB) with a reference level of 20 microPascals (2 x 10-5 N/m2). The sound pressure level generated by a steady source of sound will usually vary with both distance and direction from the source.

Sound Power Level: a measure of the total acoustic power output (in all directions) of a sound source, expressed in decibels (abbreviated dB) with a reference level of 1 picowatt (10-12 watt). The sound power level of a source is totally independent of the receiver distance and location.

Frequency Range: the range of acoustic frequencies of interest. In terms of audio recording studios, this is usually limited to the range of 20 Hz to 20,000 Hz, which represents the full range of human hearing.

Room Background Noise: any and all noise in a room when the primary source of sound is off. This is also called ambient noise.

Noise Criteria: the highest level of background noise that is acceptable for a given space. This depends on frequency and the intended use of the space. Recommended background noise criteria for various spaces can be found in the 2003 ASHRAE Applications Handbook.

NRC Rating: a single number rating that describes a material's ability to absorb sound. The NRC rating considers only the frequency range from 250 Hz to 2,000 Hz. Values typically range from 0.05 to 1.0, with higher values representing better sound absorption.

Octave: the audible frequency range is often divided into bands of frequencies because sound transmission through solid barriers can vary dramatically with the frequency of the sound. The broadest bandwidth commonly used is an octave. An octave is any band where the highest included frequency is exactly two times the lowest included frequency. For example, the frequency band that covers all frequencies between 707 Hz and 1,414 Hz is an octave band. The band is identified by its center frequency, which is defined as the square root of the product of the highest and lowest frequency. In the above case the center frequency is 1000 Hz. The entire frequency range of human hearing can be covered in the following 10 standard octave bands: 31 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz, and 16000 Hz.

One-Third Octave: A frequency band that has a width (in Hz) that is only 1/3 of the width of an octave. It takes 31 one-third octave bands to cover the entire frequency range of human hearing. One-third octaves are used when octave band analysis does not provide adequate resolution in the frequency domain.

STC Rating: A single number rating describing a material's or an assembly's ability to prevent sound transmission through the assembly or the material. The STC rating only considers sound transmission loss values at frequencies from 125 Hz to 4000 Hz, and the higher frequencies are given more weight than the lower frequencies. Higher values represent better sound control, (i.e. less sound penetrating the material or assembly).

Structure-borne Sound: Sound that travels from the source to the receiving space primarily via solid materials. This includes sounds generated by impacts to a structure (e.g. hammering, drilling, etc.) and sounds from rotating mechanical equipment that are supported by the structure (e.g. pumps, motors, fans, etc.).

Subterranean Sound: Sound that travels from the source to the receiving space primarily through the ground. This includes sounds generated by any device that causes ground vibration including, street traffic, pile driving, core drilling, and earthquakes.

Transmission Loss: a value that is proportional to the common logarithm of the ratio of the incident to transmitted acoustic power, expressed in dB. A sound wave will pass through a material with a transmission loss of 0 dB without any loss in energy. A sound wave will pass through a material with a transmission loss of 10 dB with a 90% reduction in energy. If a material has a transmission loss of 20 dB, the transmitted wave will have only 1% of the energy of the incident wave.