Glossary Of Related Terms

TERMS AND DEFINITIONS

Absorbtion Coefficient: The fraction of incident sound that is absorbed by a given material, usually given in octave bands.

Ambient Sound: The combination of all near and far field sounds, none of which is particularly dominant.

A-weighted Sound Level: The sound level measure using the A-weighting network of a sound level meter. For broadband sounds, the A-weighted sound level is loosely correlated with the perception of loudness and is expressed in dBA.

Background Sound: All sounds, except sound from a particular source of interest.

Bel: One Bel =10 decibels (dB)

Decibel: The primary unit of sound measurement; used to quantify sound pressure level, sound power level, as well as several types of attenuation mechanisms.

Dynamic Insertion Loss: The sound insertion loss of a duct silencer while handling airflow.  It is expressed in dB.

Frequency: The number of oscillations per second, generally expressed in hertz (Hz) or cycles per second (cps).

Insertion Loss: The sound level reduction at a given location due to the insertion of a noise control device, such as a duct silencer or acoustical louver.  It is expressed in dB.  See also dynamic insertion loss.

Noise: Undesirable sound that interferes with rest, sleep, mental conentration, or speech communication.

Noise Reduction Coefficient: A single-number rating system used to compare the approximate sound-absorbing characteristics of building materials.

Octave: A continuous range of frequncies whose upper frequency limit is twice that of its lower frequency limit.  Typical center frequencies for HVAC analyses are 62.5, 125, 250, 500, 1k, 2k, 4k, 8k hertz, although the 8k hertz band is often ignored and the 31.25 hertz band is sometimes considered when investigating noise and vibration complaints.  The lower frequency limit for each octave band is 70.7% of the center frequency, while the upper frequency limit for each octave band is 70.7% of the center frequency, while the upper frequency limit is 141.4% of the center frequency.  For example, the 1k hertz octave band contains noise energy at all frequencing from 707 to 1414 hertz.

Octave Band Level: The sound pressure level of the sound energy within a single octave band.  It is expressed in dB.

Resonance: Of a system in forced oscillation, a phenomenon such that any change, however small, in the frequency of excitation results in a decrease in the response of the system.

Resonance Frequency: The frequency at which resonance occurs, Unit: hertz. Unit symbol: Hz.