Glossary of Common Acoustical Terms

Absorption Coefficient:  The energy of sound waves being taken in (entering) the surface of any material rather than being bounced off or reflected; usually given in octave bands. Materials are rated in terms of their ability to absorb sounds.

Acoustical Louver: A specially built louver using sound-attenuating baffles instead of single-thickness bent steel or extruded aluminum blades.

Acoustical Material: Materials which have high absorption coefficients.  

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

Amplitude: The maximum variation of any wave from its mean value. Increasing a sound wave's amplitude increases its loudness.

Attenuation: The reduction of sound level per unit distance by divergence, diffusion, absorption, or scattering.  In acoustics, the diluting or holding back of the energy of sound waves as they pass through a material.  Materials are rated for their ability to prevent sounds from traveling through them.

A-weighted Sound Level:  The sound pressure level which has been filtered or weighted to quantitatively reduce the effect of low frequency noise. It was designed to approximate the response of the human ear to sound. A-weighted sound pressure level is measured in decibels with a standard sound level meter which contains the "A" weighting network. A-weighted decibels are abbreviated dBA and indicates approximate relative loudness.

Ceiling Attenuation Class:  Rates a ceiling's efficiency as a barrier to airborne sound transmission between adjacent closed offices. Shown as a minimum value, previously expressed as CSTC (Ceiling Sound Transmission Class). A single-figure rating derived from the normalized ceiling attenuation values.

Ceiling Sound Transmission: In a suspended ceiling construction, the transmission of sound between adjoining rooms by way of the path consisting of the ceiling of each room and the continuous plenum over, and common to, both rooms.

CNEL: Community Noise Equivalent Level;  the average (on an energy basis) noise level measured in A-weighted sound pressure level for a twenty-four (24) hour period with different weighting factors for the noise levels occurring during the day, evening, and nighttime periods.

Decibel (dB): The primary unit of sound measurement; used to quantify both sound pressure level and sound power level.  In acoustics, equal to ten times the logarithm of the ratio of one sound and a lower-intensity reference sound. One decibel indicates a difference of about 26% and is about the smallest change the ear can detect. The dB level is a logarithm quantity; the maximum normal level is approximately 120dB.

Field Impact Insulation Class (FIIC): A single-number rating for impact sound insulation, calculated from measured values of normalized impact sound pressure levels.

Field STC (FSTC): Sound Transmission Class based on field measurements of sound transmission loss of a partition.

Flanking Path: An indirect sound transmission path, such as the structure-borne path between two adjacent rooms, that bypasses a transmission barrier. 

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

Fundamental Frequency: The lowest frequency of a vibrating system.

Hertz (Hz): One cycle per second.

Insertion Loss: The sound level reduction at a given location due to the insertion of a noise control device, expressed in decibels.

L_dn: Day-Night Average Sound Level - A-weighted equivalent continuous sound exposure level for a 24-hour period with a 10 dB adjustment added to the sound levels occurring during nighttime hours (10 p.m. to 7 a.m.)

L_eq: Equivalent sound level. The dBA level of a steady state sound which has the same dBA weighted sound energy as that contained in the actual time-varying sound being measured over a specific time period.

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

Noise Criteria (NC): A single-number noise rating system published in 1957 to rate steady-state continuous noise in a room from all types of equipment, including fans, mixing boxes, diffusers, etc.

Noise Isolation Class (NIC): A single-number rating for noise isolation calculated from measured values of noise reduction between two enclosed spaces.

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

Octave: A range of frequencies whose upper frequency limit is twice that of its lower frequency limit. For example, the 1000-hertz octave band contains noise energy at all frequencies from 707 to 1414 hertz.

1/N Octave Analysis: Spectrum analysis using filters whose bandwidth is a fractional ratio of the center frequency of the filter. For example, a 1/3 octave filter centered at 1000 Hz would have a bandwidth of 260 Hz (26% equals 1/3 octave). Bandwidth (relative to a normalized center frequency of 1) is computed as 2^(1/N)-1. The typical bandwidths used (primarily for acoustical and vibration analysis) are 1/1 octave, 1/3, 1/12, and 1/24 octave.

Resonance: The natural oscillation of a construction assembly or air column that persists after the shutoff of an outside excitation. The ringing that you hear after hitting a bare round sheet metal duct is an example of resonance.

Reverberation: Persistence of reflected sound in a room after its source has stopped emitting sound.

Reverberation Time:  Time required for the sound pressure level in a room to decay to a value one millionth of its original intensity, or to drop 60 decibels.

Room Criteria (RC): A single-number noise rating system developed in 1981 to diagnose and rate the HVAC noise exposure in a room. This system is more effective than the Noise Criteria (NC) System in rating noise with strong low-frequency content.

SENEL: Single Event Noise Exposure Level;  also referred to as SEL; the time-integrated A-weighted sound pressure level of a single aircraft flyover (which exceeds a threshold noise level) which is expressed by the level of an equivalent one-second duration reference signal.  SENEL provides a measure which quantifies the effect of duration and magnitude for a single event measured above a specified threshold.

Sound Attenuation: The reduction in the intensity or in the sound pressure level of sound which is transmitted from one point to another.

Sound Leaks: Cracks under doors, openings in a wall, pipe or wiring holes, etc., which allow sound to escape through a structure from one room to another.

Sound Level Meter: A meter, usually handheld, that is used to measure sound pressure levels.

Sound Power Level (L_W or PWL): The level, in dB, at which a source produces sound, usually given in octave bands.

Sound Pressure Level (L_p or SPL): The level of sound energy, measured in dB, at a specific location. The frequency range of the measurement or calculation must be indicated along with the sound level in dB.

Sound Transmission Class (STC): A single-number rating system used to compare the sound-isolating characteristics of partitions used to separate occupied spaces. Assemblies are rated in terms of Sound Transmission Classifications. An STC value of 20-25 would indicate that even low speech would be audible in an adjoining room. An STC value of 50-60, on the other hand, would indicate that loud sound would be heard only faintly or not at all.          

Sound Transmission Loss: The amount of sound lost as the sound travels through a material.

Structure-borne Sound: Sound that radiates from a construction assembly after traveling through a building's structure in the form of vibration.