Glossary

Common Acoustical Terms

Absorption Coefficient:  The fraction of energy of sound waves being absorbed on the surface of any material rather than being bounced off or reflected in a specific frequency octave band.  Materials are rated in terms of their ability to absorb sound.

Acoustical Sealant:  Caulking product designed to remain flexible to prevent sound leaks through cracks or gaps in a wall and/or floor/ceiling assembly.

Acoustical Underlayment:  A product designed for reducing impact noise in floor/ceiling assemblies. Types of underlayments can vary, although they are typically placed either directly below hard surface finish flooring or beneath a layer of structural lightweight concrete.

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

Attenuation:  The reduction in sound pressure level as sound is transmitted from one point to another.

Average Sound Level (Leq):  Also known as equivalent sound level and expressed in dBA. The A-weighted sound level of a steady state sound which has the same sound energy as that contained in the actual time-varying sound being measured over a specific time period.

A-weighted Sound Level (dBA):  Designed to approximate the response of the human ear to sound.  A sound pressure level which has been filtered or weighted to quantitatively reduce the effect of low frequency noise.

Community Noise Equivalent Level (CNEL):  The 24-hour weighted average noise level calculated as A-weighted sound pressure levels with different weighting factors for the noise levels occurring during the evening and nighttime periods.  This weighting is applied to account for an individual’s increased sensitivity to noise during these times.  Sound levels during evening hours of 7 p.m. to 10 p.m. have an added 5 dB weighting, and sound levels during nighttime hours of 10 p.m. to 7 a.m. have an added 10 dB weighting.

Day-Night Average Sound Level (Ldn):  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.).

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.

Field Impact Insulation Class (FIIC):  A single-number rating for establishing impact sound insulation in the field (versus a laboratory setting), calculated from measured values of impact sound pressure levels.  Although this metric is no longer recognized by ASTM, it still may appear in Building Code and/or homeowners’ association documents.  What the California Building Code refers to as FIIC is now known as Impact Sound Rating (ISR).

Field Sound Transmission Class (FSTC):  A single-number rating for establishing sound transmission class in the field (versus a laboratory setting), calculated from field-measured values of sound transmission loss of a partition, in accordance with ASTM Classification E413.  As true FSTC measurements require the appropriate blocking and treatment of all flanking paths, more commonly seen metrics for determining noise isolation between two interior spaces are Noise Isolation Class (NIC) or Normalized Noise Isolation Class (NNIC).

Flanking Path:  An indirect sound transmission path, such as the structureborne 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).

Impact Insulation Class (IIC):  A single number rating calculated in accordance with ASTM E492 and E989 that is used to describe the transmission of impact noise through floor/ceiling assemblies, caused primarily by footsteps, from one space to another.

Impact Sound Rating (ISR):  A single-number rating for establishing impact sound insulation in the field (versus a laboratory setting), calculated from field-measured values of impact sound pressure levels of a partition, in accordance with ASTM E1007.  Equivalent to what the California Building Code refers to as Field Impact Insulation Class (FIIC).  If normalized to a standard reverberation time, this metric is known as Normalized Impact Sound Rating (NISR).

Insertion Loss:  The sound level reduction at a receiver that occurs when a sound-attenuating device, such as a silencer or barrier, is inserted in the path between source and receiver.  Expressed in decibels at a specific frequency octave band.

Noise:  Unwanted sound.

Noise Criterion Curves (NC):  A single-number noise rating system used to rate steady-state continuous noise in a room.  Typically used to describe mechanical noise.

Noise Isolation Class (NIC):  A single-number rating calculated in accordance with ASTM Classification E413 for noise isolation calculated from measured values of noise reduction between two enclosed spaces which is not normalized to a standard reverberation time.  If normalized, this metric is known as Normalized Noise Isolation Class (NNIC).

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.

Resilient Channels:  A method of decoupling used for installing gypsum board designed for increased sound isolation.  The channel is attached to the framing members and the gypsum board is then installed directly to the channel.  Designed to prevent rigid contact between the gypsum board and framing.  When properly installed, resilient channels can increase the STC and/or IIC ratings of a partition.

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

Reverberation Time (RT60):  Time required for the sound pressure level in a room to drop 60 decibels after the source has stopped.

Room Criterion Curves (RC):  A single-number noise rating system used to diagnose and rate the HVAC noise exposure in a room. This system is often considered to be more effective than the Noise Criterion (NC) rating system as the numerical rating is accompanied by a “quality” descriptor which explains how the sound might be subjectively described by a listener.

Sound Level Meter:  An instrument, usually handheld, that is used to measure sound pressure levels with averaging capabilities and standard frequency-weighting.

Sound Power Level (LW or PWL):  A level, in dB, used for describing sound intensity that is not accompanied by a reference distance.  Measures all of the energy emitted by a sound source, per unit of time, regardless of the space in which the source is located or distance from the source.  Commonly used by manufacturers of mechanical equipment.

Sound Pressure Level (Lp or SPL):  The level of sound energy, measured in dB, at a specific location.  In order to be meaningful, a sound pressure level measurement must be accompanied by a reference distance at which the sound source was measured.

Sound Transmission Class (STC):  A single-number rating system calculated in accordance with ASTM Classification E413 used to compare the sound-isolating characteristics of partitions used to separate occupied spaces.  Can be used to describe walls, floors, ceilings, windows, or doors.

Structureborne Sound:  Sound that radiates from a construction assembly after traveling through a building’s structure in the form of vibration.

Transmission Loss:  The amount of sound lost, in a specified frequency octave band, as the sound travels through a material.

Vibration Isolator:  A mount or hanger on which equipment may be installed to reduce the amount of vibration transmitted to the structure.