Acoustical Terms and Definitions

Acoustical terminology can be confusing because laboratory ratings, field ratings, environmental-noise metrics, occupational-noise metrics, and speech-intelligibility metrics are not interchangeable. This glossary summarizes terms that commonly appear in On-Site Acoustic Testing reports and project discussions.

Definitions below are intended as practical explanations. The governing test standard, project specification, code requirement, or regulatory definition should be used when a formal definition is required.

A

Acceleration

Acceleration is the rate of change of velocity. In vibration work it is commonly expressed in metres per second squared (m/s²) or in units of g, where 1 g is approximately 9.80665 m/s².

Accelerometer

An accelerometer is a vibration sensor or transducer whose electrical output is proportional to acceleration. Accelerometers are widely used for building, machinery, floor, and structure-borne vibration measurements.

Acoustic Absorption / Absorption Coefficient

Acoustic absorption is the conversion of incident sound energy into other forms of energy, primarily heat, rather than reflection back into a room. The absorption coefficient, commonly written as α, ranges from approximately 0 for a highly reflective surface to 1 for an ideally absorbing surface at a stated frequency.

Acoustic Calibrator

An acoustic calibrator is a reference sound source used to verify the sensitivity of a sound level meter or measurement microphone before and after measurements.

Acoustics

Acoustics is the science of sound and vibration, including their generation, propagation, measurement, control, perception, and effects.

A-Weighting

A-weighting is a standardized frequency weighting intended to approximate the sensitivity of human hearing at moderate sound levels. A-weighted sound levels are commonly reported as dBA or with notation such as L_Aeq, L_AFmax, or L_A90. A-weighting is widely used for environmental and occupational noise assessment.

Apparent Impact Insulation Class (AIIC)

AIIC is a single-number field rating of impact sound isolation through a floor-ceiling assembly. Unlike laboratory IIC, an AIIC measurement reflects the installed building condition and can include the effects of flanking paths and construction details.

Apparent Sound Transmission Class (ASTC)

ASTC is a single-number field rating of airborne sound insulation for a separating assembly. It is derived from apparent transmission loss measured in a building and includes the influence of flanking transmission through the supporting structure. ASTC is distinct from laboratory STC.

Apparent Transmission Loss (ATL)

ATL is the frequency-dependent field measure used to characterize the apparent sound insulation of a separating partition, including sound transmitted through the partition and flanking paths attributed to that partition. ASTC is a single-number rating calculated from ATL data.

B

Background Noise

Background noise is the sound present in a space or at a measurement location apart from the specific source or event being evaluated. Background level can affect the accuracy and usable dynamic range of acoustical measurements.

C

C-Weighting

C-weighting is a standardized frequency weighting with substantially less low-frequency attenuation than A-weighting. It is commonly used for high-level sound, peak-related assessment, and situations where low-frequency energy is important. Results may be reported as dBC or with notation such as L_Ceq or L_Cpeak.

Ceiling Attenuation Class (CAC)

CAC is a single-number laboratory rating used to describe the ability of a ceiling system to reduce airborne sound transmission between adjacent spaces sharing a common ceiling plenum.

Common Intelligibility Scale (CIS)

CIS is a scale used to express speech intelligibility and is mathematically related to the Speech Transmission Index (STI). Higher CIS values indicate better speech intelligibility.

Community Noise Equivalent Level (CNEL)

CNEL is a 24-hour energy-average environmental-noise metric that applies penalties to evening and nighttime sound levels to account for greater community sensitivity during those periods. The exact evening and nighttime periods should be taken from the applicable jurisdiction or specification. CNEL is similar to, but should not automatically be treated as identical to, L_den.

D

Decibel (dB)

The decibel is a logarithmic unit used to express ratios of sound pressure, sound power, vibration, and other quantities. Because the scale is logarithmic, decibel values should not be added or averaged using ordinary arithmetic unless the underlying quantities are first converted appropriately.

Day-Night Average Sound Level (Ldn / DNL)

Ldn, also written DNL, is a 24-hour A-weighted energy-average sound level with a 10 dB penalty applied to nighttime sound. The specific nighttime interval is defined by the applicable standard or jurisdiction.

F

Fast, Slow, and Impulse Time Weighting

Sound level meters can apply standardized time weightings that control how quickly the displayed or stored level responds to changes in sound. Fast weighting responds more quickly than Slow weighting; Impulse weighting is intended for rapidly changing impulsive sounds. The required time weighting depends on the measurement standard and purpose.

Flanking Transmission

Flanking transmission is sound that travels between spaces by paths other than directly through the separating element being evaluated. Common flanking paths include floors, ceilings, structural framing, ducts, penetrations, curtain walls, and adjoining partitions. Flanking is one reason field performance may differ from laboratory ratings.

Frequency / Hertz (Hz)

Frequency is the number of cycles per second of a periodic sound or vibration and is measured in hertz (Hz). Low frequencies are associated with lower-pitched sound and longer wavelengths; high frequencies are associated with higher-pitched sound and shorter wavelengths.

I

Impact Insulation Class (IIC)

IIC is a laboratory single-number rating of the impact sound isolation performance of a floor-ceiling assembly. Field testing of installed assemblies is commonly reported as AIIC.

Impact / Impulsive Noise

Impact or impulsive noise consists of brief, high-level events with rapid rise times, such as hammering, dropped objects, stamping operations, or certain construction activities. Specialized measurement descriptors may be required depending on the applicable standard.

L

Equivalent Continuous Sound Level (Leq / LAeq)

Leq is the constant sound level that would contain the same sound energy as a fluctuating sound over the stated measurement period. When A-weighting is used, the result is commonly written L_Aeq. The averaging period should always be identified when it is relevant.

Maximum and Minimum Sound Level (Lmax / Lmin)

Lmax and Lmin are the highest and lowest sound levels measured during a stated period using the specified frequency and time weightings. Examples include L_AFmax and L_ASmax.

Percentile Sound Level (Ln, L10, L50, L90, etc.)

Ln is the sound level exceeded for n percent of the measurement period. L10 is exceeded 10% of the time, while L90 is exceeded 90% of the time and is often used as an indicator of relatively steady background sound in environmental-noise work.

Logging

Logging is the storage of acoustical or vibration data at regular intervals so changes over time can be reviewed after a measurement. Logging intervals may range from fractions of a second to minutes or longer depending on the purpose of the survey.

N

Noise Criteria (NC)

NC is a family of octave-band curves used to characterize background sound in occupied spaces, particularly sound from HVAC and mechanical systems. A room’s NC rating is determined by the highest NC curve reached by the measured octave-band spectrum.

Noise Dosimeter

A noise dosimeter is a small personal exposure monitor worn by a worker to measure accumulated occupational noise exposure over a work period.

Noise Dose / Time-Weighted Average (TWA)

Noise dose represents accumulated occupational noise exposure relative to a specified criterion and exchange rate. TWA expresses exposure as an equivalent level over a stated work period. OSHA and NIOSH use different criteria and exchange rates, so the governing program must be identified rather than treating all 8-hour noise limits as equivalent.

Noise Isolation Class (NIC)

NIC is a single-number field rating of airborne sound isolation between two spaces based on measured noise reduction. Unlike ASTC, NIC is influenced by the acoustic absorption and furnishings in the receiving room and is not a property of the separating partition alone.

Noise Reduction (NR)

Noise reduction is the difference in sound pressure level between a source room and receiving room at a given frequency. It is a room-to-room result and should not be confused with transmission loss.

Noise Reduction Coefficient (NRC)

NRC is a single-number laboratory rating that summarizes the sound absorption of a material over selected mid-frequency octave bands. It is primarily used to describe absorptive finishes and products and is not a sound-isolation rating.

Normalized Noise Isolation Class (NNIC)

NNIC is a single-number field rating derived from normalized noise reduction. It adjusts measured room-to-room sound isolation to a standardized receiving-room condition, reducing the influence of unusually high or low room absorption.

Normalized Noise Reduction (NNR)

NNR is noise reduction adjusted to a standardized receiving-room condition. NNIC is the corresponding single-number rating.

O

Octave Bands and One-Third-Octave Bands

Octave-band analysis divides the frequency spectrum into bands whose upper frequency is twice the lower frequency. One-third-octave analysis divides each octave into three narrower constant-percentage bands and provides greater frequency resolution. Many architectural-acoustics ratings are calculated from one-third-octave-band data.

Outdoor-Indoor Transmission Class (OITC)

OITC is a single-number rating used to characterize the reduction of outdoor transportation and similar exterior noise through building façade elements such as walls, windows, and doors. OITC uses a different reference spectrum and frequency range than STC and should not be treated as an interchangeable rating.

OSHA Action Level

For general-industry occupational noise, OSHA’s hearing-conservation action level is an 8-hour TWA of 85 dBA. This action level is distinct from OSHA’s 8-hour permissible exposure limit (PEL). Project-specific occupational-noise evaluations should identify the applicable regulation, criterion level, and exchange rate.

P

Peak Sound Pressure Level

Peak sound pressure level is the highest instantaneous sound pressure reached during a measurement. It is different from Lmax, which is affected by the selected time weighting. Peak measurements are commonly important for impulsive or high-level sound.

R

Reverberation Time (RT, T20, T30, RT60)

Reverberation time describes the rate at which sound decays in an enclosed space after the source stops. RT60 represents the time corresponding to a 60 dB decay. Because a full 60 dB decay is often not measurable above background noise, T20 or T30 decay ranges are commonly measured and extrapolated to an equivalent 60 dB reverberation time.

Room Criteria (RC)

RC is a method of evaluating room background noise, particularly HVAC noise, using octave-band sound levels. RC ratings may include descriptors indicating perceived spectral character such as rumble, hiss, or vibration/rattle.

Weighted Sound Reduction Index (Rw)

Rw is an ISO single-number laboratory rating of airborne sound insulation. It serves a role similar to STC but is calculated using ISO methods and should not be assumed to be numerically identical to STC.

S

Sabin

A sabin is a unit of sound absorption. One square foot of perfectly absorbing surface represents one imperial sabin; one square metre of perfectly absorbing surface represents one metric sabin.

Sound Power Level (Lw)

Sound power level describes the acoustic energy emitted by a source and is a source property under stated operating conditions. Unlike sound pressure level, it does not directly depend on the measurement distance or room acoustics, although determining sound power from measurements requires an appropriate test method.

Sound Pressure Level (SPL / Lp)

Sound pressure level is a logarithmic measure of sound pressure relative to the standard reference pressure in air. It is the basic quantity measured by sound level meters and varies with source strength, distance, environment, reflections, and other conditions.

Sound Transmission Class (STC)

STC is a laboratory single-number rating of airborne sound insulation for partitions and assemblies. Field-installed performance should not be described as laboratory STC unless the applicable field method explicitly permits that terminology; current building field work commonly uses ratings such as ASTC, NIC, or NNIC depending on the measurement objective.

Speech Transmission Index (STI)

STI is a standardized index used to quantify speech intelligibility based on the effects of noise, reverberation, and transmission-system characteristics. STI ranges from 0 to 1, with higher values indicating better intelligibility.

STIPA

STIPA is a measurement method derived from STI and widely used for evaluating speech intelligibility of public-address, voice-alarm, and communication systems. It uses a specialized modulated test signal and analysis procedure.

T

Transmission Loss (TL)

Transmission loss is the frequency-dependent reduction in sound power as sound passes through a building element under defined test conditions. Laboratory TL data are used to calculate STC. In field measurements, apparent transmission loss (ATL) may be used when the installed construction and flanking paths are part of the measured condition.

V

Vibration

Vibration is oscillatory motion about an equilibrium position. Building and machinery vibration may be described in terms of acceleration, velocity, displacement, frequency, and spectral content depending on the application.

Z

Z-Weighting

Z-weighting is a nominally flat frequency weighting over the specified audio-frequency range and is used when a substantially unweighted sound spectrum is required. Measurements may be reported with notation such as dBZ or L_Zeq.

Occupational Noise: OSHA and NIOSH Are Not the Same

For U.S. occupational-noise work, it is important to distinguish regulatory requirements from recommended exposure criteria. OSHA’s hearing-conservation action level is 85 dBA as an 8-hour TWA. NIOSH also uses 85 dBA as an 8-hour recommended exposure limit, but NIOSH applies a 3 dB exchange rate. These values should not be substituted for one another without identifying the applicable program and measurement settings.

Related OSAT Services

• Building acoustics testing: ASTC, AIIC, OITC and reverberation time
• Specialized noise testing and source localization
• Industrial and occupational noise testing
• Secure facility, SCIF, SAPF and VTC testing

Terminology and rating methods change as standards are revised. OSAT reports identify the specific test method, descriptor, weighting, averaging interval, and rating procedure applicable to the project.