Acoustical Terminology

Absorption Coefficient (a):  indicates how much sound is absorbed by the material.  A perfectly absorptive material would have a coefficient of 1.0 while a perfectly reflective surface would have a coefficient of 0.0.  Often published by acoustical product manufacturers in octave bands.

Accelerometer:  a transducer whose output is directly proportional to acceleration.

Acoustics:  the science of mechanical waves dealing with the generation, transmission and effects of waves including vibration, sound and ultrasound.

Airborne Sound:  sound that arrives at the point of interest by transmission through air.

Ambient Sound:  all sounds that exist in a particular area, from near and far, including the sound source of interest.  Usually consists of sound from many sound sources.

Apparent Sound Transmission Class (ASTC):  a field rating of airborne sound isolation determined from measurements in a building instead of a laboratory.  See Sound Transmission Class (STC).

A-weighted Sound Level (measured in dBA):  a single number rating of the broadband sound spectrum.  The sound pressure level determined by using the A-weighting scale.   The A-weighting scale is designed to approximate the response of the human ear by altering (weighting) the lower and higher frequencies.  It is commonly used for the measurement of environmental noise and industrial noise.  The effect of low frequency noise is reduced when using the A-weighting scale.

Background Sound:  all sounds from sources other than the sound source of interest.

Ceiling Attenuation Class (CAC):  rates how efficiently a ceiling acts as a barrier to airborne sound transmission between two rooms.  A higher value indicates a higher degree of sound insulation.  The CAC is determined by measuring the sound transmitted from one room to another, through the ceiling, over a partition through a ceiling plenum, and down through the ceiling on the other side

Community Noise Equivalent Level (CNEL):  the A-weighted average sound level over a 24 hour period.  The CNEL incorporates weighting factors for both evening and nighttime noise levels.  Hourly Leq values for the evening period (7 p.m.-10 p.m.) are increased by 5 dB, while Leq values for the nighttime period (10 p.m.-7 a.m.) are increased by 10 dB.   The CNEL is then computed from the 24 one-hour measurements.

Damping:  the characteristic of a mechanical system that gradually reduces the amplitude of vibration.

Day-Night Average Sound Level (DNL or Ldn):  Average sound level over a 24-hour period, calculated from hourly Leq values with the nighttime levels from 10:00 pm to 7:00 am increased  by a “penalty” of 10 dB.

Daytime Equivalent Sound Level, Leq (day):   The hourly Equivalent Sound Level averagedduring the fifteen daytime hours between 7:00 a.m. and 10:00 p.m.

Dead Room:  a “dead” room is not very reverberant.  It has a short reverberation time and usually has many sound absorbing surfaces.

Decibel (dB):  a logarithmic unit of sound measurement, used to quantify sound pressure levels and sound power levels relative to a reference level.

Most people have difficulty distinguishing the louder of two noise sources if they differ by less than two decibels. Subjectively, most people perceive changes in sound levels as follows.

  • A 2-3 dB change is just perceptible.
  • A 5-6 dB change is clearly perceptible.
  • A 10 dB change is perceived as being twice-as-loud or half-as-loud

End Reflection:  the reduction of  low frequency noise resulting from the rapid change or size of an HVAC duct or pipe termination.

Equivalent Sound Level (Leq):   the sound level which, in a stated period of time, would contain the same acoustical energy as the time varying sound during the same period.  The actual instantaneous noise levels typically fluctuate above and below the measured LEQ during the measurement period.  The A-weighted LEQ is a common descriptor for measuring environmental noise.

FHWA:  Federal Highway Administration

Field Impact Insulation Class (FIIC):  see Impact Insulation Class (IIC)

Field Sound Transmission Class (FSTC)  see Sound Transmission Class (STC)

Flanking Path:  an indirect sound transmission path that bypasses the primary wall or other barrier.  For example, a structure borne path through the floor or ceiling can be a flanking path around a wall separating two rooms.

Frequency:  the number of cycles per second of a periodic wave, usually expressed in Hertz (Hz).

Impact Insulation Class (IIC):  a single-number rating of impact sound insulation of a floor-ceiling assembly.  A higher value indicates a higher degree of sound insulation.  An IIC is determined by laboratory tests performed in accordance with ASTM E492 Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.  A Field Impact Insulation Class (FIIC) is determined by tests performed on floor-ceiling assemblies in a building according to ASTM E1007 Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-Ceiling Assemblies and Associated Support Structures.  The IIC or FIIC rating is determined from the test results using the procedures of ASTM E989 Standard Classification for Determination of Impact Insulation Class (IIC).

An IIC value of 50 is the minimum required by the International Building Code (IBC) to separate dwelling units.

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

Inverse Square Law:  where sound is free to propagate in any direction, the sound pressure varies inversely with the square of the distance from the source.  This is equivalent to a decrease of 6 dB for each doubling of distance from the source.  For example, a sound level of 60 dB at a distance of 100 feet from a source will be 54 dB at a distance of 200 feet from the same source.  It will be 48 dB at a distance of 400 feet, and 42 dB at a distance of 800 feet.  This relationship does not hold true at distances very close to the source.

Live Room:  a “live” room is highly reverberant.  It has a long reverberation time and usually has mostly hard surfaces.  A room with high reverberation is also referred to as “bright”.

Masking Sound:  sound that is introduced into an environment to cover up an unwanted sound. Masking sound typically refers to sound that is added by an electronic sound system into an office or other controlled environment.

Maximum Sound Level (LMAX):  the maximum sound level that occurred during the measurement period.

Minimum Sound Level (LMIN):  the minimum sound level that occurred during the measurement period.

NEPA:  National Environmental Policy Act

Nighttime Equivalent Sound Level, LEQ (night):   the hourly Equivalent Sound Level averagedduring the nine nighttime hours between 10:00 p.m. and 7:00 a.m.

Noise:  Any unwanted or undesirable sound.

Noise Abatement Criteria (NAC):  Absolute noise levels used to determine that a noise impact occurs. Often used when assessing transportation noise or other environmental noise.

Noise Barrier:  a solid obstacle blocking the line of sight from a noise source to a listener, designed to protect sensitive areas from unwanted noise.

Noise Criterion (NC):  is a single-number noise index commonly used to define design goals for the maximum allowable mechanical HVAC noise in a room, or to quantify existing noise levels.

Noise Isolation Class (NIC):  A single-number rating derived from the measured noise reduction between two rooms.

Noise Reduction Coefficient (NRC):  A single-number rating of the acoustical absorption performance of materials.  A higher number means more sound absorption.

Noise Sensitive Area (NSA):  is a term used in environmental noise assessment.  It is an area where noise interferes with the normal activities and uses of the area.  Noise-sensitive areas can include residential buildings (single family homes, condominiums, townhomes, and apartments), schools and day care facilities, churches, campgrounds, recreation and wilderness areas, hotels and motels, hospitals and assisted living facilities.  The governing agency or municipality may have its own specific guidelines further defining NSAs and how they are treated.

Octave Band:  a frequency band that is one octave wide.  A standard series of octave bands is used in practical acoustics and are identified by their center frequencies.  An octave is the interval between two sounds having a frequency ratio of two.

One-third Octave Band:  for increased accuracy sound and vibration are often analyzed in one-third octave bands.  Each octave band is divided into thirds.

Percentile Sound Level (LNN):  the sound level that was exceeded NN% of the time over a given time period.  For example, the L90  is the decibel level that is exceeded 90% of the time.  The L90 is often used as a measure of the “residual” sound level, or the relatively steady sound level excluding short term events such as an occasional car passing or aircraft over flight.  Other percentile sound levels such as the L1 and L10 are used to analyze sound environments.

Pink Noise:  random noise that contains equal noise energy in each octave band and each one-third octave band.  Pink noise is often used for acoustical testing.

Sound Transmission Class (STC):  single number rating of airborne sound insulation of a partition such as wall, floor, roof, or door.  The sound transmission loss of a partition is measured in a laboratory according to ASTM E90 Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.  The STC is calculated from sound transmission loss data using ASTM E413 Classification for Rating Sound Insulation.  A higher STC rating indicates better sound insulation.

An STC value of 50 is the minimum required by the International Building Code (IBC) to separate dwelling units.

Reverberation Time (T60):  the time it takes for sound to decay by 60 decibels after the sound is stopped in the room.

White Noise:  random noise that contains equal noise energy in any fixed width frequency band.