A-Weighting (dBA)

The filtering of sound that replicates the human hearing frequency response. The human ear is most sensitive to sound at mid frequencies (500 to 4,000 Hz) and is progressively less sensitive to sound at frequencies above and below this range. A-weighted sound level is the most commonly used descriptor to quantify the relative loudness of various types of sounds.


The attenuation (or reduction) of sound level that results when sound is controlled by a sound absorptive material such as glass fiber or open-cell urethane foam. In the case of sound absorptive, attenuation of sound is produced by the conversion of molecular motion, into thermal energy due to friction of air molecules with fibrous or cellular materials.


Absorption coefficient (<)

The degree that sound is absorbed when struck by a sound wave. The scale goes from 0 to 1.0 being no absorption, 1 being perfect absorption. Absorption Coefficients may be reported per frequency or as a one-number average. See NRC or Noise Reduction Coefficient.


  1. Acoustics is the science of sound, including its production, transmission and effects.
  2. The acoustics of a room are those qualities that together determine its character with respect to the perception of sound.

Ambient Noise

Ambient noise encompasses all sound present in a given environment, being usually a composite of sounds from many sources near and far.


The method of determining a person’s threshold of hearing. The amount of hearing loss can be determined from an audiogram. The audiogram is typically the detection of a tone at its lowest perceivable level. The tones are at industry-designated frequencies.


An obstacle, such as a wall, that blocks the line-of-sight between a sound source and a receiver, thereby providing a barrier attenuation, i.e., a reduction of sound level at the receptor. Sound attenuation provided by barriers is principally related to the sound transmission loss through the barrier material.

Decay Rate (dB/sec)

The rate at which sound will get quieter when the noise source is removed.

Decibel (dB)

Unit for reporting level (magnitude) of sound. A change of 10 decibels is generally considered a doubling or a halving of the perceived loudness of a sound.

Exchange Rate

The louder the sound, the lower the allowable exposure time to it. The exchange rate is the relationship between exposure time and noise level. The exchange rate halves the exposure time for every 5 dB increase in noise level.


The transmission of sound around the perimeter or through holes within partitions (or barriers) that reduce the sound isolation between areas. Examples of flanking paths within buildings are ceiling plena above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.

Hearing Conservation Program

Monitoring of the workplace noise level and workers hearing sensitivity. When noise levels in the workplace exceed 85 dBA over an eight-hour period, a hearing conservation program is required.

Hearing Protection

Also known as Personal Hearing Protection. Commonly used hearing protection is ear plugs and ear muffs. Hearing protection is required to be issued to workers in a work environment with a time weighted average noise level of 85 dB.

Hertz (Hz)

Frequency measure in units of cycles per second. See frequency.

Leq (Equivalent Noise Level)

Measure used to express the average sound level (typically expressed in dBA) over a given period of time.

Ln Statistical Noise Levels

Sound levels (typically expressed in dBA) that is exceeded n percent of the time. Common statistical levels are L10 (level exceeded 10% of the time), L50 (level exceeded 50% of the time), and L90 (level exceeded 90% of the time).

Octave Band

Groups of frequencies defined by standards where the upper frequency of each band is equal to twice the lower frequency of the next higher band. Octave bands are usually named by their center frequency. The full complement of octave bands in the audible frequency range is as follows: 31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000 and 16,000 Hz.

1/3 Octave Band

Frequency band encompassing 1/3 of one octave. Three successive 1/3 octave bands make one full octave.


A sound that typically does not carry any positive benefits and typically produces annoyance.

Noise Criteria (NC)

A one-number rating for the quietness of a room. Typically associated with HVAC noise but includes all ambient noise present in room when measured.

Examples of typical NC ratings:

  • Below NC 15: Quiet studio
  • NC 15 – NC 25: Quiet library or classroom
  • NC 25 – NC 35: Typical Office
  • NC 35 – NC 45: Noisy Office

Noise Induced Hearing Loss

Hearing loss caused by exposure to high noise levels, versus hearing loss due to aging or other means.

Noise Reduction (NR)

The reduction of sound, expressed in dB, from one side of a barrier or material to the other.

Noise Reduction Coefficient (NRC)

The average of the absorption coefficients (à) for the frequency bands 250 Hz, 500 Hz,1000 Hz, and 2000 Hz. Commonly used to describe the average absorption of acoustical materials such as ceiling tiles and wall panels.


Reverberation is the persistence of sound in an enclosed space resulting from multiple reflections from space surfaces.

Reverberation Time (RT60)

The reverberation time of a room is the time it takes for sound to decay by 60 dB once the source of sound has stopped. Reverberation time is inversely related to sound absorption and is a way to measure the amount of absorption in a room.


A unit of absorption having the dimensions of square feet or square meters, as appropriate. A quantity of the amount of absorption equal to the surface area (SA) of a material multiplied by its absorption coefficient (à).


A sound carrying useful information or information of interest. The signal could be a test tone, speech, or music.

Signal to Noise Ratio (SNR)

A measure of useful signal to unwanted noise. When computed using sound levels in units of dB, it is a subtraction and not a ratio. SNR = LS – LN where LS is the level of the signal and LN is the level of the noise.

Sound Absorber

An apparatus or material that reduces the amount of sound reflected back into the environment once the incident sound wave hits it. The most common type of absorber are porous materials. Once the sound enters the porous material, its sound energy is dissipated as heat. Also see absorption.

Sound Isolation

Ability of an object or material to inhibit sound from passing through it. Typically isolating material should be nonporous. Mass increases the isolation of materials.

Sound Pressure Level, (SPL or L)

Measure of sound in units of decibels. 0 dB is considered to be the lower threshold of human hearing. Most often Sound Pressure Levels are expressed in A-weighted SPL (dBA). See also A-weighted.

Sound Reinforcement

Electronically amplifying an acoustic sound source such as a talker. Sound reinforcement systems typically use microphones, amplifiers, and loudspeakers.

Sound Transmission Class (STC)

Overall single number performance rating of transmission loss. See transmission loss.

Spectrum – Spectra

Sound can be composed of a multitude of tones of different frequencies. The decomposition of the sound into its frequency components is the sound spectrum. The sound spectrum will show the levels within each frequency band. This allows one to determine what frequencies are problems to focus noise control efforts on.

Speech Intelligibility

The ability to understand spoken words in a particular environment.

Speech Transmission Index (STI)

Measure of the intelligibility of speech in an environment. Scale of 0 to 1. An STI of 0 is completely unintelligible. An STI of 1 would be perfectly intelligible. STI can be measured in the field or can also be calculated using computer models of architectural spaces.

Speed of Sound

Sound travels at different speeds in different physical media. For air, the typical speed is 350 m/s (1100 ft/s).

Sound Reinforcement

Electronically amplifying an acoustic sound source such as a talker. Sound reinforcement systems typically use microphones, amplifiers, and loudspeakers.

Threshold Shift

After exposure to noise of high levels over a period of time, the sensitivity of the ear is reduced. The threshold shift of hearing sensitivity can be temporary or permanent.

Time Weighted Average

Because the noise level is not constant over time, a method of computing the entire equivalent noise dose is required. This is the time weighted average.

Transmission Loss (TL)

Similar to Noise Reduction but removes influence of size of test sample and amount of sound absorption of test room. TL is reported, in decibels for individual octave or 1/3 octave frequency bands.


The distance a wave travels in the time it takes to complete one cycle. Wavelength is inversely related to frequency. As frequency increases, wavelength decreases and vice versa.

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