Acoustical Terms

Absorption – Occurs when a material reduces the energy of sound reflected off of it.

Articulation Index (AI) – A measurement of the intelligibility of speech.  The Articulation Index is rated from 0.00 (no intelligibility) to 1.00 (perfect intelligibility).  The range is broken into four qualitative privacy categories including Confidential (0.00 to 0.05), Normal (0.05 to 0.20), Marginal (0.20 to 0.30), and None (0.30 to 1.00).

A-Weighted Sound Level (dBA) – A standardized measure of the sound pressure level that approximates the level perceived by the human ear.  The A-Weighted Sound Level de-emphasizes high and low frequencies since these are poorly perceived by the ear.

Background Noise – The noise present in an environment other than the desired or foreground sound.  Also referred to as Ambient Noise, it is the combination of all sounds generated near and far.  The ambient noise level can also be defined as the noise level in an area that is exceeded 90% of the time.

Decibel (dB) – The measurement unit for sound pressure level (volume).

Equivalent A-Weighted Sound Level (Leq) – The equivalent constant sound level for a varying sound level measured over a period of time.  Otherwise referred to as the equivalent average sound level.

Dynamic Range – The variation in sound levels over time or the difference between the loudest and quietest sounds measured over a period of time.

Frequency – The number of cycles of sound waves per second.

Hertz (Hz) – The number of cycles of sound waves per second.  Hertz is the measurement unit for frequency.  

Impact Insulation Class (IC) – A measurement of the impact sound insulating characteristics of a floor or ceiling treatment.

Loudness – A human’s subjective perception of the volume of sound.  A 10dB increase in sound energy is generally perceived as a doubling in loudness.

Masking – The effect of reducing or eliminating the ability to hear a sound due to the presence of a masking sound.

Noise – Any unwanted sound.

Noise Reduction Coefficient (NRC) – A measure of acoustical absorption calculated at specific mid-range frequencies.  Commonly used to rate acoustical ceiling performance.

Octave Band – Bands of frequency into which audible sound is divided for frequency analysis.  One-third octave bands further divide each octave into three segments.

Reverberation – The persistence of a sound in a space, due to many reflections of the sound from the surfaces in the space, once the sound source has been stopped.  Reverberation can be compared to a multitude of indistinguishable echoes within a space.

Room Criteria (RC) – A quantitative and qualitative assessment of noise.  The numerical component is the average of the sound pressure levels at the 500, 1000, and 2000 Hz octave bands, while the qualitative component classifies the noise as containing Rumble (R), Hiss (H), Vibration (V), or as being Neutral (N). The RC curves approximate the balanced spectrum found acceptable by most people.

Sound Level – A measure of the sound pressure level using specific weightings that emphasize certain frequencies.  Weightings include designations such as A, B, and C.

Sound Level Meter – A device used to measure sound levels.  Meters consist of a microphone, amplifier, output meter, and frequency-weighting protocols.

Sound Pressure Level (SPL) – A measurement of the magnitude or volume of sound.  It is measured in decibels (dB).

Sound Transmission Class (STC) – A classification of the sound insulating properties of a material or structure.  Commonly used to rate office walls and furniture partitions.

Transmission Loss (TL) – A measure of the sound insulation properties of a material or structure expressed as the number of decibels a sound is reduced in passing from one side to another.

Spectrum – The composition of a sound expressed in frequency and amplitude (volume).

Sound Absorption Coefficient – The fraction of incident sound power absorbed or otherwise not reflected from a surface.