Glossary

of Acoustical Terms

Absorption in acoustics is the energy of sound waves entering the surface of any material rather than being bounced off or reflected off the surface. Materials are rated by their ability to absorb sounds.

Acoustical Efficiency is how well a particular wall space absorbs, blocks and covers sound.

Acoustical Preferred Curve is a range of background sound masking spectra at octave-band frequencies from 250 Hz to 800 Hz. The spectra are where the sound levels should drop off by 5 dB per octave. These spectra provide a pleasant acoustical background that masks unwanted, intruding speech and sounds.

Amplitude is the maximum variation of any wave from the mean value. As the amplitude of a sound wave increases, so does its loudness.

Articulation class (AC) measures sound reflected from the ceiling to adjacent work spaces in open-plan environments.

Articulation index (AI) represents how all elements in and properties of a space affect the ability to understand speech. Al is expressed as a decimal value between 0 (renders speech unintelligible) and 1.00 (Causes no interference with speech clarity). By absorbing sound within a space, reducing the amount of sound transmitted to adjacent areas and rendering conversation less intelligible to non-participants, quality ceiling panels and sound masking provide a solid foundation for an effective acoustical management system.

Ceiling Attenuation Class (CAC) indicates the ability of a ceiling panel to reduce sound transmission. Measured in decibels (dB), CAC represents how much sound will be prevented, or attenuated, from transmitting between rooms with a common ceiling plenum. For example, a ceiling panel with a CAC of 40 will reduce transmitted sound by 40 dB. A ceiling panel with a CAC of 35 or higher is considered to offer significant sound attenuation properties.

dBA (A-weighted decibel) is a single number measurement based on the decibel however weighted to approximate the response of the human ear with respect to frequencies.

Decibel (dB) is a unit to express variations in power. In acoustics, equal to ten times the logarithm of the ratio of one sound and a lower-intensity reference sound. One decibel indicates a difference of about 26% and is about the smallest change the ear can detect. The dB level is a logarithm quantity; the maximum normal level is approximately 120 dB.

Flanking Sound Path is termed as a sound transmission path that bypasses a transmission barrier. An example of a sound transmission path is a structural path.

Frequency is cycles per unit of time. Frequency is usually denoted in Hertz (Hz). The frequencies of audible speech lie in the range of 400-200 Hz.

Hertz (Hz) is one cycle per sound.

Reverberation is the reflection of sound off a hard surface.

Reverberation time is the time required for a sound to drop 60 decibels or to decay to a value one millionth of its original intensity.

Sound Attenuation means reducing the intensity or the sound pressure level of sound, which is transmitted from one point to another.

STC (Sound Transmission Class) is the ability of a barrier to stop sound from passing through it. A material with an STC of 21 will prevent 21 decibels of sound from passing through it. The most sound that can be expected to be blocked between open workstations is 21 decibels. This is because sound will diffract or bend, over the top and around the sides of partial height partitions.

Normal Speech Privacy is where the Articulation Index (AI) is less than .20.

Confidential Speech Privacy is where the Articulation Index (AI) is less than .05.

Speech Privacy is measured by ASTM E-1130 and rated using an Articulation Index that classifies speech privacy levels as minimal, normal or confidential.

NRC or Noise Reduction Coefficient measures the degree to which a surface or material absorbs sound. NRC is represented by a number between 0 and 1.00, which indicates the percentage of sound reaching the panel that will be absorbed. Acoustical panels convert sound energy into mechanical energy and as the sound waves impact the material, it responds by vibrating the fibers in the material. The ability of a material to convert sound energy to mechanical energy is measured in a test that provides the Noise Reduction Coefficient or NRC. An NRC of 85 means that the material absorbs 85% of the sound that hits it and reflects back 15%.