Acoustic Glossary

Absorption Class

Classification of sound absorbers into Sound Absorption Classes A-E, according to EN ISO 11654, including frequencies 200-5000 Hz.



“Acoustics” is the term for the study of sound and how sound is experienced. The field of acoustics is divided into several specialist areas. The word acoustics comes from the Greek akoustikos (“to do with hearing”) and akouo (“to hear”).

Articulation Class (AC)

A classification of suspended ceilings according to their ability to contribute to the acoustic privacy between work stations. AC is calculated from the Interzone Attenuation according to ASTM E-1110. The Articulation Class (AC) can be calculated from the interzone attenuation. This can then be used as a tool to classify and compare acoustic ceiling systems. The higher the AC the better the speech privacy in an open plan situation. We recommend that you use a suspended ceiling with minimum Articulation class 180 in order to achieve acceptable speech privacy. A high Articulation Class gives some privacy, a lower Articulation Class means lower privacy. Confidential conversation and work tasks demanding concentration need good privacy.


Articulation Loss of CONSonants (%-Alcons)

One method of objectively measuring speech intelligibility is Articulation Loss of CONSonants (%-Alcons), showing the number of consonants being missed as a percentage. Consonants play a much more significant role in speech intelligibility than vowels. If the consonants are heard clearly, the speech can be understood more easily.

Background noise (dB)

For example, speech, scraping chairs, humming ventilation, traffic, machinery and equipment, sound from corridors, adjoining rooms, playgrounds. Increased background noise can have long-term negative effects, such as illness, fatigue, decreased productivity and efficiency. Therefore it is not recommended to attempt to obtain better daily speech privacy by increasing the ventilation noise, or use other sound masking systems. Another thing to bear in mind is that people are differently sensitive to sound and noise in general. Privacy and seclusion in open plan spaces can only be solved satisfactory by creating separate rooms for confidential discussions and work tasks needing higher concentration.

Ceiling Attenuation Class (CAC)

Single value for the laboratory sound attenuation of a suspended ceiling between two rooms according to ASTM E 1414. This measurement takes only into account the sound transmission through the suspended ceiling.
Dn,f,w (dB)
Single value, according to EN ISO 717-1, for the laboratory sound insulation of a suspended ceiling between two rooms, measured according to ISO 10848-2. This measurement takes only into account the sound transmission through the suspended ceiling.

Flutter echo

Occurs when noise bounces between parallel surfaces in a room.



Stated in Hz (hertz). The higher the value, the lighter the tone (bass – treble). The frequency of speech lies primarily between 125 and 8000 Hz, while audible sound lies between 20 and 20 000 Hz.

Interzone Attenuation of Ceiling Systems

The measurement of interzone attenuation demonstrates the sound reflective characteristics of ceiling systems when used in conjunction with partial-height space dividers. This arrangement is commonly used in offices in order to achieve speech privacy between working places.

  1. Plenum
  2. Ceiling test specimen
  3. Loudspeaker
  4. Reference location (source)
  5. Screen
  6. Measurement positions
    The measurement set up for interzone attenuation is fixed with a defined layout, a given 1.5 meter high space divider and defined positions for source and receiver. The interzone attenuation is calculated as the difference in sound pressure level between source and receiver for each one-third octave band in the frequency range 200-5000 Hz and at each distance.

Just like waves in water, sound waves move at a certain speed past a xed point. The frequency of the waves is the number of peaks or troughs which pass this point in one second. The frequency is expressed in hertz (Hz). The higher the number, the lighter the tone and vice versa (treble and bass, respectively).

The frequency range which can normally be heard by humans lies between 20 and 20,000 Hz.


Unwanted sound. Noise can often be the individual perception of a particular sound, e.g. a background noise.

Noise Reduction Coefficient (NRC)

Single value for sound absorption according to ASTM C 423, derived as the mean value of 4 frequencies in the range 250-2000 Hz.


Acoustic privacy between working places in open plan offices is expressed with the Articulation Class (AC).

Rapid Speech Transmission Index (RASTI)

RASTI is an objective way of measuring speech intelligibility. It is measured at two frequencies, 500 and 2000 Hz, by placing a loudspeaker, which transmits sound from the location of the person speaking, and a microphone where the listeners are situated. (See also STI).

Reverberation time, (T or RT)

The time it takes for the sound pressure level to fall by 60 dB after the sound has been turned off. Measuring the reverberation time allows us to calculate the total sound absorption. The reverberation time varies according to the frequency.


The physicist Wallace Clement Sabine (1869-1919) created in Riverbank, west of Chicago, the well known Sabine formula (T=0,16V/A), showing the relationship between reverberation time (T s), room volume (V m³) and the amount of absorption (A m²).

Signal to noise ratio (S/N)

Another important parameter influencing speech intelligibility is the background noise level or, more specifically, the signal to noise ratio (S/N). This is the ratio between signal (e.g. speech) and background noise (e.g. ventilation noise). To achieve good speech intelligibility a signal is considered to be at least 15 dB above noise level. For hearing- impaired people the need is even greater; a ratio of at least 20 dB is often referred to.
On the other hand, if the signal to noise ratio is much less, or if the signal is lower than the noise, the signal will be partly masked. Thus some privacy can be achieved.

Sound absorbers

Materials and structures with the ability to take up sound energy and convert it into other forms of energy. They improve room acoustics by removing sound reflections, thus reducing the noise and the reverberation time.

Sound absorption

Means that sound energy is converted into mechanical vibration energy and/or heat energy. Sound absorption is expressed as the sound absorption coefficient α or the sound absorption class (A-E) according to EN ISO 11654 or NRC/SAA according to ASTM C 423.

Sound Absorption Average (SAA)

Single value for the sound absorption according to ASTM C 423, including the third octaves in the frequency range 200-2500 Hz.

Sound Absorption Class

Classification of sound absorbers into Sound Absorption Classes A-E, according to EN ISO 11654, including frequencies 200-5000 Hz.

Sound insulation

The ability of a building element or building structure to reduce the sound transmission through it. The sound insulation is measured at different frequencies, normally 100-3150 Hz. Airborne sound insulation is expressed by a single value, Dn,f,w, Rw or R’w. Impact sound insulation is expressed by a single value Ln,w or L’n,w.

Sound pressure level (dB)

The pressure variations caused by sound waves in air are called sound pressure. The lowest sound pressure level which can be heard is 0 dB, known as the hearing threshold. The highest level which can be tolerated is called the pain threshold and is around 120 dB.

Sound strength (dB)

Measured in dB (deciBel). dB is measured at different frequencies.
dB(A) (or LpA) is a single-figure value used to describe the total sound strength for all frequencies in a way similar to the sensitivity of the ear.
dB(C) (or LpC) particularly focuses on low frequencies and better reflects how a sound is perceived by people with impaired hearing.

Speech intelligibility

Speech intelligibility is directly dependent on the level of background noise, reverberation time and the shape of the room. Different methods are used to evaluate speech intelligibility, the most common ones are RASTI, STI and %-Alcons.

Speech Transmission Index (STI)

Similar to the RASTI method but a more complete form of measuring speech intelligibility by measuring all octave bands in the frequency range 125-8000 Hz.