What is acoustics?

Acoustics is the term for the study of sound and how sound is perceived. Acoustics is about how sound behaves in space.

People find loud noise disturbing. Research shows that the most disturbing sounds in the office are the voices of others. Concentration is therefore dependent on minimising disruptive sounds.

The sound environment is important at home, in the workplace and wherever you are travelling.
The sound level where you are located is determined by the amount of absorption in the area.


There are essentially three different acoustic room types:

  • rooms with hard surfaces (reflective surfaces)
  • rooms with absorbent ceilings
  • rooms with open-plan layouts

Different room types require different acoustic solutions. Sometimes it is not enough just to use absorbent ceilings, in which case the use of suspended ceiling tiles or wall absorbers is recommended
as a supplement.

Sound damping

Most people will perceive discomfort from a lot of sound associated with reverberation (echo) or loud basic sound (during) as noise, so it is normally appropriate to implement soundproofing measures.

Acoustic concepts

Sound reflection When a sound wave hits a surface, it will be reflected, absorbed or scattered. What determines the strength of the reflection is the distance the sound travels and the acoustic properties of the reflecting surface.

Speech perception Speech Transmission Index, STI, is a number between 0 and 1 that indicates how well a receiver perceives speech from a speaker or a public address system at a position in a room.

Speech transmission index is typically used to characterise speech perception when using sound reinforcement systems. Advanced measurement instrumentation is required. Values higher than 0.6 indicate reasonably good speech perception, values lower than 0.3 indicate very poor speech perception.

Articulation index, AI, is a number between 0 and 1 that indicates how much of the information from the speaker is transmitted to the receiver in a room. There are several models for calculating AI in a room. It can be calculated from the percentage of perceived words in listening tests, from the measured difference between speech level and background noise level in frequency bands, or with more advanced methods based on the room's pulse response, among other things. Values higher than 0.6 indicate relatively good speech perception, values lower than 0.3 indicate very poor speech perception.

Sound field General: A sound field is an area in which there are sound waves. Direct sound field: is a sound field where the sound pressure level is determined by the direct radiation from the source. In a direct sound field, the sound pressure level drops 6 dB per doubling of distance from the source when the extent of the source is small in relation to the distance to the source.

Reverberant sound field/reflection sound field: is a sound field in a room where the sound pressure level is determined by reflected sound from the room's boundary surfaces. Diffuse sound field: is a reflection sound field that, averaged over time, is independent of room position and direction. Sound space: is a room with an approximately diffuse sound field. A sound space can be achieved by sound-reflecting boundary surfaces and sound-diffusing elements. The room is used to measure the absorption factor and sound power radiated from the source. Anechoic chamber: is a room with near total absorbing boundary surfaces. The room enables indoor measurements in a direct sound field.

Sound absorption The absorption factor, α, is the ratio between non-reflected and incident sound power. Most commonly used is the absorption factor for diffuse sound incidence. The theoretical absorption factor can be at most equal to 1, while the practical one, due to edge effects, can be greater than 1.

absorption factor Factor describing the degree to which a material is acoustically sound absorbing. Specified as an unnamed number between 0 and 1. The specified limit values apply to the average absorption factor determined from data for the room's individual surfaces in unfurnished rooms. The limits for the absorption class apply to the mean absorption factor for floors, walls and ceilings in each of the 1/1 octave bands 500Hz, 1000Hz, 2000Hz and 4000Hz.

Absorption class Absorption class, A, B etc., is a classification of sound absorbers based on measured absorption factor. The class for an absorber in question is obtained by calculating the octave values from 250 Hz up to and including 4,000 Hz for the absorption factor from the 1/3 octave values, and rounding off in steps of 0.05. Frequency is a measure of the number of times an event repeats in a unit time. To calculate frequency, set a fixed time interval, count the number of times an event occurs and divide by the length of the time interval. These values are plotted in the diagram below and the absorption class is determined.

graph_classes( According to EN-ISO 11654 )

Absorption classes for work premises Lowest basic values for mean absorption factor α. Premises for industry, crafts, shops, canteens, restaurants, etc.

Class A

Class B

Class C

Class D

0,30 α

0,25 α

0,20 α

0,15 α

In such environments, Class A absorbers should be used to create a less stressful soundscape. You should also strongly consider using wall absorbers as additional absorption in such premises. In canteens, it can be an advantage to use sound-insulating screens between the different groups of tables.

Reverberation time (echo) The time it takes for the sound pressure level to decrease 60 dB after the sound source is stopped. Indicated in seconds. Short reverberation time indicates high acoustic absorption in the room. The specified limit values apply to the room-averaged reverberation time in each of the octave bands 125Hz, 250Hz, 500Hz, 1000Hz and 2000Hz, determined according to NS 8173, and for specialised rooms according to NS-EN ISO 3382. Alternatively, methods according to NS-EN ISO 18233 can be used. For 1/1 octave band 125Hz, the reverberation time may exceed the basic values in the table by up to 40%. For gymnasiums and classrooms, the reverberation time limits apply from 250Hz and for stairwells from 500Hz. Reverberation time is determined by several factors, including the absorbers used and their placement, the amount of furniture, shelves, etc. and the size and layout of the room. Regardless of the materials used, a lot can be gained in reverberation time if wall absorbers are used. This is particularly effective in small rooms where the walls make up a large proportion of the total area.

Definitions reverberation time

  • Long: 0.8 - 1.5s Take measures to diffuse sound in ceilings and on walls.
  • Normal: 0.6 - 0.8s Perform measures in the ceilings.
  • Short: 0.4 - 0.6s Absorbing and diffusing measures in ceilings and on walls.
  • Ultra short: 0.2 - 0.4s Absorbing and diffusing measures in ceilings, walls and floors.

Reverberation time = RT1 (measured reveberation time) = Tsek (measured reverberation per second).

Tsek is the term for measured reverberation time. Tsec is also what is stated in NS8175 as a requirement for different types of rooms and areas, and is the time it takes the sound/impulse noise to reduce by 60 dB in the measured area. It is based on an RT60 measurement based on impulse noise (shot, bag, balloon) in the area and that this is measured with approved equipment for such.

Airborne sound insulation A structural ability to insulate against airborne sound transmission in the building.

Sound damping - silencers

Terms normally used in preference to absorption and absorbents.