Below is a selection of terms relating to acoustics and sound. Most of the definitions were compiled by the experts at Akustikbüro Oldenburg.
Auralisation (aural simulation)
Auralisation allows the acoustic conditions of a room to be made audible during the building design phase. This process is based either on computer simulations of a virtual room or on measurements taken from a real space. Auralisation helps demonstrate the acoustic experience in advance and enables planners to compare different design options more intuitively than by comparing numeric parameters alone.
A‑weighted sound pressure level – dB(A)
The A‑weighted sound pressure level is the frequency-weighted average of the sound pressure level (dB), adjusted to reflect the sensitivity of human hearing. This weighting accounts for the fact that we perceive sounds of different frequencies at different loudness levels, with the human ear being most sensitive to mid-range frequencies, especially those typical of speech. Almost all regulations and guidelines specify values in dB(A).
Background noise level
Background noise refers to low-information sounds such as air conditioning or distant traffic. The background noise level is measured in dB or dB(A), depending on whether frequency weighting is applied. It has a direct impact on speech intelligibility.
Building acoustics
Building acoustics is a field of building physics and acoustics that deals with how constructional elements affect the transmission of sound between rooms or between interior and exterior spaces.
Decibel (dB)
A logarithmic unit used to express sound pressure levels. The relevant scale for human hearing ranges from 0 dB to 140 dB. A level of 0 dB corresponds to a sound pressure of 20 µPa.
Diffraction sound
When a sound wave encounters an obstacle, it bends around the edges of the object. This redirected part of the sound wave allows sound energy to reach the shadow zone behind the obstacle. This edge-deflected portion is referred to as diffraction sound.
Equivalent sound absorption area
The equivalent sound absorption area A is the sum of all surface areas S within a room, each multiplied by its respective sound absorption coefficient α: A = α1S1 + α2S2 + α3S3 + … + αnSn
Flutter echo
Flutter echo occurs when a sound wave bounces back and forth multiple times between two or more hard, reflective surfaces. It can be perceived as a rapid, decaying series of echoes resembling a machine-gun effect when clapping hands or producing a sharp sound. Flutter echo is usually perceived as disturbing and should be avoided, for example by using non-parallel surfaces or placing absorptive materials on some of the reflective areas.
Frequency
Frequency refers to the number of sound pressure changes per second. High-frequency sounds are perceived as high-pitched tones, while low-frequency sounds are perceived as deep or low-pitched. Everyday noises such as traffic or white noise consist of a mix of many frequencies. The unit of frequency is the hertz (Hz), where 1 Hz = 1 cycle per second. Human speech generally falls between 250 Hz and 2,000 Hz, while the full range of human hearing spans from 20 Hz to 20,000 Hz.
Isophones
Isophones are curves that indicate the sound pressure level required at each frequency to produce the same perceived loudness in the human ear.
Maskers
see Sound masking
Noise
Noise refers to sounds that, due to their volume or structure, are perceived as harmful, disturbing, or burdensome to humans or the environment. Whether a sound is experienced as noise depends on a person’s condition, preferences, and mood. The perception of noise and its impact on individuals is influenced both by objectively measurable factors – such as sound pressure level, pitch, tonality, and impulsiveness – and by subjective factors. For example, noise is perceived as particularly disruptive during sleep or while performing tasks that require high levels of concentration. Sounds a person likes are generally not perceived as disturbing, even at high volumes, whereas disliked sounds may be bothersome even at low volumes (e.g. certain types of music). A person’s general state of wellbeing also affects their sensitivity to noise. We speak of noise disturbance when one or more sounds interfere with or interrupt an activity. People tend to be especially sensitive when verbal communication is disrupted – for example, when a loud conversation at a neighbouring table makes it difficult to follow what someone is saying, especially during mentally demanding tasks or when trying to sleep.
Octaves
Acoustic parameters such as sound pressure level or absorption coefficient are typically specified in octave or third-octave bands. Detailed acoustic planning requires precise information about a material’s or room’s acoustic properties across frequency ranges. Relevant octave frequencies in room acoustics are 125 Hz, 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz and 4,000 Hz. Each octave step results from doubling the preceding frequency. Each octave contains three third-octave bands (see also “Single-number sound absorption ratings”).
Omnidirectional sound sources
Omnidirectional sound sources emit sound evenly in all directions. Since most loudspeakers do not behave omnidirectionally at close range, special measurement devices such as dodecahedron loudspeakers (with twelve drivers arranged in a spherical layout) are used in acoustic testing, providing nearly uniform radiation between 100 Hz and 4,000 Hz.
Porous absorbers
Porous absorbers include materials such as mineral wool, foams, carpets, and fabrics. Their effectiveness relies on sound entering the open structure of the material, where the sound energy is converted into heat through friction between air particles and pore surfaces. Porous absorbers are primarily effective at mid to high frequencies.
Psychoacoustics
Psychoacoustics is a branch of acoustics and noise impact research that deals with the subjective perception of objectively measurable sound signals. It also considers the influence of personal attitudes and expectations on how sound is perceived.
Rating level (Lr)
The rating level Lr (L for “level”, r for “rating”) is the key parameter for objectively assessing noise exposure in workplaces. It is based on the A‑weighted sound pressure level but includes adjustments for the nature of the sound (e.g. impulsiveness, prominence of specific tones) and the duration of exposure. The rating level is also given in dB(A).
Resonant absorbers
This term covers all types of absorbers that utilise a resonant mechanism, such as an enclosed air volume or a vibrating surface. They are mainly used to absorb sound at medium to low frequencies and tend to be effective in narrow frequency ranges (see also “Porous absorbers”).
Reverberation room
A reverberation room is a special type of laboratory space in which the walls reflect nearly all incident sound waves. These rooms have very long reverberation times across the entire frequency range.
Reverberation room method
This method is used to determine the frequency-dependent sound absorption coefficient. A sample of the material to be tested is placed in a reverberation room. The change in the reverberation time is used to calculate the material’s absorption performance.
Reverberation time
This refers to the time it takes for a sound to decay by 60 dB after the source has stopped. It is a key parameter in room acoustics and is denoted by T.
Room acoustics
Room acoustics focuses on how a room’s architectural features affect sound events occurring within it. This applies to spaces such as concert halls, theatres, classrooms, studios, churches, offices, call centres, or conference rooms — anywhere sound performances (speech or music) or verbal communication take place. The key question is which surfaces and materials can be used to create optimal listening conditions. The critical material property in this context is sound absorption.
Sabine’s reverberation formula
When the room volume and total equivalent absorption area are known, the reverberation time can be estimated using Sabine’s formula:
T = 0.163 × V / A
where T is the reverberation time, V is the room volume (in m³), and A is the total equivalent sound absorption area (in m²). This formula is based on the findings of physicist Wallace Clement Sabine (1868–1919), who discovered the relationship between a room’s volume, its surface absorption, and reverberation time.
Single-number sound absorption ratings
To simplify the representation of the frequency-dependent absorption coefficient and to enable basic comparison between different sound absorbers, various single-number ratings are used. In Europe, the weighted sound absorption coefficient (αw) is defined by DIN EN ISO 11654. Common single-number ratings in the US are the NRC (Noise Reduction Coefficient) and the SAA (Sound Absorption Average). All of these are derived from measured absorption data in third-octave or octave bands. Accurate acoustic planning requires detailed knowledge of absorption values, preferably in third-octave bands (see also “Octaves”).
Sound absorbers
Sound absorbers are materials that reduce incident sound by damping it or converting it into other forms of energy, typically heat. There are two main types: porous absorbers and resonant absorbers, as well as hybrid types combining both mechanisms.
Sound absorption coefficient α
The sound absorption coefficient α of a material indicates the proportion of sound energy it absorbs. α = 0 means no absorption (full reflection), α = 0.5 means 50% absorption and 50% reflection, and α = 1 means full absorption (no reflection).
Sound attenuation
Sound attenuation describes the ability of materials to absorb sound or convert sound energy into other forms (mainly heat). It differs from sound insulation (see “Sound insulation”).
Sound barriers
Sound barriers are used to interrupt the direct transmission of sound from a source to a receiver. Examples include screens or desk-mounted elements. Furniture such as cupboards or large shelving units can also act as sound barriers. These may be equipped with sound-absorbing surfaces to further reduce sound propagation.
Sound events
A general term referring to tones, music, bangs, noise, crackles, etc.
Sound insulation
Sound insulation refers to the limitation of sound transmission through building elements. It is a measure used to acoustically separate rooms from unwanted noise originating from adjacent spaces or from outside the building. This is not to be confused with the required in-room sound absorption (see also “Sound absorption”). Sound insulation is a fundamental concept in building acoustics.A distinction is made between airborne sound insulation and impact sound insulation. Airborne sound is generated by sources within a room that are not directly connected to the building structure, such as people speaking. Impact sound, on the other hand, results from structure-borne noise (e.g. footsteps or knocking), which causes walls or ceilings to radiate airborne sound. There are legally defined requirements for both airborne sound insulation and impact sound insulation in buildings.
Sound masking
Sound masking refers to the intentional use of natural (e.g. birdsong) or artificial (e.g. broadband noise) sounds to mask unwanted noises. It is often used to reduce speech intelligibility in open-plan offices when the background noise level is otherwise too low.
Sound pressure
All sound events involve small fluctuations in air pressure that propagate through elastic media such as air or water. These fluctuations are referred to as sound pressure. The greater the fluctuation, the louder the sound. The faster the fluctuation, the higher the frequency.
Sound pressure level (Lp)
The sound pressure level (L for “level” and p for “pressure”) is a logarithmic measure used to describe the strength of a sound event. It is often – though not entirely correctly – referred to simply as the “sound level”. Sound pressure level is expressed in decibels (abbreviated as dB). Sound pressure is measured using microphones. The typical measurable range starts just above 0 dB and extends up to approximately 150 to 160 dB.
Sound spectrum
The sound spectrum describes the frequency content of a sound. Pure tones consist of a single frequency, while complex sounds such as noise or music are made up of multiple frequencies.
Speech intelligibility
Speech intelligibility refers to how well speech can be understood in a given space. It depends on the room’s surfaces (walls, ceiling, floor), the furnishings, and the presence of people.
Terz (third-octave bands)
See “Octaves”.
Wavelengths of sound
Wavelengths of sound are the fluctuations in air pressure caused by sound events. The length of a sound wave determines its frequency, and the height (amplitude) determines its level. Long waves have low frequencies (low-pitched sounds), short waves have high frequencies (high-pitched sounds). A 100 Hz wave has a wavelength of approx. 3.40 metres in air; a 5,000 Hz wave has a wavelength of about 7 centimetres.
