The unit (abbreviated dB) used to describe the level of sounds and noises. The total range between inaudible and painful (damaging!) sound levels is from about 0 to 140 dB. The decibel scale is logarithmic in order to better describe the changes in perceived loudness. Thus, increasing the value by 10 dB, say from 40 dB to 50 dB corresponds to an increase in the physical energy by a factor of 10, but is perceived as a “doubling” of the level (at mid frequencies). An increase by another 10 dB from 50 dB to 60 dB will be perceived as a similar relative step in perceived level, although the physical energy has actually been increased by a factor of 100 from the 40 dB level.

The experience of the sound being repeated with a delay relative to the first perceived impression of the sound. This occurs when strong, individual reflections of the sound is delayed more than 50 to 100 milli seconds (depending on the temporal nature of the original sound). Impulsive sounds like hand claps are sensitive to the detection of echoes in rooms.

Repeated reflections often between two parallel, hard and smooth surfaces. The phenomenon is heard as a repeated train of echoes in cases, where the distance between the parallel surfaces is large (more than say 10 metres). When the distance is smaller such as in small meeting rooms, it is heard more as a coloration of the sound spectrum like a “rough” distortion of the sound.

The number of oscillations per second. The unit is called Hertz (Hz).

A system consisting of a closed, limp surface, membrane, in front of a confined air volume (often with a porous absorber inside). Such a construction will often have a pronounced resonance frequency at which the surface is vibrating strongly when hit by sound containing this very frequency. The absorption occurs when the vibration energy of the sound wave is transferred to vibration of the membrane and eventually into heat due to losses in the membrane and in the air-porous material interaction in the cavity. Typical examples are a timber floor on joists, a gypsum board wall or simply a drum. The resonance frequency can be revealed by hitting the surface with the fist. Like drums, deeper cavities and heavier membranes result in a lower resonance frequency and vise versa. Normally, membrane absorbers absorb within a fairly narrow range of frequencies (in the order of one octave) in the low tone register.

Unwanted sounds. Noise can cause fattique, stress, poor intelligibility, cardio vascular deceases. Exposure to high noise levels over long periods of time can also cause permanent damage to hearing.

A material typically consisting of thin fibers which are not densely connected so that a very large part of the space occupied by the material is actually consisting of air. Typical such materials are woven cloth, mineral wool or synthetic foam materials. The absorption effect is due to friction between the moving air molecules in the sound wave and the very large total inner surface of the fibre material. Porous absorbers normally absorb sound energy in the middle and high frequency range (depending on thickness of the porous layer or its distance from a hard surface such as the wall or a ceiling slab).

The phenomenon that certain systems vibrate strongly at a specific frequency when excited either by a short impulse or continuously by a source containing that same specific frequency. Examples are vibrating strings, air in tubes of given lengths – or rooms, where both one, two and three dimensional resonances can occur.

A system consisting of a (often stiff) surface with holes or slits in front of a confined air volume (often with a porous absorber inside or just behind the perforated surface). Such a construction will exhibit a tendency to resonate in a certain frequency range where the air in the holes vibrate strongly when hit by sound containing this frequency range. The absorption occurs through friction between the edges of the holes/slits and the strongly vibrating air molecules or between the vibrating air and a porous material behind the perforated surface. Typical examples are perforated gypsum ceilings or perforated metal sheets placed at a distance from a wall or ceiling slab. Normally the frequency range of the absorption is larger (two to four octaves) and placed higher in the sound spectrum than found for membrane absorbers, but the absorption effect at high frequencies is normally lower than that of porous absorbers.

The decay of sound in a room detected after a sound source has stopped. In rooms with little absorption and large dimensions, the reverberation time is larger than in smaller rooms or rooms with larger areas covered with sound absorbing materials. In mathematical terms, the reverberation time in a room (measured in seconds) is roughly proportional to the room volume and inversely proportional to the total absorption area in the room (as described by the so called Sabine formula). The total absorption area is the sum of the absorption areas of all surfaces and items in the room plus a little absorption in the air itself at high frequencies.

The phenomenon that some of the reflected energy is dispersed into many different directions after having hit a boundary surface or obstacle. This occurs when the surface of the hitted surface is irregular measured with a scale comparable to the wave length (scattering) or when the extension of the reflector is small relative to the wavelength (diffraction).

Wave energy in the form of varying pressure and movement of the air travelling through atmospheric air (and sometimes liquids and solid structures as well) perceivable by the human ear.

A (solid) material characterized by its ability to absorb sound.

The phenomenon that part of the sound energy is removed from the sound field (and transferred into another form of energy - usually heat) when the travelling sound wave hits a surface or body. Hereby the level of the sound – and in closed rooms also the reverberation time – is reduced. If all sound energy hitting the surface is absorbed, we say that the absorption coefficient is one. If no sound is absorbed the coefficient is zero.

The energy represented in a sound field. The amount of energy in – even very loud - sounds is much smaller than those we normally deal with as “energy”. The maximum sound energy produced by a 100 piece symphony orchestra is in the order of one Watt, although this will result in a level above 100 dB everywhere in a large concert hall.

Sound energy (under observation) within a certain part of space.

Amplitude/intensity.

The phenomenon that one (loud) sound, the masker, can make another (weaker) sound inaudible, when the two sounds are present at the same time. The masking effect is more pronounced when the frequency content of the masker is equal to or slightly lower than the sound which is being masked. When the frequency contents of the two sounds are far apart (on the tone scale), masking is less likely to occur.

The phenomenon that part of the sound energy hitting a surface or body is NOT absorbed, but reflected back into the sound field. If all the incoming energy is returned to the sound field, we say that the absorption coefficient is zero. When the travel directions of the incoming and most of the reflected energy are similar to those of a ball reflected by the barrier of a billiard table, we call it a specular reflection.

An obstacle which is capable of obstructing the transmission of sound (either through reflection or absorption), so that a sound “shadow” is established behind the barrier.

A system capable of emitting sound, e.g. the human voice, an instrument, a loudspeaker, a vibrating machine, strong air currents hitting trees etc.

The phenomenon that in some cases some of the sound energy is neither reflected/scattered nor absorbed, but travelling through the absorber and continuing its travel from the rear side.

A sound wave consists of elastic oscillations (in air) which means that the air pressure varies between being (slightly) higher and slightly lower than the static, atmospheric pressure (on average one Bar at sea level). A simple visual analogy is the waves observed on a water surface when a stone is dropped in water. The source in this case is the stone hitting the water surface.

The degree to which speech is understandable to a listener. The most common objective measure of speech conditions in a room is the Speech Transmission Index (STI), which can be measured as a number between 0.0 and 1.00. Subjective testing of speech intelligibility is performed as a percentage of consonants or words being correctly understood. However, such tests are very cumbersome and mainly carried out in laboratories. Objective STI testing can be performed in minutes and reveal results which are highly correlated with subjective impressions as indicated below:

<STI – skala>

About 340 meters per second in atmospheric air at normal temperatures (20 degrees Celsius).

The physical distance between two peaks in a complete period of an oscillation (from neutral over high over neutral to low and back to neutral). The wavelength is equal to the speed of sound divided by the frequency. Thus, the wave length in air of a 100 Hz tone is about 3.4 m, and that of a 1000 Hz tone is 34 cm.