Acoustic glossary
The practical absorption coefficient αp
Weighted absorption coefficient αw
Absorption
The absorption of a surface is usually referred to as A = α x S, where S is the surface area in m², α is the absorption coefficient and A is the surface absorption/m².
Different materials and constructions absorb sounds with different results, depending on what frequency the sound has.
Medium high and high frequencies are absorbed primarily of porous materials such as textiles and mineral wool. Typical of high frequent absorbents is that they have a relatively low flow resistance – you can, by mouth, blow air through the material with some resistance. Low frequencies – bass notes, are mainly absorbed by thin plate materials or membranes such as panes of glass – or by gaps and cracks.
A consequence of the absorption frequency is due to that the reverberation time in a room will be different at different frequencies. An appropriate choice of materials in floors, walls, ceilings and interiors affords reverberation times at different frequencies that are the most suitable to its function and use.
One important factor is where the distance between the absorbent’s surface and underlying hard surface, such as the distance between a curtain and a wall. Each doubling of that distance expands the absorbent’s area of use towards lower frequencies by an octave. The absorbent’s “break frequency” will be halved.
The practical absorption coefficient αp
αp is used for to calculate reverberation times. The absorption coefficient is measured according to the international standard ISO 354. αp is given for each full octave, 125 Hz, 250 Hz, 500 Hz, etc. as an average value of three one third octave measuring. The calculated and accounted absorption diagram shows the absorption’s ability to attenuate sound reverberations in different frequencies, per unit area (m²) of the material. When you know the absorption coefficient one can calculate the surface area of absorbent required to achieve the desired reverberation time.
Attenuation of sound reverberations will be perceived by the ear as a lowering of the noise level. It might be good to know that a change in sound level of 10 dB is perceived as a halving of the sound.
Weighted absorption coefficient αw
αp is compared to a reference curve according to ISO 11654 where maximum negative deviation is allowed to be 0.10. αw is thereafter given as a value, which is the reference curve’s value at 500 Hz.
Absorption classes
To simplify the description of a material’s ability to absorb sounds, we divide it into different classes according to ISO 11654 for that reason, as is represented from A to E, where A is the group that has the best absorption.
In order to be classified under any of the classes A to E, it is required to attain certain absorption values within the frequencies between 200 – 4000 Hz according to the following chart.
1. Absorption A 2. Absorption B 3. Absorption C 4. Absorption D
5. Absorption E 6. Unclassified 7. Frequency Hz 8. αp Practical sound absorption coefficient
Reverberation time
The time it takes for the sound to subside so that it no longer is heard is called the reverberation time – which physically is defined as the time it takes until the level has dropped 60 dB.
The reverberation time is influenced by the room volume and by the amount of absorption in the room. In rooms with similar shape and interior the reverberation time increases proportionally with the size of the room if the proportions of the room are unchanged. Even the shape of the room has a particular significance. The choice of materials in floors, walls, ceilings and interiors determine the amount of absorption in the room and is thus a factor of importance for the reverberation time. Since the volume of the room is usually given, it is the absorption that is employed to achieve the desired reverberation time. The importance of the choice of materials in building and interior design is primarily due to that different materials absorb sounds with different results and in different range of frequencies. Large areas with high absorption in a room afford a short reverberation time and the opposite affords a long reverberation time. The reverberation time is thus a measure of the acoustical absorption in a room.
Sound
Sounds are pressure waves, a wave motion which follows the same laws, such as water waves. There are long waves – bass tone, and short waves – treble tones. Sound waves most often consist of both long and short waves at the same time; just as a large water wave which in its turn is curled by smaller waves. Low tones are influenced by high tones in the same way. One important quality of sounds is that low and high tones spread in different ways in a room. Low tones are behaving almost like gas; they move in circles around objects and corners and fills up the entire room. High tones behave differently; they can be shadowed by barriers, and be reflected by surfaces. Highs tones can be both directed by reflective surfaces, and be absorbed. The low tones can only be influenced by absorbing them away.
If the sound level of a bass tone and a tone of medium height changes, the level of the higher tone has to be changed even more in order for the ear to still be able to perceive the tones of equal strength. Physically/for measuring it is therefore required a major change of the level at higher frequencies. The perception of the sound level of sounds composed of different high tones corresponds, partly because of this, not easily with the levels such as measured with a sound level meter. Technical readings provide answers that are only valid under given conditions.
Soundproofing
Soundproofing is another term, which refers to a design’s ability to prevent sound energy to pass from one room to another. When an airborne sound hits a confined area, some parts of the sound energy reverberate against the surface and some parts passes through the confined area. By that, the construction reduces the transferred sound energy to adjacent spaces.
It is said that the design has a “reduction rate”, R. The reduction rate depends on both sound frequency and angle of incidence, and partly on the construction’s build-up.
Acoustic pressure level
The acoustic pressure level is measured in dB and it is a measure of the acoustic level in the room. There is a relationship between sound absorption and an acoustic pressure level that can be expressed by the following formula:
Lp = Lw x 10 x log (A/4)
Where:
Lp = Acoustic pressure level
Lw = The acoustic power level of the source
A = Acoustical absorption amount, i.e. the selective coefficient of absorption x the surface.
High acoustical absorption in a room affords a lower acoustic pressure level at the same time.
Audibility
Audibility indicates how clear the sound from the transmitter is perceived by the recipient. What a reverberation time the room has and the amount of disturbances such as noise will affect the audibility. There is a demand for excellent audibility in e.g. classrooms for remedial teaching, auditoriums etc. Short reverberation times from 0.4 to 0.6 sec customized for the actual room is normally sufficient, however, in some cases it is necessary to strengthen the reverberations so that the sound will not be absorbed in such high degree that the audibility deteriorates. The acoustician will play an important part of this.