Absorption
Acoustical absorption is the energy transformation from acoustic energy pressure waves) to thermal energy in a porous material like mineral wool.
Absorption Coefficient α
The absorption coefficient α is a material property, which describes how well a material absorbs sound waves... α=1 means 100% absorption, α=0 means 100% refl ection.
When building materials for inner boarding are chosen, the function of the room and the absorption coeffi cient of the boarding must be harmonized.
A high absorption coefficient is not necessarily a good one – it depends on the function of the individual room. The absorption coefficient has an impact on reverberation time.
A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. Unless otherwise specified, a diffuse sound field is assumed. The values at the sound-absorption coefficient usually range from about 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms.
The sound absorption coefficient can be measured in 1/3 octave bands according to ISO 354 and weighted as αw acc. to ISO 11654.
Acoustic impedance
The acoustic impedance Z (or sound impedance) is the ratio of sound pressure p to particle velocity v in a medium or acoustic component and is usually represented in complex notation as Z=R + iX. The real and imaginary components are called, respectively, acoustic resistance and acoustic reactance.
See also characteristic impedance.
Airflow resistance
See specific airflow resistance.
Airflow resistivity
See specific airflow resistance.
Air absorption
Air absorption is usually neglected in calculations of reverberation times for auditoriums, but for large enclosures it may become signifi cant. Air absorption is greater for high frequencies and is dependent upon air temperature and relative humidity.
In calculations of reverberation times the air absorption can be included by adding an equivalent absorption area for the sound attenuation by air as (acc. to EN 12354-6):
Aair=4*m *V
V - room volume
Airborne sound
Airborne sound is the sound propagation in air as pressure waves. The sound propagation in solid materials is called structure borne sound propagation.
Airborne sound insulation
The reduction of the sound intensity for a sound wave passing through a building element. (level difference for the incoming airborne sound wave and transmitted airborne sound wave).
The sound insulation is normally expressed as the sound reduction index R.
R depends on the frequency of the sound passing through the element and is measured in 1/3 octave bands.
Ambient noise
The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.
Amplitude
The instantaneous magnitude of an oscillating quantity such as sound pressure.
The peak amplitude is the maximum value.
Anechoic Chamber
An anechoic chamber is a room where there is no reverberation and no echo.
An acoustic space without echo or reverberation. Often used for the acoustic testing (e.g. microphones and loudspeakers) or sound recordings without any refl ections for use in auralization examples.
ASTM E90 Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions
This test method covers the laboratory measurement of airborne sound transmission loss of building partitions such as walls of all kinds, operable partitions, floor-ceiling assemblies, doors, windows, roofs, panels, and other space-dividing elements.
ASTM E413
Classification for Rating Sound Insulation
The STC value (Sound Transmission Class) is calculated from 125Hz to 4000 Hz. The Weighted Sound Reduction Index (Rw) is the ISO equivalent of STC.
It is determined in accordance with ISO 717, Rating of Sound Insulation in Buildings and of Building Elements. The reference contour extends from 100 to 3150 Hz and there is no 8 dB rule. Usually STC and Rw are approximately equal.
ASTM C423
Standard test method for sound absorption and sound absorption coefficients by the reverberation room method Measurement of sound absorption coefficient a in a diffuse field (room methode) and calculation of the NRC value (Noise Reduction Coefficient)
Attenuation
When sound travels through a medium, its intensity diminishes with distance. This weakening in the energy of the wave results from two basic causes, scattering and absorption. The combined effect of scattering and absorption is called attenuation
Audible frequency range
A human ear can normally perceive frequencies from 20 Hz to 20.000 Hz. This is called the audible frequency range.
Aural
Relating to the ear or to the sense of hearing. From Latin auris, ear.
Auralization
Auralization is the process of rendering audible, by physical or mathematical modelling, the sound field of a source in a space, in such a way as to simulate the binaural listening experience at a given position in the modelled space. [Kleiner et al 1993].
A-weighting
A frequency-response adjustment of a sound measurement that conditions the output signal to an approximate human response (fits best to the human ear at a sound level of app. 40 ... 50 dB(A)).
The A – filter values [dB] in 1/1 octave bands from 31 HZ to 8 kHz are:
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