Acoustics - Sound-scattering properties of surfaces - Part 2: Measurement of the directional diffusion coefficient in a free field
1 Scope
This document specifes a method of measuring the directional diffusion coeffcient of surfaces.
The diffusion coeffcient characterizes the sound refected from a surface in terms of the uniformity of the refected polar distribution. The diffusion coeffcient is a measure of quality designed to inform producers and users of surfaces that, either deliberately or accidentally, diffuse sound. It can also inform developers and users of geometric room acoustic models. The diffusion coeffcient is not suitable for direct use as an input to current diffusion algorithms in geometric room acoustic models.
This document details a free-feld characterization method.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 266 Acoustics - Preferred frequencies
IEC 61260 Electroacoustics - Octave-band and fractional-octave-band flters
3 Terms and defnitions
For the purposes of this document, the following terms and defnitions apply.
3.1
sound ray
line following one possible direction of sound propagation from a source point
3.2
specular refection
refection that obeys Snell’s law, i.e. the angle of refection is equal to the angle of incidence
Note: Specular refection can be obtained approximately from a plane, rigid surface with dimensions much larger than the wavelength of the incident sound.
3.3
specular zone
area contained by imaginary lines that are constructed from the image source, which is created about the plane of a specifed reference fat surface via the edges of that surface to the receiver arc or hemisphere
Note 1: The reference fat surface is a plane and rigid surface, with the same projected shape or footprint as the test surface.
Note 2: The position at which an imaginary line from the image source to a receiver crosses the diffuser is the specular refection point (see Figure 1).
3.4
far feld
region in which the refected sound pressure level from the test surface decays by 6 dB per doubling of distance
Note: In the near feld, the shape of the angular feld distribution is dependent on the distance from the diffuser.
3.5
single plane diffuser
surface that displays distinct anisotropic behaviour, as can be the case for a cylinder or a one-dimensional Schroeder diffuser
Note: For these surfaces, the diffusion is measured in the plane of maximum diffusion.
3.6
multiple-plane diffuser
surface that is expected to display more approximately isotropic behaviour, as can be the case for a hemisphere or a two-dimensional Schroeder diffuser
Note: For these surfaces, hemispherical evaluation is appropriate, yielding a single diffusion coeffcient. Alternatively, measurements can be done in two orthogonal planes.
3.7
semicircular polar response
sound pressure level created by energy scattered from the surface as a function of angle measured about the reference normal, generated under free-feld or pseudo-free-feld conditions, in a specifed plane, on a semicircle centred at the reference point, at an appropriate radial distance
Note: The reference normal is an outward-pointing vector perpendicular to the front face of a reference fat surface. The reference point is the geometric centre of gravity of the reference fat surface.
3.8
hemispherical polar response
sound pressure level scattered from the surface as a function of spherical coordinates measured about the reference normal, generated under free-feld or pseudo-free-feld conditions, on a hemisphere centred at the reference point
3.9
directional diffusion coeffcient
dθ, φ
measure of the uniformity of diffusion produced by a surface for one source position
Note: The value of dθ, φ is bounded between 0 and 1. When complete diffusion is achieved by the surface, the diffusion coeffcient is 1. However, real diffusers rarely have diffusion coeffcients higher than 0.7. If only one receiver receives non-zero scattered sound pressure, the diffusion coeffcient is 0. The subscript θ is used to indicate the angle of incidence relative to the reference normal of the surface. The φ indicates the azimuth angle.
3.10
random incidence diffusion coeffcient
d
measure of the uniformity of diffusion for a representative sample of sources over a complete semicircle for a single plane diffuser, or a complete hemisphere for a hemispherical diffuser
Note: A mean or a weighting of the directional diffusion coeffcients for the difference source positions is used to calculate the diffusion coeffcient, as specifed in 8.4. A guideline to achieve a representative sample of sources is given in 6.2.2. The lack of a subscript for d indicates random incidence.
3.11
normalized directional diffusion coeffcient
dθ, φ, n
directional diffusion coeffcient of the test specimen normalized to that of the reference fat surface
3.12
normalized diffusion coeffcient
dn
random incidence diffusion coeffcient determined from the normalized directional diffusion coeffcient
3.13
physical scale ratio
1:N
ratio of any linear dimension in a physical scale model to the same linear dimension in full scale
Note: The wavelength of the sound used in a scale model for acoustic measurements obeys the same physical scale ratio. Therefore, if the speed of sound is the same in the model as in full scale, the frequencies used for the model measurements are a factor of N times higher than in full scale.
4 Measurement principle
The diffusion coeffcient quantifes how the energy refected from a surface is spatially distributed. This spatial distribution is described by polar responses of the refected sound pressure level. A source is used to irradiate the test surface, and microphones at radial positions in front of the surface are used to measure the sound. The refected sound is extracted from the microphone signals using the process outlined in Clause 7. The diffusion coeffcient is then calculated from the refected sound pressure levels using the equations shown in Clause 8. To remove fnite-panel effects, which cause the diffusion coeffcient to decrease as the frequency increases, a normalized diffusion coeffcient is calculated.
The microphone positions should map out a semicircle or hemisphere, for a single plane or hemispherical measurement, respectively. Single-plane diffusers can be measured using a two-dimensional goniometer, either using a boundary plane measurement (see Figure 3) or in an anechoic chamber. A multi-plane diffuser can be characterized by making two single plane measurements in orthogonal planes in a two-dimensional goniometer - this is the quickest and easiest approach. Alternatively, a hemispherical measurement can be done using a three-dimensional goniometer (see Figure 2).
