ISO 17497-2:2012

INTERNATIONAL ISO
STANDARD 17497-2
First edition
2012-05-15
Acoustics — Sound-scattering properties
of surfaces —
Part 2:
Measurement of the directional diffusion
coefficient in a free field
Acoustique — Propriétés de dispersion du son par les surfaces —
Partie 2: Mesurage du coefficient de diffusion directionnel en champ libre
Reference number
©
ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measurement principle . 3
5 Frequency range . 4
6 Test arrangement . 4
6.1 Measurement environment . 4
6.2 Measurement field . 5
6.3 Test specimen . 6
7 Test procedure . 7
7.1 Test signals . 7
7.2 Source and receiving equipment . 7
7.3 Measurements . 7
7.4 Polar response processing . 8
8 Expression of results . 11
8.1 Directional diffusion coefficient . 11
8.2 Normalized directional diffusion coefficient .12
8.3 Calculation of area factors .12
8.4 Random incidence diffusion coefficient .12
8.5 Presentation of results .13
9 Test report .13
Annex A (normative) Qualification of a measurement space .14
Bibliography .15
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
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Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 17497-2 was prepared by Technical Committee ISO/TC 43, Acoustics, Subcommittee SC 2, Building acoustics.
ISO 17497 consists of the following parts, under the general title Acoustics — Sound-scattering properties of surfaces:
— Part 1: Measurement of the random-incidence scattering coefficient in a reverberation room
— Part 2: Measurement of the directional diffusion coefficient in a free field
iv © ISO 2012 – All rights reserved

Introduction
The degree of acoustic scattering from surfaces is very important in all aspects of room acoustics, e.g. in concert
halls, sound studios, industrial halls and reverberation chambers. The degree of scattering and absorption in
a room are important factors related to the acoustic quality of the room. This part of ISO 17497 addresses the
measurement and characterization of scattering surfaces.
The scattering coefficient is introduced in ISO 17487-1. In this part of ISO 17487, a measurement method for
the directional diffusion coefficient is introduced. The diffusion coefficient is different from, but related to, the
random incidence scattering coefficient. While the scattering coefficient is a rough measure that describes the
degree of scattered sound, the diffusion coefficient describes the directional uniformity of the scattering, i.e.
the quality of the diffusing surface. Consequently, there is a need for both concepts, and they have different
applications.
The work has been coordinated with the working group of the Audio Engineering Society, AES SC-04-02 for
the Characterization of Acoustical Materials.
INTERNATIONAL STANDARD ISO 17497-2:2012(E)
Acoustics — Sound-scattering properties of surfaces —
Part 2:
Measurement of the directional diffusion coefficient in a free field
1 Scope
This part of ISO 17497 specifies a method of measuring the directional diffusion coefficient of surfaces.
The diffusion coefficient characterizes the sound reflected from a surface in terms of the uniformity of the
reflected polar distribution. The diffusion coefficient 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 coefficient is not suitable for direct use as an input to
current diffusion algorithms in geometric room acoustic models.
This part of ISO 17497 details a free-field 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 filters
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
sound ray
line following one possible direction of sound propagation from a source point
3.2
specular reflection
reflection that obeys Snell’s law, i.e. the angle of reflection is equal to the angle of incidence
Note 1 to entry Specular reflection 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 specified reference flat surface via the edges of that surface to the receiver arc or hemisphere
Note 1 to entry The reference flat surface is a plane and rigid surface, with the same projected shape or footprint as the
test surface.
Note 2 to entry The position at which an imaginary line from the image source to a receiver crosses the diffuser is the
specular reflection point (see Figure 1).
Key
1 source
2 specular zone
3 diffuser
4 image source
5 receiver arc
Figure 1 — Representation of specular zone
3.4
far field
region in which the reflected sound pressure level from the test surface decays by 6 dB per doubling of distance
Note 1 to entry In the near field, the shape of the angular field 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 1 to entry 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 1 to entry For these surfaces, hemispherical evaluation is appropriate, yielding a single diffusion coefficient.
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-field or pseudo-free-field conditions, in a specified plane, on a
semicircle centred at the reference point, at an appropriate radial distance
Note 1 to entry The reference normal is an outward-pointing vector perpendicular to the front face of a reference flat
surface. The reference point is the geometric centre of gravity of the reference flat surface
2 © ISO 2012 – All rights reserved

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-field or pseudo-free-field conditions, on a hemisphere centred at the
reference point
3.9
directional diffusion coefficient
d
θ,φ
measure of the uniformity of diffusion produced by a surface for one source position
Note 1 to entry The value of d is bounded between 0 and 1. When complete diffusion is achieved by the surface,
θ,ϕ
the diffusion coefficient is 1. However, real diffusers rarely have diffusion coefficients higher than 0,7. If only one receiver
receives non-zero scattered sound pressure, the diffusion coefficient 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 coefficient
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 1 to entry A mean or a weighting of the directional diffusion coefficients for the difference source positions is used
to calculate the diffusion coefficient, as specified 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 coefficient
d
θ,φ,n
directional diffusion coefficient of the test specimen normalized to that of the reference flat surface
3.12
normalized diffusion coefficient
d
n
random incidence diffusion coefficient determined from the normalized directional diffusion coefficient
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 1 to entry 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 coefficient quantifies how the energy reflected from a surface is spatially distributed. This spatial
distribution is described by polar responses of the reflected 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
reflected sound is extracted from the microphone signals using the process outlined in Clause 7. The diffusion
coefficient is then calculated from the reflected sound pressure levels using the equations shown in Clause 8.
To remove finite-panel effects, which cause the diffusion coefficient to decrease as the frequency increases, a
normalized diffusion coefficient 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).
5 Frequency range
The measurements shall be performed in one-third-octave bands with centre frequencies covering the
frequency range from 100 Hz to 5 000 Hz, in accordance with IEC 61260 and ISO 266. This refers to full-scale
measurements. If a physical scale factor of 1:N is used, the centre frequencies should cover the frequency
range from N × 100 Hz to N × 5 000 Hz.
If the scale model is filled with a gas in which the speed of sound is different from that in atmospheric air, the
measurement frequencies shall be chosen in such a way that the wavelength obeys the physical scale ratio 1:N.
High frequencies may be omitted from the measurements if the attenuation in the air is too high.
6 Test arrangement
6.1 Measurement environment
Annex A describes the measurement environments that shall be used. A qualified anechoic chamber can be
used. An implementation of such a set-up is illustrated in Figure 2. Alternatively, a large non-anechoic space
can be used to simulate a reflection-free environment if certain techniques described in Annex A are used.
Figure 2 — Three-dimensional measurement goniometer
Boundary measurements may also be carried out to remove the necessity for a space to be anechoic in one
plane provided conditions in Annex A are satisfied. An implementation of such a set-up is illustrated in Figure 3.
4 © ISO 2012 – All rights reserved

Figure 3 — Two-dimensional boundary measurement technique
Scale models may be used to evaluate the diffusion from test surfaces. If the speed of sound is the same in the
model as in full scale, then the frequencies used for the model measurements shall be a factor of N higher than
in full scale. For scale models, the absorption properties shall be the same for both the full-scale surface at
full-scale frequencies and the test surface sample at the equivalent model-scale frequency. When considering
absorption from samples, losses due to viscous boundary layer effects shall be included. This inclusion can
limit the useable model scales.
6.2 Measurement field
6.2.1 Near-field versus far-field measurements
Diffusers may be applied in situations where some or all sources and receivers are in the near field. In such
cases, measurements to determine the diffusion coefficient should take place both at application-realistic
near-field positions and in the far field. The tests in the far field monitor the amount of diffusion achieved,
measurements in the near field shall be used to check for near-field aberrations, particularly focusing.
An exception to the preceding rule occurs if the diffuser is to be applied only for far field sources and receivers,
in which case, diffusion coefficient measurements may be undertaken only in the far field.
When comparing test surfaces, the same geometry shall be used in each case to avoid errors. Full geometry
information, source locations, receiver positions, and test surface dimensions and construction shall be
quoted in reports.
6.2.2 Far-field measurements
Approximate far-field conditions can be achieved if at least 80 % of the receiver positions are outside the
specular zone, see Figure 4. The source to reference-point distance should be 10 m and the receiver’s semi-
circle or hemisphere should have a radius of 5 m.
Measurements shall be made with a maximu
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