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The
sound that we hear in a room is a combination of the direct sound and
the indirect reflections from the room's boundary surfaces and contents.
Hence, one of the central topics in room acoustics is how to manipulate
these indirect reflections that affect the way we perceive sound.
Sound striking a surface is either transmitted or reflected. The reflected
sound can either be attenuated by a sound absorbing surface, re-directed
by a reflecting surface or scattered by a diffusing surface. When a significant
portion of the reflected sound is uniformly dispersed, we call the surface
a diffusor. The past 100 years, since the founding of architectural acoustics
by Sabine, have largely been devoted to studying how absorption affects
sound. Over this time a library of absorption coefficients have been tabulated
based on accepted standards and a reasonable understanding of how absorption
should be applied in room designs has been achieved.
By contrast, scientific knowledge on the role of diffusing surfaces has
only been developed much more recently. Over the past 20 years significant
research on methods to design, predict, measure, and quantify sound diffusing
surfaces by RPG has resulted in a growing
body of information on this topic. For example, RPG is working with
international standards organizations in an attempt to quantify the diffusivity,
or the degree to which a surface uniformly scatters incident sound, and
the random incidence scattering coefficient.
To provide a fast and accurate measurement of the directional scattering
properties of architectural surfaces, RPG research developed the first
2-dimensional 1:5 scale, high angular resolution, fixed multi-microphone
polar boundary measurement system using a maximum-length sequence (MLS)
fast Hadamard approach, which offers 70 dB of dynamic range. The measurement
hardware is based on the TEF-20 DSP, which samples at 48 kHz.
Figure 1 shows a sample at the center of two concentric semicircles. The
inner semicircle has a radius of 1m and contains 37 fixed Crown GLM-100
microphones providing 5 degree angular resolution. The outer semicircle
has a radius of 2m and contains fixed positions to vary the angle of incidence
of the loudspeaker.
Data Reduction
Measurement
hardware and computer control |
Figure
2:
Select image to enlarge
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Under
computer control 37 MLS stimuli are emitted in turn. A microphone switcher
selects the appropriate microphone and 37 impulse responses are recorded.
The data analysis is shown in Figure 2. To isolate the scattering from the
sample under test (rectangle in B), a background measurement containing
the direct sound and the room interference is subtracted. The speaker /
microphone response is then deconvolved from the measurement and the scattered
data are windowed. These windowed data for all of the angles of observation
are concatenated in Figure 2C in the form of a "temporal" angular response.
Each impulse response is transformed into its frequency response, Figure
2D, and 1/3 octave polar responses are obtained, Figure 2E. The auto correlation
of these polar responses is used as a diffusion coefficient metric and is
plotted versus frequency to obtain the diffusion response, Figure 2F.
3D Goniometer
For one dimensional scattering surfaces like cylinders and 1D QRDs, the
2D diffusion apparatus is useful. For two dimensional surfaces, a 3D goniometer
is needed to measure the backscattering on the surface of a hemisphere.
Such an apparatus has been designed and built as part of a grant funded
by the Engineering Physical Sciences Research Council and RPG Diffusor Systems.
This 3D goniometer is shown in Figure 3. The sample is placed at the center
of the 1m radius microphone arc and the 2m speaker arc. For a given loudspeaker
azimuth and elevation angle, measurements are made for various azimuth and
elevation positions of the microphone to uniformly describe the performance
on the surface of a hemisphere.
Figure 3 |
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Home:
Research & Development: The
Evolution of the Diffusion Coefficient
Measuring
Diffusion
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