2014_Summer_LARSS_Technical_Paper_O'Meara_Cody

Rotating acoustic diffusor
Implementation of a rotating diffusor in the
NASA LaRC Structural Acoustic Loads and
Transmission (SALT) facility
Cody A. O’Meara
_________________________
Albert R. Allen
_________________________
Research Directorate, Structural Acoustics Branch
August, 2014

Implementation of a rotating diffusor in the NASA LaRC
Structural Acoustic Loads and Transmission (SALT) facility
Cody A. O’Meara
Albert R. Allen
Research Directorate, Structural Acoustics Branch
Aeronautics Research Directorate

Abstract
Measurements carried out in the NASA LaRC SALT facility reverberation room have
recently been used to evaluate noise control concepts targeting lower frequencies
typically below 300 Hz. The testing involves the measurement of changes in the space
averaged room sound pressure level (SPL) or transmission loss (TL) due to the addition
of a noise control concept, such as an array of resonant absorbers. The purpose of this
project is to design, model, and implement a prototype rotating diffusor in the SALT
facility’s reverberation room in order to reduce the variance of measurements by
modulating the room modes during data acquisition. The benefit of using the rotating
diffusor was realized in the form of reduced variance in the measured data.
In order to achieve this goal, background research was performed on the various
designs of acoustic diffusors, and their effectiveness at modulating room modes. Based
on time restrictions and ease of fabrication, it was determined that a rotating planar
diffusor panel would be built, as opposed to more complex conical shaped designs. In
order to remain practical, the panel must be able to be easily handled by two people
during installation, while also maintaining a size that allows it to be rotated without
rearranging the existing microphone array. Once the dimensions were determined, the
materials were ordered, and the prototype diffusors were built using steel strut channels
as a frame, and covered with corrugated PVC panels on either side. After the fabrication
was completed, the panels were then evaluated by performing measurements with and
without the rotating diffusor. A basic model of the room was created in parallel using the
Finite Element (FE) modeling software Cubit. Initially, the reverberation room was
modeled as a rectangular prism with similar room volume. Subsequently, the
reverberation room model was recreated using existing building blueprints in order to
more accurately represent the effects of the oblique angles presented by the facility’s
splayed walls. The resulting room models were then used to predict the acoustic room
modes for various diffusor configurations using the Salinas FE solver. The ability of each
diffusor configuration to effectively modulate room modes was assessed by viewing
changes in the room mode Modal Assurance Criterion (MAC) values at various diffusor
angles relative to a fixed angle.
The rotating diffusor appeared to improve the quality of the test data by reducing the
variance in the noise reduction (NR) and insertion loss (IL) spectra attributed to
introducing a resonator array. Choice of drive signal configuration also influenced the
results. Further analysis of the data is currently underway. Due to the promising initial
results of the prototype diffusor panel and its ability to reduce variance in measurements,
further investigation into a more permanent diffusor panel and rotational drive system
seems advantageous.

Introduction
Recently, experimental evaluation of resonant, narrowband noise control concepts
have been performed in the 278 reverberation room at the NASA LaRC SALT
facility. The typical approach taken when evaluating noise control devices is to determine
the absorption area of the device by measuring the band-limited reverberation time (i.e.
T60) in the room with and without the device present or active. This results in fractional
octave, e.g. 1/3 octave, band-limited data and often precludes the evaluation of
narrowband performing resonant noise control devices, which are currently of interest
[1]. Due to this limitation, a non-standard approach is taken, whereby the narrowband
Noise Reduction (NR) attributed to the noise control device is measured directly. A NR
test involves the measurement of the space averaged room level during steady state
ensonification of the room with and without the noise control device present in a two-step
process. The difference of the two resulting room level spectra provide the NR attributed
to the concept.
At low frequencies, the variance of the NR spectrum makes it difficult to evaluate the
performance of a resonant noise control device, especially when the NR attributed to the
device is particularly low or narrowband in character. In order to confidently evaluate a
device such as this, the measurable performance can be increased by increasing the test
article size, which can be costly, or the variance of the measurement can be reduced by
introducing a rotating diffusor to increase the effective room diffusivity in a time
averaged sense. The benefit of the rotating diffusor becomes particularly apparent during
the direct measurement of NR due to a resonant noise control device. Specifically,
introducing a strong, local impedance change in the room in the form of a resonant noise
control device has been found to “shift” or “split” the otherwise fixed room modes. It is
postulated that this increases the variance of the resulting NR spectrum and is an artifact
of the room and not indicative of the performance of the noise control concept on
average. While the diffusor panel rotates, it continuously warps the room shape in time,
while also continually redirecting the energy flow in space. The combination of these two
events helps to create a more diffuse field by continuously shifting both modal
frequencies and incidence angles during testing [2].This effect has been found to average
out the mode splitting effect produced by the resonator. A schematic of the rotating
diffusor concept is shown in Figure 1.
Figure 1: Rotating Planar Diffusor

The methodology of this project can be summarized in six steps. The first
consisted of performing background research on diffusors used in reverberation rooms.
Next, 10’ x 6’ diffusor dimensions were established based on practical limitations such as
maintaining clearance with the microphone array in the reverberation room. This was
followed by material ordering and fabrication of the diffusors. The diffusors were then
evaluated in the reverberation room by conducting room level acquisitions at fixed
diffusor positions and NR tests with and without the diffusor actively rotating during
acquisition. A numerical model of the room was created in parallel using the Finite
Element (FE) modeling software Cubit, and the Salinas FE solver was subsequently used
to assess the ability of various diffusor configurations to effectively modulate room
modes by viewing relative changes in the low frequency room modes during simulated
diffusor rotation.
Summary of Research
Referring to the original project goals, the mission was to design, model, and
implement a prototype rotating diffusor in the SALT facility’s reverberation room in
order to reduce the variance of measurements by modulating the room modes during data
acquisition. Because of the complexity of this project, it was necessary to split the tasks
into two different subsets: namely the design and fabrication of the diffusors, and the
modeling. This report focuses on the design, fabrication, and experimental evaluation of
the diffusors.
Several factors were involved when determining what type of diffusor to build.
Two main constraints were immediately apparent when this project was assigned: time,
and ease of fabrication. With only a ten week period to design, build, test, and analyze the
effectiveness of the diffusors, it was necessary to choose a design that would allow for a
quick and simple fabrication without taking away from its ability to modulate the room
modes. Because of this, it was determined that a rotating planar diffusor panel would be
built, as opposed to more complex conical shaped designs.
Next, the dimensions needed to be determined. The panel needed to be large enough
to reflect low frequencies well, but also small enough to be able to be easily handled by
two people during installation, while also maintaining a size that allows it to be rotated
without rearranging the existing microphone array. Because of the restrictions provided
by the microphone array in the room, it was determined that the width would be ten feet,
as this was the maximum width that could be obtained without having to rearrange the
microphones. A height of six feet was also determined, as anything taller would have
been difficult to move in and out of the reverberation room’s door.
Material choice was also researched, as the materials must be light enough to install
and remove from the room, while also remaining acoustically solid in order to reflect the
sound waves. Because of these factors, a steel strut channel frame was built for stability,
while corrugated PVC panels were attached to both sides by punching holes around the
perimeter of the PVC panels and attaching them to the steel frame with zip ties, as seen in
Figure 1. No filler material was installed in the space between the panelings, however, it
may be advantageous to do so in the future.

Tests Conducted
Once the diffusor panel was complete, it was then installed into the SALT facility’s
278 reverberation room as shown in Figure 3. In order to properly test the panels, the
microphones in the room had to be calibrated using a Brüel & Kjær model 4134 piston
phone microphone calibrator. This calibrator sends a pure tone to the microphone, which
is then matched in the system to ensure an exact calibration. The equipment and
instrumentation used for testing can be seen in Table 1 and is further described in [3].
Room sound pressure level (SPL) and noise reduction (NR) measurements were taken
with the panel either stationary or rotating in order to determine its effectiveness at
modulating the room modes, as well as its ability to average out the mode splitting effect
produced by a resonant noise control device evaluated during NR measurements.
Initially, the diffusor was rotated manually at approximately 5 RPM before the use of
steady aerodynamic force via fixed fans was found to be more favorable. The diffusor
panel was hoisted near the center of the room from a chain that is mounted to a swivel as
shown in Figure 2. An arrangement of three or more box speakers, as shown in Figure 3,
were used to excite the room with steady state noise while the sound pressure levels were
measured with twelve microphones placed randomly throughout the room. Tables 2-4
show the types of tests that were run. When evaluating a resonant noise control device,
such as the resonator array panel shown in Figure 4, room sound pressure levels were
acquired with both open and closed inlets in order to determine the NR attributed to the
resonator array.
Figure 4: Resonator array panel with open (left) and closed (right) inlets.
Figure 2: Diffusor panel installed
in center configuration, 0°,
mounted on swivel
Figure 3: SALT facility with
speaker box locations in
reverberation room

Table 1: Instrumentation and equipment used for testing.
Description Manufacturer Model/Type
½-inch Prepolarized Condenser Microphones G.R.A.S. 40AO
½-inch Microphone Preamplifiers G.R.A.S. 26CA
ICP Signal Conditioner PCB Piezotronics 584A
Multi-Channel Amplifier Rane MA6S
Signal Switching System Precision Filters PF 464K
Chassis with Dynamic Signal Acquisition Modules National Instruments NI PXI-1045 NI PXI-4472B (3)
Acoustical Calibrator Brüel & Kjær 4231
Two-way Speaker Box JBL Professional JBL JRX115
Table 2: Fixed diffusor angle tests.
Output File Drive Signal Diffusor Angle
roomSPL_10_Jul_2014_14_01_46 Uncorrelated Random 30°
roomSPL_10_Jul_2014_14_09_06 Uncorrelated Periodic Random 30°
roomSPL_10_Jul_2014_14_11_36 Correlated Periodic Random 30°
Table 3: Cases acquired during fixed or rotating diffusor, for placement of the resonator
array test article during ensemble averaging procedure
Output File Diffusor Angle Resonator Location Inlet Open/Closed
roomSPL_11_Jul_ 2014_10_49_59 0° 1 Closed
roomSPL_11_ Jul_2014_11_08_43 Rotating 1 Closed
roomSPL_11_ Jul_2014_11_19_36 0° 2 Closed
roomSPL_11_ Jul_2014_14_54_05 0° 3 Closed

Table 4: Series of fixed vs. rotating NR tests with no resonant test article present.
Output File Diffusor Angle
roomSPL_24_Jul_2014_09_51_22 0°
roomSPL_24_Jul_2014_09_51_22 0°
roomSPL_24_Jul_2014_10_04_52 0°
roomSPL_24_Jul_2014_10_13_19 0°
roomSPL_24_Jul_2014_10_19_11 0°
roomSPL_24_Jul_2014_10_25_02 0°
roomSPL_24_Jul_2014_10_30_54 0°
roomSPL_24_Jul_2014_10_36_46 0°
roomSPL_24_Jul_2014_10_42_38 0°
roomSPL_24_Jul_2014_10_49_13 0°
roomSPL_24_Jul_2014_11_10_15 Rotating
Data Reduction and Results
In order to evaluate the effectiveness of the diffusor, the raw data had to be compiled
into an interpretable format. The first data that is used relates to the space averaged room
pressure power spectral density (PSD) measured when the PVC panel diffusor is mounted
in the center of the room and fixed at different angles (refer to Table 2). The pressure
PSD is determined by
where df is the spectral resolution, which is 0.1 Hz, and is the time and space
averaged mean squared pressure acquired from 12 reverb room microphones during
steady state random noise excitation, as seen in Figure 5. The modes begin to be effected
at higher frequencies, typically above 200 Hz. However, the effects can still be seen at
these lower frequencies.

When measuring the NR spectrum of the resonator array shown in Figure 2, the space
and time averaged room sound pressure spectra are measured with and without the
resonator inlets closed. The NR spectrum is the dB difference between these two
measurements and shows the relative change in room level with the resonator array open
and closed. The noise reduction is calculated using
where is the space and time averaged mean square pressure acquired from 12
reverb room microphones during steady state random noise excitation and averaged over
3 test article locations. Figure 6 and Figure 7 shows the results. The resonator array used
in this example exhibits positive narrowband NR at the first and second “quarter wave”
resonances near 95 Hz and 280 Hz respectively. The ability to perceive the resonator
array NR is improved by the reduced variance, as shown in Figures 6 and 7, especially at
higher frequencies.
Figure 6: The NR spectrum for a fixed diffusor and a rotating diffusor, 50-150 Hz.
(a) Fixed diffusor, 0° (b) Rotating diffusor
Figure 5: PSD spectrum for fixed diffusors ranging from 0°-90°, and 30-160 Hz.

While the NR spectrum shows an apparent reduction in variance that increases with
frequency, it can still be difficult to quantify the effect of the rotating diffusor. In an
attempt to further characterize the frequency dependent effect that using the diffusor has
on the variance of measured NR spectra, a series of additional tests were conducted (see
Table 4) with no test article to the room to provide a set of many NR spectra with and
without use of the rotating diffusor. These NR measurements are referred to as zero-NR
measurements, because the expected NR from subtracting out two measurements of room
level spectra with no test article in place during either measurement is zero. In order to
have a better understanding of the variance in the NR spectra , it helps to make
histograms of the narrowband NR values within specific frequency bands, namely 1/3
octave bands. Fixed and rotating diffusor histograms were calculated in this way from
both the resonator array NR results and the zero-NR measurements and are shown in
Figures 6-7. To more clearly view the relative difference in spread between the fixed and
rotating diffusor NR values, vertical dashed lines are also shown in Figures 8-9
representing +/- one standard deviation, which can be found using
∑
⁄
Where n is the number of elements in the sample, x is the vector of all points in the
NR spectrum within each 1/3 octave band with 0.1 Hz resolution, and ∑ . The
y axes are normalized by dividing the bin counts by the total area under all the bins..
(c) Fixed diffusor, 0° (d) Rotating diffusor
Figure 7: The NR spectrum for a fixed diffusor and a rotating diffusor, 200-400 Hz.

Figure 8: Histograms showing the NR in the room at specific frequencies with no
resonator present. and when the resonator is present (right).

Due the NR's being determined from acquisitions using no test article, the mode shifting
due to the test article's effect on the room modes were not there, so the variance was
relatively small, and the effect of the diffusor was not as significant.
Conclusions
An experimental assessment of the ability of a rotating diffusor to reduce the variance
of reverberation room measurements by modulating the room modes during data
acquisition has been carried out on a prototype diffusor panel that is either held stationary
or rotated at approximately 3 RPM using a fan during data acquisition. Measurements of
space and time averaged room sound pressure levels acquired during steady state noise
excitation was used to analyze the effectiveness of the prototype diffusor panel when
evaluating a candidate resonant noise control device. Diffusor rotation appeared to
improve the quality of the test data by reducing the variance in the noise reduction (NR)
attributed to introducing a resonator array, especially at higher frequencies. Interestingly,
the variance reduction when using the rotating diffusor was found to be much less
significant when no test article was present, which suggests that the diffusor is most
effective at reducing variance in the NR spectrum due to the mode shifting effect of local
impedance boundary condition changes introduced by a resonant noise control device.
Due to the promising initial results of the prototype diffusor panel and its ability to
reduce variance in measurements, further investigation into a more permanent diffusor
panel and rotational drive system seems advantageous.
Figure 9: Histograms showing the NR in the room at specific frequencies with the
resonator present.

References
[1] Allen, A. R., & Schiller, N. H. NASA LaRC, (2014).Transmission loss and
absorption of corrugated core sandwich panels with embedded resonators. Fort
Lauderdale, FL: Noise-Con.
[2] Schultz, T. Diffusion in reverberation rooms. Journal of Sound and Vibration, 17-28.
[3] Grosveld, F. W. (2013). Characterization of the reverberation chamber at the NASA
Langley structural acoustics loads and transmission (SALT) facility (Report No.
NASA/CR--2013-217968). Hampton, VA: NASA.

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