Experimental validation of methods to predict radiated noise is presented. A combined finite element and boundary element model was used to predict the vibration and noise of a rectangular box excited by a mechanical shaker. The predicted noise was compared to sound power measured by the acoustic intensity method. Inaccuracies in the finite element model shifted the resonance frequencies by about 5 percent. The predicted and measured sound power levels agree within about 2.5 dB. In a second experiment, measured vibration data was used with a boundary element model to predict noise radiation from the top of an operating gearbox. The predicted and measured sound power for the gearbox agree within about 3 dB

Oswald, Fred B.

Seybert, A. F.

English

NASA Technical Reports Server

NASA
	
AVSCOM
Technical Memorandum 105729 	 Technical Report 92—C-013
Experimental Validation of
Boundary Element Methods
for Noise Prediction
A.F. Seybert
Department of Mechanical Engineering
University of Kentucky
Lexington, Kentucky
and
Fred B. Oswald
Lewis Research Center
Cleveland, Ohio
Prepared for the
Inter-Noise '92
sponsored by the International Institute of Noise Control Engineering
Toronto, Canada, July 20-22, 1992
SYSTEMS COMMANDNASA
US ARMY
AVIATION
https://ntrs.nasa.gov/search.jsp?R=19930001778 2020-03-17T09:42:48+00:00Z
Experimental Validation of Boundary Element Methods
for Noise Prediction
A. F. Seybert
Department of Mechanical Engineering
University of Kentucky
and
Fred B. Oswald
NASA Lewis Research Center
ABSTRACT
Experimental validation of methods to predict radiated noise is presented in this
paper. A combined finite element and boundary element model was used to predict
the vibration and noise of a rectangular box excited by a mechanical shaker. The
predicted noise was compared to sound power measured by the acoustic intensity
method. Inaccuracies in the finite element model shifted the resonance
frequencies by about 5 percent. The predicted and measured sound power levels
agree within about 2.5 dB. In a second experiment, measured vibration data was
used with a boundary element model to predict noise radiation from the top of an
operating gearbox. The predicted and measured sound power for the gearbox agree
within about 3 dB.
INTRODUCTION
The prediction of noise in the design stage is important for building low-noise
machinery. In the past this was seldom done because the analysis techniques
lacked the necessary accuracy and detail. Except for the use of simple rules of
thumb, the designer's best guide was often years of experience and a large
database of old noise data. To shorten the time to bring a product to market,
companies are now beginning to rely less on testing and more on analysis.
However, the designer must have confidence in the analysis tools used.
Confidence may be achieved in a variety of ways including comparison of predicted
data with test results. In this paper the acoustic boundary element (BE) method
is validated by comparing the predicted sound power of a test gearbox with that
measured using the sound intensity method. 	 The vibration of the gearbox was
measured and used as input to the BE model.
An additional step in the prediction of noise occurs if the vibration is
predicted rather than measured using, for example, a finite element (FE) model
of the structure. In this paper a simple box-like structure was used to test the
feasibility of using a combined FE/BE model to predict both structural vibration
and radiated sound power.
PREDICTION OF NOISE OF TEST GEARBOX
Figure 1 shows the NASA Lewis gear noise rig. The rig consists of a
single-mesh gearbox driven by a 150 Kw variable speed electric motor and loaded
by an eddy-current dynamometer [1]. Figure lb shows the measurement of vibration
and sound intensity on the top surface of the gearbox. Also shown in this figure
is the sound measuring apparatus. Only the top surface of the gearbox was used
to simplify and shorten the validation study. A pair of accelerometers was used
to measure vibration; one accelerometer was moved from point to point and the
second accelerometer was fixed to maintain a constant phase datum.
The acceleration data were acquired by a two-channel spectrum analyzer,
transferred to a desktop computer, and converted to real and imaginary components
of velocity. These velocity components were used to produce an input file for
the BEMAP program [2]. For this work, a public-domain PC version of BEMAP was
used.
The vibration measurements were made on a rectangular grid of 7x9 points,
as shown in Fig. lb. This grid was used in BEMAP to construct a boundary element
mesh of 48 linear elements; each element consisting of four nodes, one at each
corner. Although BEMAP also contains the more efficient quadratic elements, the
linear quadrilateral element was selected because it matched the measurement grid
on the top surface of the test gearbox. The rule of thumb for linear elements
is that the mesh size should not exceed one-quarter of a wavelength for the
highest frequency of interest. The nodal spacing was 38 mm; hence, the highest
frequency meeting this requirement strictly is 2250 Hz.
Figure 2 shows the comparison of the sound power measured using the sound
intensity system in Fig lb [3] and that predicted from measured vibration using
BEMAP. The data in Fig. 2 were obtained for a running condition of 3000 rpm and
68 Nm torque. Agreement is generally good throughout the frequency range, even
above 2250 Hz, the limit suggested when using linear boundary elements.
A COMBINED FE/BE MODEL FOR NOISE PREDICTION
A box-like apparatus (Fig. 3) resembling the gearbox in Fig. 1 was
constructed to evaluate the feasibility of using a combined FE/BE model for noise
prediction. The apparatus consisted of sides and bottom of 12.7 mm steel plate
and a top surface of 1.6 mm aluminum plate [4].
Random excitation was applied to the apparatus in Fig. 3. The sound
intensity was measured on a hemispherical surface over the apparatus which was
placed on the floor of the laboratory. In addition, the force applied to the top
surface of the apparatus by the shaker was measured, and the measured sound power
was normalized by this force for comparison to the predicted sound power based
on a unit force.
An FE model was constructed for only the top surface of the apparatus in
Fig. 3. The sides and bottom were not modeled as they were assumed to be
perfectly rigid. The vibration level of the top surface was calculated for a
unit input force using the ANSYS program. A software interface was used to
transfer the FEM geometry and vibration data from ANSYS to BEMAP for the
prediction of sound power per unit force. A BE model (Fig. 4) for the apparatus
was constructed using quadrilateral and triangular elements. The BE model
included the sides of the apparatus, even though the FE model did not. Thus, the
three-dimensional geometry of the apparatus was modeled, but any sound radiated
from the sides was neglected in the FE/BE model (i.e., in the BE model the nodes
on the sides of the apparatus had zero velocity). Measurements showed that the
vibration amplitude of the sides of the box was approximately 10 dB below that
of the top surface.
Figure 5 shows a comparison between the measured sound power and that
predicted from the FE/BE model. Note the approximately 5 percent shift in the
resonance frequencies due to the overly stiff FE model. Clamped boundary
conditions were assumed in the FE model of the top surface of the apparatus. In
the actual apparatus the plate forming the top surface was restrained along its
perimeter by 12.7 mm square steel rods.
Figure 6 shows the measured and predicted sound power data from Fig. 4
summed into one-third octave bands. Also shown in this figure is the sound power
predicted from measured vibration. The measured vibration data were obtained
only for the top of the apparatus in a manner similar to that described for the
gearbox. The table below shows the overall linear and A-weighted sound power
levels for the three spectra in Fig. 6.
A-weighted	 Linear
Measured	 113.5	 120.5
Predicted (measured vibration)
	 112.0
	 119.0
Predicted (FE vibration) 	 109.5
	 118.0
2
DISCUSSION OF RESULTS
It may be seen from the results that it is feasible to predict sound power
quite reliably if the vibration data are known accurately. For example, when
measured vibration data is used in BEMAP, the sound power may be predicted with
an error of less than 2 dB overall. This is seen in the table above and in the
results of Figs. 2 and 5. These predictions are quite good when one considers
the host of experimental errors due to changes in operating conditions, drift in
calibration, etc., over the several hours needed to measure the vibration data
and the fact that the sound radiated by the sides of the apparatus in Fig. 3 was
neglected in the prediction.
The results also demonstrate the need for accurate FE models for accurate
prediction of machinery noise. Reliable estimates of damping on a per modal
basis are important if the modes are underdamped as is the case for the apparatus
in Fig. 3.	 (In practice, however, the damping is usually much higher for most
built-up structures, thereby making the resonant response less critical). In
addition, the structural boundary conditions must be known accurately. For the
FE model of the apparatus in Fig. 3, it was assumed for simplicity that the sides
of the box were rigid and the boundary conditions of the top plate were clamped.
Clearly, these boundary conditions are only approximately fulfilled in the real
apparatus.
REFERENCES
1. "Comparison of Analysis and Experiment for Gearbox Noise," Fred B. Oswald,
A. F. Seybert, T. W. Wu, and William Atherton, NASA TM-105330, 1992.
2. "BEMAP User's Manual," A. F. Seybert, T. W. Wu, and G. K. Wan, Spectronics,
Inc., 1991.
3. "Automated Acoustic Intensity Measurements and the Effect of Gear Tooth
Profile on Noise," W. J. Atherton, A. Pintz and D. G. Lewicki, NASA TM-
100155, 1987.
4. "Validation of FEM and BEM to Predict Noise in Gearbox Structures," X. F. Wu,
Ph.D. dissertation, University of Kentucky, 1992.
3
f ^ItI ^^^i
^^-^rfr.^rrr
Y
C-91-058031
Dynamometer -1
Optional
Optional output flywheel-iinput flywheel	 Test
	
Torque	 gearbox 7 i
Speed	 4 \ meters
increaser -I	i
Drive
motor?
(a) Layout.
(b) Test gearbox showing measurement grid and RAIMS robot with
sound intensity probe.
Figure 1.—NASA gear-noise rig.
80
3
N
b 70
m 60
a
a 50
a
0 40
W
30	 -	 —
400	 800	 1200 1600	 2000 2400 2800 3200
Frequency, Hz
Figure 2.—Measured and predicted sound power levels for top
of gearbox.
Microphones
Personal
computerFlexible top plate	 r Accelerometer	 ^—+
Microphone
Impedence	 AMP
head
Shaker
Stiff box
Power AMP
Signal
generator
Charge AMP I	 I	 1 Charge AMP
Spectral analyzer
Figure 3.—Apparatus to verity FEMBEM model.
Figure 4.—Boundary element mesh
for apparatus in Fig. 3.
	120 r	 BEM using FEM vibration
......••••••	 Measured
	
m 110 F–	{
a	 I	 i	 =
100
'•,
so
CO
70
so
	
100	 200	 300	 400	 500
Frequency, Hz
Figure 5.—Measured and predicted sound power level using
FE/BE model.
E BEM/FEM vibration
® BEM/measured vibration
E3 Measured
120
CO
110
0
3
a 100
a
c
N 90
CU 80
0 70
60
100 125	 160 200 250 315 400 500
Center frequency of one-third octave band
Figure 6—Measured and predicted sound power levels in one-third
octave bands.
6
Form ApprovedREPORT DOCUMENTATION PAGE OMB No. 0704-0188
Public reporting burden for this collection of information is estimated toaverage 1 hour per response, including the time for reviewing instructions, searching existing data sources,
gathering and maintaining the data needed, and completing and reviewing the collection of information.
	 Send comments regarding this burden estimate or any other aspect of this
collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports, 1215 JeffersonDavis Highway, Suite 1204, Arlington, VA	 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC
	
20503.
1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE TYPE AND DATES COVERED
JIf Iv1992
7137REPO11`r
Technical Memorandum
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
Experimental Validation of Boundary Element
Methods for Noise Prediction
WU-505-63-366. AUTHOR(S)
1L162211A47A
A.F. Seybert and Fred B. Oswald
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
National Aeronautics and Space Administration
Lewis Research Center E-7132
Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
National Aeronautics and Space Administration
Washington, D.C. 20546 -0001 NASA TM-105729
AVSCOM TR-92—C-013
11. SUPPLEMENTARY NOTES
Prepared for the Inter-Noise '92 sponsored by the International Institute of Noise Control Engineering, Toronto, Canada, July 20-22, 1992.
A.F. Seybert, Department of Mechanical Engineering, University of Kentucky, Lexington, Kentucky 40506, and Fred B. Oswald, NASA
Lewis Research Center, Cleveland, Ohio. Responsible person, Fred B. Oswald, (216) 433-3957.
12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Unclassified -Unlimited
Subject Category 37
13. ABSTRACT (Maximum 200 words)
Experimental validation of methods to predict radiated noise is presented in this paper. A combined finite element and
boundary element model was used to predict the vibration and noise of a rectangular box excited by a mechanical
shaker. The predicted noise was compared to sound power measured by the acoustic intensity method. Inaccuracies in
the finite element model shifted the resonance frequencies by about 5 percent. The predicted and measured sound
power levels agree within about 2.5 dB. In a second experiment, measured vibration data was used with a boundary
element model to predict noise radiation from the top of an operating gearbox. The predicted and measured sound
power for the gearbox agree within about 3 dB.
14. SUBJECT TERMS 15. NUMBER OF PAGES
Noise; Vibration; Acoustic intensity; Boundary element method; Finite element method 8
16. PRICE CODE
A02
17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT
OF REPORT OF THIS PAGE OF ABSTRACT
Unclassified Unclassified Unclassified
NSN 7540-01-280-5500 	 Standard Form 298 (Rev. 2-89)
Prescribed by ANSI Std. Z39-18
298-102


Experimental validation of boundary element methods for noise prediction

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server