anechoic chamber

Minneapolis, Minnesota

NOISE-CON 2005 2005 October 17-19

Design and Analysis of a Hemi-Anechoic Chamber at Michigan Technological University Jason Dreyer Ashish S. Jangale Mohan D. Rao Michigan Technological University 1400 Townsend Drive Houghton, MI 49931 [email protected]

ABSTRACT A four-wheel chassis roll dynamometer test facility is being installed on the campus of Michigan Technological University (MTU). The chassis dynamometer will be enclosed in a soundproof hemi-anechoic room in order to conduct noise radiation measurements on test vehicles. All surfaces of the room, except the floor and control room window, will be acoustically treated with tetrahedral acoustic cones. The acoustic absorption properties of these materials were characterized through reverberation chamber testing. The expected reverberation time and background sound pressure level (SPL) of the treated chamber were predicted based on untreated chamber acoustic properties, absorptive material properties, and the amount of acoustic material that could be applied to given chamber dimensions and would still preserve the functionality of the room.

1. INTRODUCTION AND BACKGROUND

The objective of this research was to design and analyze an acoustic treatment to provide a hemi-anechoic environment inside a chamber containing a chassis-roll dynamometer at MTU. The main purpose of any anechoic chamber is sound field free of reflections. The design must be both practical and functional. Factors, like cost and ease of construction, often dictate the range in which the anechoic chamber will be effective.

As an experimental testing chamber, the background noise level should be minimized to ensure the sound comes from the sources of interest. The ideal anechoic environment would be an infinite field absent of any sound sources and in which no barriers or obstacles can reflect the sound. This type of environment is nearly impossible to achieve indoors without the presence of absorptive materials to absorb the sound energy. Anechoic chambers typically line the walls with absorbent materials comprised typically of porous foams.

The classical reference for design and construction of anechoic sound chambers is a 1946 paper by Beranek and Sleeper [1]. In this paper, the authors evaluated a constructed chamber by plotting the SPL with respect to the distance from a sound source. They verified the presence of a direct field by the plots exhibiting a decrease in SPL by 6-dB for each doubling of distance from the source, also known as the inverse-square law. They also characterized absorption materials using simple impedance measurements and suggested methods to employ such materials in a way to maximize their absorption characteristics, such as mounting the absorptive materials in a frame to exploit the air gap between the treatments and walls.

Design and Analysis of a Hemi-Anechoic Chamber Dreyer, Jangale, & Rao

Noise-Con 2005, Minneapolis, Minnesota, October 17-19, 2005 2 of 8

Another article by Biesel and Cunefare [2] outline a test system and procedure to qualify the free-field characteristic of anechoic chambers. They advocate for a standard devise for laboratories to qualify their measurement environment. The devise measures SPL in a room at continuous transverse paths radially outward from the source. The degree at which the SPL measurements follow the inverse-square law would then be quantified and subjected to a pass/fail criteria suggested by the authors.

National Instruments website [3] also suggests tests to quantify the free-field characteristics of an anechoic chamber. In addition to the inverse square law, they also suggest the use of Noise Criteria (NC) to quantify background level.

An article about the design and validation of an anechoic test facility at the University of Florida [4] suggests the use of reverberation time in the 100-Hz third-octave band as a metric to meet. Through practical experience, frequencies below 100-Hz are difficult totally to remove from any type of building without isolation of the room with respect to the building. They also suggest noise reduction (NR) measurement through the walls and doors of the chamber to evaluate possible noise path leaks into the chamber.

The effectiveness of the hemi-anechoic chamber treatment will be evaluated based on the following four metrics assembled from the literature: • Reverberation Time (T60) must be minimized at 100-Hz third-octave band to ensure that all

sound frequencies above and including 100-Hz would be absorbed into the room treatment and not reflected. A T60 of 0.1-s was set as the design target to achieve since the equipment used can not effectively quantify reverberation times below this value.

• Sound field of the treated chamber should follow the inverse-square law, a characteristic of a direct field with a decrease in SPL by 6-dB for each doubling of distance from the source.

• Noise Reduction (NR) through the surfaces of the chamber (walls, doors, etc.) must be maximized in order to effectively isolate the chamber from the building environment.

• Noise Criteria (NC) for rooms should be in the very quiet range, corresponding to an NC-Rating between NC-15 and NC-20 for octave bands between 63-Hz and 8000-Hz. The background SPL must be minimized at 100-Hz third-octave band to ensure that all sound frequencies above and including 100-Hz would be minimized.

2. UNTREATED CHAMBER ANALYSIS

Before treating the chamber with acoustic materials, the sound field in the chamber must be characterized. Four different tests were done in the chamber to characterize the acoustic properties of the untreated room, characterized by the layout schematic in Figure 1.

A. Background SPL Measurements These measurements were used to quantify the average background SPL of the room at

the center of the dynamometer. A 0.5-in-diameter 01dB MEC210 omni-directional condenser microphone with PRE12N preamplifier was oriented to face each wall. SPL measurements were made according to ANSI S1.13-1971 [5] in each third-octave band using the 01dB SYMPHONIE data acquisition system and software, averaged over 10-s with a bandwidth of 20-Hz to 20-kHz.

Figure 2 is the background SPL measurement for each octave band. The NC-Rating for this SPL is between NC-30 and NC-35. In the 100-Hz third-octave band, the SPL is consistently 38-dB. This level will be later used to estimate the SPL of the treated chamber.



Figure 1: Chamber Layout Schematic (Not to Scale).

Figure 2: NC-Rating for Chamber Background SPL

B. NR Measurements The B&K HP-1001 sound source with Type-4205 sound power source was placed in the

center of the dynamometer track, with the same settings as the sound field measurements. A 01dB SdB01+ sound level meter (SLM) with 0.5-inch-diameter 01dB MCE210 omni-directional condenser microphone with PRE12N preamplifier was used to measure the average SPL in the 125-Hz octave band along the inside and outside of each wall, once with the source on and once with the source off. The resulting measurements were then recorded according to ANSI S1.13-1971 [5].

Figure 3 is the summary of the NR measurements. The range of uncertainty was +/- 1-dB for the inside measurements and +/- 3-dB for the outside measurements, from observation of the SLM behavior. Applying these ranges to the measured SPL inside and outside the walls with and without the source on, the south and north walls do not seem to have any significant sound leaks. Sources of potential leaks on the east and west walls were noted for future treatment and as possible contributors to measurement error.

NC Rating for Chamber Background

0

10

20

30

40

50

60

70

80

63 125 250 500 1000 2000 4000 8000

Octave Band (Hz)

SP

L (

dB

)

Hearing ThresholdNC-15

NC-20

NC-25

NC-30

NC-35

NC-40

NC-45

NC-50

NC-55

NC-60

AverageBackground

Side (South Wall)

30’-8”

44’

North

1’-3”16’-8”

10’

7’-4”

10’

Plan

7’-4”

Side (East Wall)

Cone DepthPitPlywood BaseNo Cones

Wall/RafterWindow

Entrance

Gar

age

Doo

r

Gar

age

Doo

r

Side (South Wall)

30’-8”

44’

North

1’-3”16’-8”

10’

7’-4”

10’

Plan

7’-4”

Side (East Wall)

Cone DepthPitPlywood BaseNo Cones

Wall/RafterWindow

Entrance

Gar

age

Doo

r

Gar

age

Doo

r



Figure 3: Room Noise Reduction Measurements.

C. Sound Field Measurements These measurements were used to characterize the sound field in the chamber according

to ANSI S1.13-1971 [5]. Each measurement was taken, using the 01dB SYMPHONIE data acquisition system and software, at horizontal distances of 1, 2, 4, 8, and 16-ft away from the source and at heights of 2, 4, and 8-ft. The B&K HP-1001 sound source with Type-4205 sound power source was placed in the middle of the dynamometer track plates, and the measurements were taken along each diagonal of the room with the 0.5-inch-diameter 01dB MCE210 omni-directional condenser microphone facing the source. The sound power source was set to broadband random (100-Hz to 10-kHz) at 90-dB power level, and the SPL measurements were averaged over 10-s with a bandwidth of 20-Hz to 20-kHz.

Figure 4 shows the sound field measurements summaries with respect to horizontal/vertical distance for the chamber with a covered dynamometer pit. A nearly constant SPL regardless of distance from a source characterizes a reverberant field. Assuming that a change in SPL of less than 3-dB is not significant, at heights of 8-ft, the SPL seems to be rather constant. For the other heights, the SPL seems to level off past 8-ft from the source. Thereby, for T60 measurements, the microphone must be at least 8-ft from the source at a height of 4-ft.

D. Reverberation Time Measurements In order to characterize the existing room absorption, the reverberation time for the room

was measured according to ASTM C423-90a [6] with the 01dB SYMPHONIE data acquisition system and software. The 0.5-inch-diameter 01dB MCE210 omni-directional condenser microphone was set on a tripod at 4-ft in height. The tripod was placed at five different locations in the chamber, at the four corners and center of the dynamometer track plates. The microphone was oriented toward the source for one set of measurements and oriented away from the source for the other set. Each set of measurements included five averages. High quality speakers with subwoofer were used to generate the pink noise used in this study. The volume of the speakers was set to same subjective loudness as the B&K HP-1001 source in the sound field measurements.

125-Hz Octave Band SPL for Inside of Chamber and Outside of Walls

0

10

20

30

40

50

60

70

80

Chamber S-Wall W-Wall E-Wall N-WallLocation

SP

L (

dB

)

No sourceSource

WES

N

OutsideInside

Possible Leak at Garage Door

Possible Leak through Ventilation



Figure 4: Chamber Sound Field for Each Diagonal and Microphone Height.

Figure 5 is a plot of the average 60T of the room, with the associated maximum and

minimum values as the error bars. In comparison to the target specification of 60T of 0.1-s, the untreated room has many reflections. The reverberation time was then used to calculate the room absorption untreateda and average absorption coefficient chamberα for the untreated chamber, according the following expressions:

60untreated

166.0

T

Va chamber= (1)

chamber

untreatedchamber S

a=α (2)

where chamberV ( 3m-636.81 ) and chamberS ( 2m-481.94 ) are the volume and surface area of the

untreated chamber, respectively.

3. CHARACTERIZATION OF ACOUSTIC CONE TREATMENT

The primary acoustic treatment for the walls, doors, and ceiling of the chamber is a donated tetrahedral acoustic cone, shown as a pair in Figure 6. Each cone, from tip to base, is 36-in (0.91-m) thick. The lowest frequency that these cones can effectively absorb will correspond to the quarter-wavelength equal to the thickness of the cones, which therefore is 94-Hz.

Figure 7 is the plot of the absorption coefficient for one cone based on the reverberation chamber measurements, according to ASTM C423-90a [6]. Due to the size of the reverberation chamber (47.5-m3) being smaller than the recommended 200-m3, values for absorption at frequencies below 500-Hz have large experimental error. However, a trend line based on the data above 500-Hz can be drawn to estimate the absorption coefficient at lower frequencies. In the 100-Hz third-octave band, the absorption coefficient for one cone appears to be between 0.4 and 0.5.

Covered Sound Field (Diagonal # - Height)

74

76

78

80

82

84

86

88

90

92

94

1 2 4 8 16

Horizontal Distance from Source (ft)

SP

L (

dB

)

1-2'

1-4'

1-8'

2-2'

2-4'

2-8'

3-2'

3-4'

3-8'

4-2'

4-4'

4-8'

123

4

Heights of 8-ft

Flat Trend in SPL Regardless of Height or Horizontal Distance from Source



Figure 5: Average Reverberation Time and Target for Chamber with Covered Pit.

Figure 6: Tetrahedral Acoustic Cone Treatment.

The exposed surface area of the cone with dimensions given in is 720-in2 (0.465-m2),

excluding base areas. The volume of one cone with given dimensions is approximately 2304-in3 (0.0378-m3), including base volume. The room absorption in the 100-Hz third-octave band of one cone was estimated in expression (3) using an absorption coefficient Hzcone@100−α of 0.45 and

the exposed surface area coneS of one cone.

)m-(0.209 in-324 22coneHzcone@100Hz-cone@100 == − Sa α (3)

30-in

6-in

12-in

12-in

12-in

30-in

6-in

12-in

12-in

12-in

Reverberation Time for Covered Chamber (MAX/AVG/MIN)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

80 100

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

Center Frequency (Hz)

T60

(s)

AverageTarget = 0.1-s



Figure 7: Absorption Coefficient of One Cone in Reverberation Chamber.

4. PREDICTION OF TREATED ROOM ACOUSTIC PROPERTIES

The amount of absorptive material that can be applied to the chamber is limited to the given chamber dimensions with respect to preserving the functionality of the room. A CAD model of the chamber was created and treated with cones along the walls, doors, and ceiling. The minimum number of cones estimated using this model was 2790. Partial cones could eventually be used to line areas in which entire cones could not be accommodated.

The reverberation time in the 100-Hz third-octave band of the treated chamber could be estimated using this estimated number of cones and the absorption properties of both the room and cones, presented the following equations:

floorchamberconecones

conecones

SaN

VNVT

α+−

=)(166.0 chamber

60 (4)

=⋅+⋅⋅−=− 36.125114.0209.02790

)0378.02790636.81(166.0Hz60@100T 0.15-s (5)

where conesN is the number of cones and floorS is the surface area of the chamber floor.

Assuming sound field of untreated chamber is reverberant, the SPL pL can be estimated

by the following expression:

+=

aLL wp

4log10 10 (6)

where wL is the sound power level and a is the chamber absorption. By subtracting pL

expressions of treated and untreated rooms based on equation (6), the relationship between the treated and untreated chamber SPL and absorptions are represented by equation (7).

Absorption Coefficient (Alpha) of One Cone in Reverberation Chamber

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

63 80 100

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

Center Frequency (Hz)

Alp

ha



+=

treated

untreated10untreated,treated, log10

a

aLL pp (7)

Adapting equation (7), the reduction in background SPL at 100-Hz can be estimated as

untreated,treated, pp LL − , given in equations (8) and (9).

+

=−floorchamberconecones

untreated10untreated,treated, log10

SaN

aLL pp α

(8)

{ } dB4.1036.125114.0209.02790

87.5410log10100@untreated,treated, −−=

⋅+⋅=−

−Hzpp LL (9)

Therefore, the proposed treatment to the chamber would effectively decrease the background SPL by at least 10.4-dB. If the background SPL in the untreated chamber were 38-dB at 100-Hz, then the expected background SPL for the treated chamber would be 27.6-dB (8.5-dBA) in the 100-Hz third-octave band.

5. CONCLUSIONS The untreated chamber sound field, background SPL, boundary NR, and room absorption were first characterized. To achieve a sound field free of reflections, absorptive tetrahedral cones will be added to walls, ceiling, and doors of the room. Each cone adds 0.209-m2 of room absorption to each square foot of surface space. A CAD model estimates the use of at least 2790 cones in the room, yielding an estimated T60 of 0.15-s and background noise attenuation of 10.4-dB, both in the 100-Hz frequency band. For an untreated background SPL of 38-dB, the treated background level should be at least 27.6-dB (8.5-dBA) at 100-Hz. After the chamber is treated, the T60, the background SPL NC-Rating, the sound field characteristics, and NR of the chamber boundaries should be measured to ensure that the chamber is an isolated free-field environment.

ACKNOWLEDGEMENTS This work was sponsored by Ford Motor Company and Keweenaw Research Center. A special thanks to Robert Rowe, Geoff Gwaltney, Jay Meldrum, Jagdish Dholaria, Kurt Korpela, Paul Lefief, Amanda Otis, Kyle Stewart, and Chad Walber for their contributions to this project.

REFERENCES

[1] Barenek, L.L., and H.P. Sleeper, Jr. “The Design and Construction of Anechoic Sound Chambers,” Journal of the Acoustical Society of America, 18(1), pp.140-150, 1946.

[2] Biesel, Van B. and Kenneth A. Cunefare. “A Test System for Free-Field Qualification of Anechoic Chambers,” Sound and Vibration, May 2003.

[3] National Instruments Corporation. 11500 N Mopac Expwy., Austin, TX 78759-3504. (www.ni.com) [4] Jansson, D., et. al. “Design and Validation of an Aeroacoustic Anechoic Test Facility,” AIAA/CEAS

Aeroacoustic Conference. June 17-19, 2002. [5] ANSI, ANSI S1.13-1971. Method for the Measurement of Sound-Pressure Levels, Standards Secretariat,

Acoustical Society of America, Melville, NY, 1971. [6] ASTM, ASTM C423-90a. Standard Method of Test for Sound Absorption of Acoustical Materials in

Reverberation Rooms, American Society for Testing and Materials, Philadelphia, PA, 1972.

anechoic chamber

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