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Measurement of SoundAbsorption and ImpedanceD. W. Herrin, Ph.D., P.E.University of KentuckyDepartment of Mechanical Engineering
Measurement of Sound ImpedanceImpedance Tube MeasurementsASTM E1050-95 (ISO 10534-2) Test MethodmicrophonesTransfer functiondriver (loudspeaker)sampleDept. of Mech. EngineeringUniversity of Kentucky2Noise and VibrationShort Course
Coordinate System and Microphone LocationsImpedance Tube Measurementsxx2x1Material sampleDept. of Mech. EngineeringUniversity of KentuckySound source3Noise and VibrationShort Course
Plane Wave TheoryImpedance Tube MeasurementsTotal sound pressure at any point in the tube:P(x ) Ae jkx Be jkx-x traveling wave x traveling waveThe transfer function between points 1 and 2:P(x2 ) Ae jkx2 Be jkx2 e jkx2 Re jkx2H12 jkx1jkx1 jkx1P(x1 ) Ae Bee Re jkx1R Bis the pressure reflection coefficient of the materialADept. of Mech. EngineeringUniversity of Kentucky4Noise and VibrationShort Course
Solving for Material PropertiesImpedance Tube MeasurementsSolving for R:e jkx2 H12 e jkx1R H12 e jkx1 e jkx2Normalized specific boundary impedance:z 1 R ρo c 1 RSound absorption coefficient of the material for any angle of incidence :α (ϕ ) 4r! cos ϕ2(1 r! cosϕ ) ( x! cosϕ )Dept. of Mech. EngineeringUniversity of Kentucky52rwhere r! ρo cxx! ρo cNoise and VibrationShort Course
Two-Microphone StandardsImpedance Tube Measurements1. ISO 10534-2, Acoustics-Determination of sound absorption coefficientand impedance in impedance tubes - Part 2: Transfer-function method2. ASTM E1050-10, Standard Test Method for Impedance andAbsorption of Acoustical Material Using a Tube, Two Microphones anda Digital Frequency Analysis SystemDept. of Mech. EngineeringUniversity of Kentucky6Noise and VibrationShort Course
Sound Absorption MeasurementImpedance Tube MeasurementsSample holderwith rigid pistonDept. of Mech. EngineeringUniversity of Kentucky7Noise and VibrationShort Course
Cutting with Rotating BladeImpedance Tube MeasurementsInexpensive and accurate if keptsharpenedStanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky8Noise and VibrationShort Course
Water Jet CuttingImpedance Tube MeasurementsExpensive butaccurateStanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky9Noise and VibrationShort Course
Stamping Press SystemImpedance Tube MeasurementsUsed for low-density fibrousmaterialsDept. of Mech. EngineeringUniversity of KentuckyStanley, Internoise 201210Noise and VibrationShort Course
“Bad” and “Good” SpecimensImpedance Tube MeasurementsStanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky11Noise and VibrationShort Course
Stanley, 2012 Specimen PreparationImpedance Tube MeasurementsBBBBBBVery important step for good resultsGravity aided insertion and self centeringFace uniformly flush with cell lipFront surface even across lip of test cellNo facing protrusions or wrinkles interacting with cell wallExtremely small ( at most) and consistent gap betweenspecimen and cell wallB No specimen compression in cellDept. of Mech. EngineeringUniversity of Kentucky12Noise and VibrationShort Course
Comparison of 5 Samples (Water Jet)Impedance Tube MeasurementsStanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky13Noise and VibrationShort Course
Comparison of 4 SamplesImpedance Tube MeasurementsAbsorption Response of One Die Cut, and Three 99 mm Water JetCut Foam Samples of Slightly Different Diameter.Stanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky14Noise and VibrationShort Course
Effect of Impedance Tube SizeImpedance Tube Measurements8 Samples of 2 inch fiberAbsorption Coefficient10.80.60.41.375 inch tube0.203.875 inch tube0Dept. of Mech. EngineeringUniversity of Kentucky5001000Frequency (Hz)1515002000Noise and VibrationShort Course
Effect of Cutter SizeImpedance Tube Measurements8 Samples of 0.75 inch thick 0.6 lbs/ft³ MelamineAbsorption Coefficient10.80.60.41.375 inch Diameter Cutter0.21.360 inch Diameter Cutter001000Dept. of Mech. EngineeringUniversity of Kentucky20003000Frequency (Hz)1640005000Noise and VibrationShort Course
SummaryImpedance Tube Measurements“While the use of an impedance tube system to measureacoustic absorption is not an extremely precise andrepeatable process due to unavoidable variations ofspecimen cutting and cell fit, the disciplined use of theguidelines stated in this paper will help to insure that testresults maintain a consistent level of accuracy and validity.The experience gained with repeated preparation andtesting will also contribute to a better feel for more subtleaspects of preparation and specimen fitting for testing.”Stanley, Internoise 2012Dept. of Mech. EngineeringUniversity of Kentucky17Noise and VibrationShort Course
1. ISO 10534-2, Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes - Part 2: Transfer-function method 2. ASTM E1050-10, Standard Test Method for Impedance and Absorption of Acoustical Material Using a Tube, Two Microphones and a Digital Frequency Analysis System Dept. of Mech. Engineering 6File Size: 2MBPage Count: 17People also search fornoise absorption materialsnoise absorption wall
