Recent Advances in Acoustic MaterialsHBM PRODUCT PHYSICS CONFERENCE 2020, DAY 2

Ed Green, Ph.D.HBK Sound and Vibration Engineering Services

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Organizational Information

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Ed Green, Ph.D.

• Ph.D. Purdue University - Ray W. Herrick Laboratories (1995)

• Noise and Vibration Engineer in the Detroit area for the last 26 years

• Principal Staff Engineer at HBK Sound and Vibration Engineering Services for last 9 years

• MS and PHD theses on acoustic material modeling

Advances in Material Measurements - Background

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• In 1930s, Per Vilhelm Brüel performed important work on the Standing Wave Tube while at the Technical University of Denmark

• The modern descendent of the Standing Wave Tube, the B&K 4206 (T) Impedance Tube is the standard for material properties measurement

• Materials developments are never done – new requirements (EVs), new products, new technologies

B&K Standing Wave Tube

B&K 4206

Advances in Material Measurements - Outline

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• Impedance tube• Impedance/transmission tube basics• Extending measurements to low frequency• High frequency measurement to 20 kHz• Measuring small samples• Measuring high-tech fabric samples• Aircraft materials• Metamaterials

• Center point beam• Preferred sample dimensions• Issues• Temperature Limits

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Impedance Tube

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Impedance Tube Basics

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Transmission Tube Basics

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Most important is getting enough low frequency energy• Larger source• Limit bandwidth

Microphone spacing

Demonstrated measurements to 17 Hz

Extending Measurements to Low Frequency

B&K Very Low Frequency Source“Sound Cannon”

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Uses 4206 tube and software as foundation

Uses B&K 1/8” microphones

Scaled down to have minimum mic spacing and tube diameter

Anechoic termination

High Frequency Measurement to 20 kHz

HF Tube with Straight Section

HF Tube with 3D Printed Turbo Muffler

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High Frequency Measurement to 20 kHz

Low frequency peaks are the result of TL of area adapters. The size of these peaks is a function of the expansion ratio.

Three peaks due to turbocharger muffler – 7506 Hz, 8325 Hz, and 10412 Hz. This is what we are looking for.

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Measuring Small Samples and High-Tech Fabric Samples

Nonwoven Fabric

ePTFE Membrane

Earplug

Fabric Setup - Cones

Earplug Setup – Step Area Change

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Measuring small samples is challenging Key requirements:

• Correct for area changes using transfer matrix technique• Be careful of edge boundary conditions• Control sound amplitude for thin membranes

Measuring Small Samples and High-Tech Fabric Samples – cont.

Nonwoven Fabric

ePTFE Membrane

Earplug

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Measuring Small Samples and High-Tech Fabric Samples – cont.

Nonwoven Fabric

ePTFE Membrane

Earplug

Correct for area changes using transfer matrix technique

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Measuring Small Samples and High-Tech Fabric Samples – cont.

Nonwoven Fabric

ePTFE Membrane

Earplug

Be careful of edge boundary conditions

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Measuring Small Samples and High-Tech Fabric Samples – cont.

Nonwoven Fabric

ePTFE Membrane

Earplug

Control sound amplitude for thin membranes

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B&K 9737 Portable Impedance Tube is used to make specialized measurements on acoustic materials used to reduce jet engine noise• Up to 155 dB to measure nonlinear

effects• Compensate for sample curvature• Special calculations

Aircraft Materials

B&K 9737 Portable Impedance Tube

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Impedance tube used to measure TL and Absorption of Metamaterials

Metamaterials

Metamaterial provided by Zaryab Shahid, Dr. Anastasia Hnifah Muliana, and Dr. James Hubbard of Texas A&M

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Center Point Beam

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Basic procedure for Center Point Beam test described in Dowling and Saha, “A Correlation between Oberst Bar and Center Point Damping Results”, SAE 2009-01-2134

Used 1:10 aspect ratio

Recommend modifying to 1:20. Center beam is like to cantilever beams fused in the middle

Preferred Sample Dimensions

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Good experience with coated metal beams

Issues with higher frequency modes of acrylic and GFR beams

Issues worse at higher temps Test limited to -55 C to 121 C by sensors

Issues and Temperature Limits

471 Hz Mode 512 Hz Mode

Please type any questions you have into the WebEx Q&A dialog You can open the Q&A window by selecting the “Q&A” icon in the WebEx toolbar at the top

of your screen:

Today’s presentation will be E-mailed to all attendees. The webinar will also be posted on our website: http://www.hbm.com/en/3157/webinars/

If you have additional technical questions, feel free to contact our technical support team at support@usa.hbm.com

Questions?

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Thank You

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