hardware design, assembly, and instrumentation

Custom hardware design, assembly, and instrumentation for acoustic experiments, including impedance tube construction and measurement setups.

Custom-designed impedance tube for acoustic input impedance measurement of an automotive horn
Custom-designed impedance tube and experimental configuration for acoustic input impedance measurement of an automotive horn.

Custom-designed impedance tube

To measure the acoustic input impedance of an automotive horn, a custom-designed impedance tube was developed and fabricated in compliance with ISO 10534-2. This setup enabled reliable broadband impedance characterization consistent with industrial standards.

Standard

ISO 10534-2:2023 Acoustics — Determination of acoustic properties in impedance tubes, Part 2: Two-microphone technique for normal sound absorption coefficient and normal surface impedance

Experimental Setup
  • Custom-designed impedance tube complying with ISO 10534-2
  • Car shell horn (device under test)
  • Spectrum analyzer, measurement microphone
Impedance tube experimental setup photograph
Impedance tube components before assembly
Acoustic input impedance measurement results compared with FEM predictions
Experimental setup (left), components before assembly (middle), and measurement results of the custom-designed impedance tube (right).

The measured acoustic input impedance of the automotive horn showed good agreement with FEM predictions, validating the accuracy of the impedance tube design and measurement procedure.

Custom microphone holder fabricated via MJF 3D printing for sequential measurement
Custom microphone holder fabricated via MJF 3D printing for sequential measurement with different microphone types.

Custom microphone holder

To prevent acoustic interference arising when multiple microphones are placed simultaneously in the same spatial position, measurements were conducted sequentially. Custom holders were fabricated so that different microphone types could occupy exactly the same spatial location during measurements(Kim† et al., 2025; Kim et al., 2025).

Experimental Setup
  • Fabricated a custom microphone holder via MJF 3D printing to allow sequential use of 1/8-inch and 1/2-inch microphones at identical spatial positions
  • Employed a quick-release adapter to enable fast microphone exchange while maintaining alignment
Results

Achieved consistent spatial positioning of microphones, eliminated mutual interference, and ensured reliable characterization of both ultrasonic and audible fields with minimal system variability.

Hands-on Experience in Transducer Assembly

Establish executable assembly plans, acquire proper knowledge of transducer integration, and develop appropriate tools for reliable assembly.

Technical Skills
  • Utilized a 3D-printed alignment jig to precisely align transducer parts and electrodes, designed to apply radial pre-stress and secure proper concentricity during piezoelectric ultrasonic transducer assembly.
  • Designed and fabricated a wrench-flat clamping jig enabling controlled preload application using a torque wrench.
  • Applied specially designed fasteners to prevent thread stripping during assembly and to withstand both preload and dynamic loads during operation.
Results
  • Established a consistent assembly process, with assembled ultrasonic transducers demonstrating good agreement between experimental measurements and simulation results.
Stepped plate parametric array loudspeaker(Kim et al., 2025)
SPPAL transducer head part positioned in 3D-printed alignment jig
1 - Transducer head part with alignment jig
First piezoelectric stack and electrode aligned by assembly jig
2 - The first piezostack and electrode aligned by jig
Adding transducer middle part during SPPAL assembly
3 - Adding transducer middle part
Second piezoelectric stack and electrode aligned by assembly jig
4 - The second piezostack and electrode aligned by jig
Adding transducer tail part to complete SPPAL stack
5 - Adding transducer tail part
Custom-designed wrench flat clamping jig for controlled preload application
6 - Custom-designed wrench flat clamping jig
Transducer affixed with wrench flat clamping jig and mechanical vise
7 - Affix transducer with wrench flat clamping jig and mechanical vise
Tightening transducer assembly with torque wrench for controlled preload
8 - Tightening with wrench flat clamping jig and torque wrench
Fully assembled SPPAL ultrasonic transducer
9 - Final assembly
Wiring of the assembled SPPAL transducer
10 - Wiring
Metamaterial-integrated parametric array loudspeaker(Kim† et al., 2026)
MiPAL Langevin-type transducer assembly using machine vise, custom clamping jig, and aligner
1 - Assembly of the Langevin-type configuration using a machine vise, custom clamping jig, and aligner.
Preloading stud bolt and nut with torque wrench during MiPAL assembly
2 - Preloading of the stud bolt and nut using a torque wrench.
Langevin-type configuration mounted on transducer fixture with O-ring
3 - Photograph of the Langevin-type configuration mounted on the transducer fixture with an O-ring.
Coupling conical horn to Langevin transducer with controlled preload via torque wrench
4 - Coupling of the conical horn to the Langevin transducer, with controlled preload applied using a torque wrench.
Assembled Langevin transducer with conical horn
5 - Assembled Langevin transducer with the conical horn.
Fully assembled MiPAL transducer architecture
6 - Fully assembled transducer architecture.
ACEL demonstration setup with driving circuit for sound and light emission
ACEL demonstration setup

Circuit Implementation for Demonstration

To design and implement the driving circuit for demonstration, including bandpass filters and piezo amplifiers, enabling frequency-selective sound generation synchronized with light emission. The circuitry was designed such that musical inputs from a piano app on a tablet PC directly drive flexible alternating-current electroluminescent (ACEL) devices, realizing an interactive sound-and-light demonstration.(Park† et al., 2024)

Technical Skills
  • Tablet PC's output signal characterization, Circuit configuration & design, Component selection, PCB layout, Assembly, EMC consideration
Three cascaded circuit boards inside housing for ACEL demonstration
3 Cascaded circuit boards inside its housing.
3D view of demonstration circuit PCB layout
3 dimensional view of demonstration circuit.
Custom-designed MEMS microphone analog interface PCB for die characterization
Custom-designed MEMS microphone analog interface PCB for die characterization.

MEMS Microphone Analog Interface

Investigate MEMS microphone analog interfaces (voltage and charge amplifiers), evaluate the effect of parasitic capacitance from packaging and preamplifier circuits, and apply guard ring strategies to enhance sensitivity and SNR.

Technical Skills
  • Designed and fabricated custom interface PCBs for MEMS microphone die characterization.
  • Identified parasitic capacitance sources using an impedance analyzer and validated their effects through LTspice circuit simulations.
  • Implemented guard ring connection via metal lid and verified sensitivity improvement experimentally.
  • Characterized MEMS microphone performance in terms of sensitivity, noise floor, and SNR(A).
Microphone calibrator setup for low-frequency range calibration
Microphone calibrator for low-frequency range.
Microphone calibration in semi-anechoic chamber for high-frequency range
Microphone calibration in semi-anechoic chamber for high-frequency range.
Measured frequency response in sensitivity of MEMS microphone with analog interface
Measured frequency response in sensitivity of the MEMS microphone with analog interface circuitry.
Measured power spectral density of MEMS microphone with analog interface circuitry
Measured power spectral density of the MEMS microphone with analog interface circuitry.

Related works

2026

  1. Nat. Commun.
    Dual-domain metamaterials co-integrated with a compact ultrasonic transducer for highly directional audio generation

2025

  1. Appl. Acoust.
    An Accurate Measurement of Parametric Array Using a Spurious Sound Filter Topologically Equivalent to a Half-Wavelength Resonator
    An Accurate Measurement of Parametric Array Using a Spurious Sound Filter Topologically Equivalent to a Half-Wavelength Resonator
    Woongji Kim†, Beomseok Oh†, Junsuk Rho*, and Wonkyu Moon*
    Applied Acoustics, Dec 2025
    DOI Related
    Project
  2. J. Acoust. Soc. Am.
    Design, Analysis, and Experimental Validation of a Stepped Plate Parametric Array Loudspeaker
    Design, Analysis, and Experimental Validation of a Stepped Plate Parametric Array Loudspeaker
    Woongji Kim, Beomseok Oh, Chayeong Kim, and Wonkyu Moon*
    The Journal of the Acoustical Society of America, Sep 2025
    DOI Related
    Project

2024

  1. Small
    Interactive Deformable Colored Sound Display Achieved with Electrostrictive Fluoropolymer and Halide Perovskite
    Interactive Deformable Colored Sound Display Achieved with Electrostrictive Fluoropolymer and Halide Perovskite
    Doowon Park†, Woongji Kim†, Chaeyong Park, Jun Choi, Arup Ghorai, Gilwoon Lee, Seungmoon Choi, Wonkyu Moon*, and Unyong Jeong*
    Small, Sep 2024
    DOI

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