Cohu Acoustic Stimulus Module

Project: Acoustic Stimulus Module

Sponsor: Cohu Semiconductor

Figure 1: Photo of the acoustic stimulus module with a ruler for scale

Background: 

    With industry's increasing desire to pack more processing power into ever smaller packages, the popularity of MEMS technology is increasing. MEMS are MicroElectroMechanical Systems, and their construction is based off of the same semiconductor technology which allows for the creation of the integrated circuit years ago. Cohu Semiconductor is a global leader in test-handling various semiconductor components, with significant work being done in MEMS test solutions. Cohu has expressed interest in the design of a MEMS microphone testing apparatus, optimized based on space, test consistency, and scalability.

Objectives:

    Cohu's key concerns with this design focus mainly on the mechanical design of the test apparatus, as opposed to the design of the test methods themselves. Since Cohu only provides the test apparatus, the design must be flexible enough to satisfy the different test methods of Cohu's customers. The following is a list of Cohu's desired functional parameters:

• Air tight test chamber

•  Test up to four MEMS microphones

•  Include space for a reference microphone (test control) 

•  Sound Pressure Level:

  • Primary:  65 dB-110 dB 

  • Secondary: 65 dB - 130 dB

• Input Frequencies:

  • Primary: 400 Hz-12 kHz

  • Secondary: 20 Hz - 20 kHz 

• Maintain a total harmonic distortion of <=1% across the test subjects

Final Design Solution:

Figure 2: Cutaway CAD model showing acoustic stimulus module with internals exposed and labeled

    The proposed design solution relies on a piezo speaker as the acoustic stimulant. A function generator will be used to send a high fidelity sine wave to an amplifier, which will step up the voltage of the signal in order to power the speaker. The speaker will send pressure waves down through an orifice in a plate, beneath which lie the MEMS microphones. The small holes bored through the orifice plate will channel the sound right onto the MEMS acoustic ports. From here, a National Instruments DAQ board will be used to capture the MEMS output signals, and save them for later analysis. A cutaway CAD model of the final design can be seen above, with the key components clearly labeled. 

Summary of Performance Results:

    The testing of the final acoustic stimulus module has shown that the design satisfies the performance characteristics desired by Cohu. When the tests were run with a reference microphone, it was clear that the total harmonic distortion (THD) of the system was less than 1% throughout the entire frequency range of 20 Hz to 20 kHz. The same reference microphone tests were also able to show that the desired sound pressure level (SPL) of 130dB was easily attainable in the primary (400 Hz - 12 kHz) range. For frequencies above 12 kHz,  130 dB of SPL was reached, however at frequencies below 400 Hz, the SPL fails to this goal (only gets up to about 115 dB). Comparing this performance to the goals stated above, it's clear that both the primary and secondary THD goals were reached, while only the primary goal for SPL was attained.

Executive Summary:

    The following is a link to the Executive Summary: https://docs.google.com/document/d/1qqANhjpIdFujTwjErn_BSXfaVl5SXktmHbLAm81WS-k/edit?usp=sharing