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Advanced Microphone Measurement Seminar

In this pair of 60 minute online seminars, Listen’s Support Manager, Steve Tatarunis, discusses some of the more advanced aspects of microphone testing including:

Part 1: Presented on Feb 18, 2021. View Recording.

  • Substitution, Equalized Source and Transfer Function sequences in SoundCheck, including how to calibrate for these measurements. This is an expansion of material covered in the ‘basic microphone testing webinar’ and include more details about the sequences and calibration and the trade-offs of each method.
  • Advanced measurements including polar plot, self-noise, signal to noise, distortion, active/passive noise rejection

Part 2: Presented on Mar18, 2021. View Recording.

  • Phase
  • Open loop microphone testing for voice activated devices such as smart speakers, automotive audio etc.
  • Advanced hardware and test configurations for measuring microphone arrays and MEMS devices

Microphone Polar Plot: Substitution Method Using Outline ET250-3D

Microsoft Word - Mic_Polar_Plot-Substitution_Method-LinearX_LT36This sequence measures the directional response of a microphone and graphs the result as a polar plot.   A log sweep stimulus is played from 100 Hz to 10 kHz at each angular increment, and the acquired waveform is analyzed using the Time Selective Response algorithm.  This method allows the test to be performed in a non-anechoic environment by placing a window around the direct signal, eliminating the influence of reflections.  Commands are sent automatically to the Outline turntable via an RS-232 connection, instructing it to move in 10 degree increments after each measurement.  The sequence measures the response every 10 degrees from 0 to 180 and mirrors the polar image, which simulates a full 360 degree polar and saves test time.   The response at each angular increment is compared against the on-axis response to create a normalized curve.  This removes the influence of the device’s frequency response and sensitivity, such that the polar plot only shows the directional response.  The final display also contains a graph of the directivity index in decibels versus frequency.

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Microphone Polar Plot: Substitution Method Using LinearX LT360 Turntable

Microsoft Word - Mic_Polar_Plot-Substitution_Method-LinearX_LT36This sequence measures the directional response of a microphone and graphs the result as a polar plot.   A log sweep stimulus is played from 100 Hz to 10 kHz at each angular increment, and the acquired waveform is analyzed using the Time Selective Response algorithm.  This method allows the test to be performed in a non-anechoic environment by placing a window around the direct signal, eliminating the influence of reflections.  Commands are sent automatically to the LT360 turntable via an RS-232 connection, instructing it to move in 10 degree increments after each measurement.  The sequence measures the response every 10 degrees from 0 to 180 and mirrors the polar image, which simulates a full 360 degree polar and saves test time.   The response at each angular increment is compared against the on-axis response to create a normalized curve.  This removes the influence of the device’s frequency response and sensitivity, such that the polar plot only shows the directional response.  The final display also contains a graph of the directivity index in decibels versus frequency.

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Open Loop Microphone Testing – Updated

This sequence demonstrates the two most common microphone measurements, frequency response and sensitivity, on a microphone embedded in a recording device. Typically, when measuring a microphone the response of the device can be captured simultaneously with the stimulus. However, with devices such as voice recorders and wireless telephone forming a closed loop can be cumbersome or impossible. This sequence demonstrates how to measure such a device by recording the signal on the device under test, transferring that recording to the computer running SoundCheck and then using a Recall step to import the recorded waveform and analyze it.

This specific sequence, v4, is an improvement on the prior versions. The v1 release required that the audio file containing the recorded response waveform be manually windowed outside of SoundCheck before being analyzed. The v2 release utilized a new feature in SoundCheck 14, using values from the memory list to semi-automatically trim the waveform before analysis. The v3 release completely automated waveform editing through the use of an intersection level and windowing post processing steps. Currently the v4 release uses the new Auto Delay+ algorithm, exclusive to SC18 and beyond. Auto Delay+ is capable of detecting and accounting for delays of -0.5 seconds to any positive delay, nullifying the need for windowing steps in the sequence. If you are interested in learning more about this algorithm please refer to the Analysis section of the SoundCheck manual.

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