Interchannel Phase

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Display for the Interchannel Phase sequence

When measuring multiple inputs, for example testing a beamforming microphone array, it can be useful to measure the phase response in reference to a single microphone in the array. This measurement is called interchannel phase. In this sequence, a six microphone array is measured. The user can select a reference microphone (defaulted in this sequence to DUT Mic 1), and it will measure the phase difference between this microphone and the five other recordings. A limit of +/-10° is taken at 1kHz, and the magnitude comparison of all measurements is also displayed.

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Microphone Frequency Response and Sensitivity Test Sequence

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This is sequence is a basic example of the two most common microphone measurements: frequency response and sensitivity. A stepped sine sweep is played from 10 kHz to 100 Hz through a source that has previously been calibrated to produce 1 Pascal across the frequency band. The recorded signal is analyzed with a Heterodyne analysis step, which calculates the response curve. A post processing step is then used to extract the level at 1 kHz, the sensitivity value. Limits are set around both the frequency response and the sensitivity, but the default values are arbitrary and should be adapted to your particular device.

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Microphone Polar Plot Substitution Method Using Outline ET250-3D

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Microphone polar plotThis sequence measures the directional response of a microphone and graphs the result as a microphone 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 ET250-3D turntable via an ethernet 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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MEMS Microphone Test: Measuring Digital MEMS Microphones – Frequency, Sensitivity, and Power Supply Rejection (PSR) Performance

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MEMS microphone test

This MEMS microphone test suite contains 3 sequences to enable comprehensive testing of digital MEMS microphones:

  • Measurement of frequency and sensitivity using a calibrated source
  • Measurement of frequency using the ‘substitution method’, a solution for measuring frequency response where the source speaker cannot or is not equalized
  • Measurement of power supply rejection (PSR) performance

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Microphone Acoustic Overload Point (AOP)

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Microphone acoustic overload pointThis sequence measures the microphone acoustic overload point (AOP) – the SPL required to produce 10% THD @ 1kHz from the microphone’s output.
A 1 kHz amplitude sweep from a calibrated source speaker is applied across a range from 120 dB SPL to 135 dB SPL.The recorded time waveform is then analyzed using the HarmonicTrak algorithm to produce a Fundamental and THD vs. Level curve. An intersection post processing function identifies where the THD curve intersects 10% and that level is is used in a limit step to produce a Pass/Fail AOP verdict.

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Microphone SNR Measurement (Background Noise Method)

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Microphone SNR measurementThis sequence characterizes a microphone’s ability to passively and/or actively reject noise in the user’s environment. Unlike traditional microphone SNR measurement methods, which calculate a ratio based upon a reference signal and the microphone’s noise floor, this method utilizes a signal (speech played from a mouth simulator) and noise (background noise played from two or more equalized source speakers) captured by both a reference microphone and the DUT microphone.

First a recording of the baseline ambient noise in the test environment is made and a 1/3 octave RTA spectrum is calculated from the recording. Next, the speech signal (mouth simulator) and noise signals (Left and Right speakers) are played consecutively and recorded separately using the reference microphone. A 1/3 octave RTA spectrum is calculated from each recorded time waveform. Next the same measurements are repeated using the DUT microphone. The resulting RTA spectra are then post processed to produce a signal gain spectrum and a noise gain spectrum which are then used to derive the SNR spectrum of the DUT mic. For best accuracy, the Signal and Noise spectra should be at least 5 dB above the ambient noise floor of the measurement environment.

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Microphone Polar Plot: Substitution Method Using LinearX LT360 Turntable

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This 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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Smart Speaker Embedded Microphone Test Sequence

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smart_speaker_final_display_micThis sequence demonstrates a method by which SoundCheck can measure the performance of a microphone embedded in a so-called “smart speaker.” This smart speaker embedded microphone test assumes that the DUT is an Amazon Echo, but it can be adapted for use with virtually any other type of smart speaker by substituting the Echo’s voice activation phrase WAV file (“Alexa”) with one specific to the desired make and model.

The sequence begins by playing a voice activation phrase out of a source speaker, prompting the DUT to record both the voice command and the ensuing stepped sine sweep stimulus. A message step then prompts the operator to retrieve this recording from the DUT’s cloud storage system. This is accomplished by playing back the recording from the cloud and capturing it with a Triggered Record step in the SoundCheck test sequence.  The Recorded Time Waveform is then windowed (to remove the voice command) and frequency shifted prior to analysis and the result (Frequency Response) is shown on the final display step.

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Open Loop Microphone Test

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Open loop microphone test

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 telephones, forming a closed loop can be cumbersome or impossible. This sequence demonstrates how to perform an open loop microphone test 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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Measuring Digital MEMS Microphones: Frequency, Sensitivity and Power Supply Rejection (PSR) Performance

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seq_dig_mic_final_display_substitution_methodThis test suite contains 3 sequences to enable comprehensive testing of digital MEMS microphones.

The first measures the frequency and sensitivity and displays two graphs: absolute level in dBFS, and the same response curve but normalized to 0 dB at 1 kHz.

The second sequence uses the substitution method to test a digital MEMS microphone frequency response with a source speaker that is not or cannot be equalized. The MEMS microphone is simultaneously measuring with a reference microphone , and by subtracting the response of the reference microphone from the DUT microphone the response and sensitivity of the device under test is revealed.

Measuring Digital Microphone PSR (Power Supply Rejection)
The third sequence demonstrates a method for measuring a digital MEMS microphone’s power supply rejection performance (PSR). This sequence measures PSR at 217 Hz (the 217 Hz GSM TDM pulse often of concern) but is easy to modify to test at any frequency. A DC supply with a calibrated AC signal, simulating electrical interference is applied to the MEMS microphone. SoundCheck then records the audio from the DUT, analyzes it with a spectrum analyzer and extracts the RMS energy at the specific frequency of the simulated electrical interference and returns the PSR value. The setting of frequency, waveform type and amplitude of the simulated electrical interference is controlled entirely from within SoundCheck.

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ABOUT LISTEN INC

Listen is a global leader in audio measurement and analysis, with over 30 years’ experience developing innovative test methods and algorithms. SoundCheck, our powerful software-based audio analyzer, pairs with our test interfaces, precision microphones, and accessories to offer a complete solution for testing loudspeakers, headphones, microphones, smart devices, communication systems, hearing aids, automotive audio, and more. Trusted by engineers worldwide, Listen delivers the accuracy and flexibility essential for modern audio testing, from the R&D lab to the production line.

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