Microphone SNR Measurement (Background Noise Method)

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This sequence characterizes a microphone’s ability to passively and/or actively reject noise in the user’s environment.  Unlike traditional microphone SNR measurements 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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Triggered Record Using Chirp Trigger and WAV File (Version 17 and later)

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This test sequence demonstrates SoundCheck’s Triggered Record – Chirp Trigger function for open loop testing of devices without analog inputs such as smart speakers, wearables, smart home devices, tablets and cellphones.  A stimulus WAV file is created in SoundCheck and transferred to the device under test, where it is played back and the response recorded in SoundCheck as if the stimulus were played directly from SoundCheck. The Acquisition step is triggered by the chirp in the stimulus file. Chirp triggers are more robust than level and frequency triggers which are susceptible to false triggering due to background noise.

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Measuring Max SPL versus Frequency

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This sequence characterizes the performance of transducers such as speakers, microspeakers and headphones by measuring how much voltage is required at each frequency to drive the transducer to specified limits of THD, Rub & Buzz, Perceptual Rub & Buzz or Compression. This is useful for detailed transducer analysis and determination of optimal power rating.

The user selects which metric is tested (one at a time is advised), the limit value and the stimulus start frequency. The sequence then uses an iterative looping process, initially with a +3dB step and then a +0.5dB step to precisely determine the value at which the limit is exceeded. This is repeated across the frequency range to generate curves of voltage and SPL vs frequency at the specified limit.

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Triggered Record Using WAV File (Version 16.1 and later)

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This sequence allows you to test devices without an analog input such as smart speakers, tablets, cellphones and MP3 players using SoundCheck’s frequency-based trigger functionality. This method offers improved accuracy over previous level-based triggering, especially in noisy environments. A stimulus WAV file is created in SoundCheck, and copied to the device under test, where it is played and the response recorded in SoundCheck as if the stimulus were played directly from SoundCheck. The stimulus WAV file to be used on the device under test (DUT) may be customized in the stimulus step.

Note that this sequence uses the level-based trigger available in SoundCheck 16.1 and later. If you are using version 16.0 or earlier, please see the level-based trigger sequence.

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Triggered Record Using WAV File and 6 Mic Array

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This sequence allows you to measure a playback system without analog inputs using a 6 microphone array. Specifically, the sequence is designed to measure an in-car audio system. A stimulus WAV file is created in SoundCheck and transferred to the device under test (DUT) where it is played back and the response captured by SoundCheck using a triggered record function. The 6 recordings are batch analyzed to produce individual fundamental curves and the curves are post-processed to produce a single average curve from which an average sensitivity value is calculated.

Final display for Triggered Record Sequence

Final display for Triggered Record Sequence

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

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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

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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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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 example 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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Smart Speaker – Embedded Loudspeaker Test Sequence

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smart_speaker_final_displayThis sequence demonstrates a method by which SoundCheck can measure the performance of a loudspeaker embedded in a so-called “smart speaker”. This example 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 audio file (“Alexa, play Test Signal One”) with one specific to the desired make and model.

The sequence begins by playing the voice activation phrase out of a source speaker, prompting the DUT to playback the mp3 stimulus file from the cloud, followed by a pause step to account for any activation latency. Following the pause, a triggered record step is used to capture the playback from the DUT. The Recorded Time Waveform is then frequency shifted prior to analysis and the results (Frequency Response, THD and Perceptual Rub & Buzz) are shown on the final display step.

We recommend reading our AES paper on this subject prior to continuing as it contains additional details on the test methods devised for this sequence.

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Bluetooth Headphone Test

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bluetooth headset test resultThis sequence tests the send and receive performance of a stereo Bluetooth headset with a built-in microphone using a mixture of analog and digital channels. The left and right earphones are measured simultaneously with a stepped sweep from 20 kHz to 20 Hz using two Bluetooth profiles: A2DP and HFP. The mic is measured with a stepped sweep from 8 kHz to 100 Hz using the HFP profile.

A short 1 kHz tone is pre-pended to the test stimulus which serves as reference tone for resampling and frequency shift operations. Post-processing resampling and frequency shift precisely synchronizes the stimulus and response waveforms prior to analysis. In this case, the HarmonicTrak algorithm is used for frequency response and THD analysis. A2DP frequency response and THD curves are displayed on the first display, followed by A2DP & HFP curves superimposed on a subsequent display. Lastly, the Bluetooth headset’s microphone is tested with HFP and its frequency response is shown on the final display along with the previously collected data.

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