Tag Archive for: microphones

100 Things #69: Testing MEMS Microphones In SoundCheck

Testing MEMS microphones and devices in SoundCheck is as simple as any microphone test. In fact, SoundCheck’s ability to work with different devices means MEMS setup is plug and play. Just like testing an analog microphone, SoundCheck can perform standard tests using microphone substitution or microphone subtraction. SoundCheck allows up to 64 channels of simultaneous acquisition, so daisy-chaining MEMS inputs easily expands the number of hardware inputs. Testing MEMS with SoundCheck is simple for both individual components and finished products.

Testing MEMS Microphones In SoundCheck

Try MEMS microphone measurements for yourself!

Our free test sequence includes not just one, but three pre-written test sequences for testing MEMS microphones including Frequency and Sensitivity, Microphone Substitution, and Power Supply Rejection.

Video Script:

As you know, SoundCheck offers unrivaled flexibility for testing different types of devices, and testing a MEMS microphone is as simple as adding a small, stand-alone MEMS interface to your test setup.

Let’s look at how we measure a digital MEMS- or micro-electromechanical- microphone in SoundCheck from component level to a finished product.

Testing digital MEMS microphones is identical to testing an analog microphone, we simply add a MEMS interface like the DCC-1448 to convert the digital PDM signal to PCM. This interface has 2 inputs, and  two interfaces can be daisy chained together for 4 discreet MEMS inputs. Performing the measurement is the same as with an analog mic. SoundCheck calibrates the source speaker by first measuring the speaker’s response and applying the reciprocal response to the speaker, effectively flattening the speaker. This removes the speaker’s influence on the MEMS microphone measurement [diagram.. Short video B-roll]. Alternatively, we can capture the response simultaneously though the MEMS mic and a calibrated reference mic, and subtract the reference mic’s response from the MEMS response, leaving the raw response of the MEMS microphone [diagram].

SoundCheck also tests MEMS sub-assemblies like a hands free microphone array. These sub-assemblies usually have multiple MEMS microphones and onboard D/A. Since SoundCheck easily supports multiple channels and multichannel audio hardware like our own AmpConnect 621, scaling for testing 4 or 6 channel MEMS sub-assemblies is a snap.

Of course, once the MEMS sub-assembly is installed into the finished product, we can measure that too. Here’s a test setup of SoundCheck testing the hands free MEMS microphones installed in a vehicle. SoundCheck plays a test stimulus through the mouth simulator and captures the response through the on-board microphones. The microphone response is transferred to SoundCheck via a BTC-4149 Bluetooth interface paired to the vehicle’s head unit, then analyzed. 

Aside from basic audio quality tests using sine waves, SoundCheck can also measure MEMS microphones with real world signals like pink noise, speech, music and more for all types of tests including communications, voice activation and more.

Check out our website for more information and free test sequences for measuring MEMS microphone sensitivity, frequency response and power supply rejection.

Microphone Polar Plot: Substitution Method Using LinearX LT360 Turntable

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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Open Loop Microphone Testing

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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