Wednesday, August 22, 2018

Microphone Self-Noise

A hearing assistive device must not be noisy.  At minimum, noise will be annoying.  At worst, noise will harm, not help a person's ability to hear.  So, for our Tympan device, I want to make sure that it has low self-noise.  In this post, I show how the microphone that you choose can strongly affect the apparent noise of your system.  Spoiler: be careful with lapel mics because they can be very noisy!


Goal:  I want to measure the self-noise of different microphones in combination with the Tympan.

Approach:  My approach is to first calibrate the Tympan when using each microphone.  That way, when comparing between microphones, I'm comparing apples-to-apples.  Once calibrated, I will put the devices in a super-quiet environment and make ambient sound recordings.  The super-quiet environment will probably be so quiet that recordings will reveal the self-noise of the microphones.

Hardware:  As shown in the picture above, I am testing with a Tympan Rev C, which includes built-in microphones (Knowles SPH1642) on its PCB.  I also made recordings with a Sony lapel microphone (ECM-CS10) plugged into the Tympan's microphone jack.  As a reference microphone for the calibration, I used a B&K 2250 sound level meter (SLM) with its 4191 microphone element.

Setup:  I performed the recordings in our single-walled acoustic test chamber in the basement of our building.  It is fairly quiet, though it is not as quiet at the lowest frequencies (125 Hz and below).  As shown in the figure at the top of this post, I put the lapel microphone and the B&K microphone very close to the Tympan's built-in PCB mic.

Device Configuration:  The Tympan was configured to record audio straight to its SD card as 32-bit floating point samples, switching automatically between the two microphones after a fixed interval of time.  It was configured with an input gain of +15dB for all recordings.  My code is on GitHub here.  The B&K SLM was configured to record its calibrated audio to its compact flash card.

Calibration:  To calibrate the microphones, I played white noise into the sound chamber.  The B&K SLM recorded the audio in calibrated units, as shown in the first plot below.  Simultaneously, the Tympan recorded the audio (through each of the two microphones) in uncalibrated units, as shown in the second plot below.  By comparing the raw Tympan levels with the calibrated B&K levels, the bottom plot shows the sensitivity of the Tympan+microphone at each frequency.


Measuring Self-Noise:  Turning off the white noise stimulation, the sound chamber was very quiet. Again, the B&K SLM and the Tympan made recordings from their microphones.  My assumption is that, especially for the Tympan, the true background noise in the sound chamber was so low that the recordings will reveal the self-noise of the microphones.

Self-Noise Expressed as SPL:  The first plot below shows the raw, uncalibrated noise levels recorded by the Tympan.  To convert these values to SPL, I need to apply the calibration data discussed above.  There's a couple ways that I could apply the calibration.  The middle plot shows the estimated SPL if I were to have calibrated the Tympan only at a 1 kHz, which is a common, simple way to calibrate a device.  Alternatively, if we use the full frequency-dependent calibration for the Tympan, the bottom plot shows the estimated SPL.  Using either calibration approach, the conclusion is the same: the Sony lapel mic has much higher self-noise than the built-in PCB mics!

The Microphone Matters!  Getting quantitative, I can summarize these spectra by applying an A-weighting curve and computing the broadband sound pressure level (ie, "dBA").  From these recordings, the Tympan + Lapel Mic has a self noise equivalent to an ambient noise level of 41-43 dBA (depending upon the calibration approach).  By contrast, the Tympan + PCB Mics show a noise level of between 27-29 dBA.  This 14 dB difference is a big!

Conclusion:  Since low self-noise is good, the Tympan's built-in PCB microphones seem to be a better choice than this Sony lapel microphone.  I look forward to trying other microphones to see if I can get even lower self-noise.

Follow-Up: What Self-Noise Should Be Expected?  After completing this post, I realized that I should have looked at the microphone datasheets to see what the manufacturers say about each microphone's self-noise.  Here's what I found:

  • For the PCB mics (SPH1642), the self-noise is not reported directly.  But, they report the signal-to-noise ratio as 65 dBA when given a 1 kHz signal at 94 dB SPL.  This means that the noise floor for the mics is (94 dB - 65 dBA) = 29 dBA.  This is exactly the value that I found, when I used the simple calibration for 1 kHz.  This gives me additional confidence in my measurement technique.
  • For the Sony lapel mic (ECM-CS10), the noise level is reported simply as "38 dB".  Presumably, this is A-weighted, but there is no more information provided.  My own value (41 dBA, for single-frequency calibration) is 3 dB higher than the datasheet value.  The cause for the difference is unknown, though the difference is modest.

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