Calibrating My Earphones with the Tympan

At its core, a hearing aid is a microphone, an amplifier, and an earphone.  Each element is critical to the quality of the sound produced by the device.  For the Tympan, it (currently) is simply the amplifier (ie, the electronics) -- you have to bring your own microphones and speakers.  In my last post, I looked at a few different microphones.  Today, I'm going to look at some earphones.  I'm going to calibrate a set of earphones with the Tympan to measure the frequency response and overall loudness.

Earphones:  The earphones that I'm going to calibrate are my Klipsch S4 earbuds.  They are absolutely nothing special.  They are simply low-cost, consumer-level earbuds like anyone might have.  I think that inexpensive earbuds are where a lot of people will start when using the Tympan, so it's where I'm going to start with my earphone calibration.

Setup:  My overall setup is shown in the figure above.  I'm using the the Tympan electronics to drive the earbuds.  The Tympan has been programmed to generate a tone that sweeps from low frequency to high frequency .  The earbuds are fixed in a coupler, which mates to a laboratory-grade microphone (B&K 4192) and data acquisition system.  So, by generating tones at known digital level in the Tympan, I can measure the corresponding sound pressure level produced by the earbuds in the 2cc coupler.

Using the B&K 4949 2cc Coupler

Coupler:  The coupler is a very important part of this setup.  Earbuds are supposed to be worn in an ear canal.  There is very little volume in your ear canal that the earbud needs to drive.  So, if you want to measure the response of an earbud, you need to use a coupler that creates a similar small-volume environment.

Which Coupler to Use?  There are two typical choices for couplers for this kind of testing: (1) an ANSI-standard 2cc coupler, or (2) a so-called "artificial ear".  While the artificial ear is designed to give a more realistic response, a basic 2cc coupler is what the ANSI standard uses for hearing aid assessment, so that's what I'm starting with.  My 2cc coupler is a B&K Type 4946, shown above.

Fitting my earbud to the coupler's adapter.  The output of the earbud should be flush with the hole in the adapter plate.

Fitting the Earbud:  The 4949 includes different adapter pieces to help fit a variety of hearing aid styles.  For my earbud, I chose to use the broad dish-shaped adapter piece intended for in-the-ear hearing aids.  As you can see above, I used putty to hold the earbud in place and positioned the earbud such that the output of the earbud is flush with the opening of the adapter.  It isn't pretty, but I think that it gets the job done.

Completing the Setup:  Once the earbud and adapter plate were fit to one end of the coupler, I inserted the measurement microphone into the other end of the coupler.  I connected the earbuds to the Tympan electronics and programmed the electronics to produce frequency sweeps at different digital levels (my code is on my GitHub here).

My setup used to calibrate the Tympan output with my Klipsch consumer-level earbuds.

Raw Data:  Some raw data from this measurement is shown below (raw data is in my GitHub here).  It is the measured sound output from the earpbud as measured by the microphone in the 2cc coupler.  This microphone is calibrated to units of Pascals from which sound pressure level (SPL) is easily computed.  As you can see, I had the Tympan generate five frequency sweeps, each one at a louder amplitude.

Raw data recorded from the Klipsch earbuds during this calibration testing.

Actual Amplitude:  In my Arduino code, I thought that I was commanding the amplitude to step from 0.1 up to 1.0.  In later analysis, however, I discovered that the ToneSweep object from the Teensy Audio library only produces half the amplitude that you think.  So, when you command "1.0", you actually get an amplitude of 0.5.  I confirmed through by sending the audio out the USB Audio connection to the PC.  It was half.  So, instead of spanning 0.1 to 1.0, my test spanned only 0.05 to 0.5.

Low-Level Noise:  When I looked at this raw data in the frequency domain (see spectrogram below), I was surprised to see this strange, low-level background noise appear at the higher drive levels.  It first appears at a digital drive level of 0.25.  What is this?  At lower drive levels it is not there -- it just magically appears at 0.25.  This is very strange.

Spectrogram of audio produced by the Tympan via the Klipsch earbuds during the calibration tests.  When the drive amplitude is 0.25 and above, a low-level background noise appears.

[Follow-Up: Additional testing suggests that this noise might be related to the Tympan's audio codec (TI 3206) struggling to deliver the required current to these low impedance (18 ohm) earbuds.  That's my current working theory, but I cannot yet definitely confirm that that's the issue.  I'll have to do more examination of this in the future.]

Data Analysis:  From this raw data, I divided the recording to isolate each frequency sweep.  Then I measured the SPL at each frequency (analysis code here, more Matlab functions here).  For each drive amplitude, the measured response is shown in the figure below.  As expected, the frequency response is basically independent of amplitude, which is good.

Measured frequency response for different drive levels.

The Bottom Line:  The figure below is how I chose to summarize these results.  The red line is the measured frequency response and loudness when driving at an amplitude of 0.25, which is when that low-level noise/distortion appears.  The blue line is the frequency response that I measured at my strongest drive level (0.5 relative to full scale) that I then scaled up as if I had driven it at the hardware's maximum drive level (1.0 relative to digital full scale).  As can be seen, at 1 kHz, the system should be able to produce about 117 dB SPL at full drive.

Bass Boost:  A secondary result that is obvious in this graph is that these earphones do not have a flat frequency response.  Not at all.  These consumer-level earphones clearly provide a strong boost to the frequencies below, say, 500 Hz.  I'm guessing that this bass boost this was a conscious decision by the designers in order to appeal to a certain segment of the earbud market.  This is perhaps not the ideal response for a hearing instrument.  I'm curious to test other earphones to see which earbuds might have a more flat response.

Caveat:  An important caveat to this work is that the measurements were taken using a 2cc coupler.  Your ear is NOT the same as the 2cc coupler.  In a real ear, the frequency response could be quite different.  Perhaps the overal SPL produced by the earphones will be higher (or lower).  These are important questions.  So, in the future, I'd like to repeat these tests using an artificial ear, which should better simulate a real ear.  Will the results be different than on the 2cc coupler?  I look forward to finding out!

Calibrating Microphones with Tympan

One goal of our open source hearing aid platform ("Tympan") is to get people to experiment with new sound processing algorithms but another goal of the Tympan is to enable people to experiment with different microphones and speakers.  So, with regards to microphones, we designed the Tympan electronics to have several different kinds of inputs to allow it to support a range of microphones.  Today, I'll start by calibrating three different microphones with the Tympan.  Then, in a later post, I'll look at how the different microphones influence the self-noise of the system.

Microphones:  The three microphones that I'm using today are shown above.  There's the "Sony Mic", which  is a lapel electret mic intended for picking up voice.  Then, there is the "PCB Mic", which is an inexpensive surface-mount silicon MEMS mic that we've included on the Tympan PCB.  And, finally, there's the "Knowles Mic", which is a high-sensitivity, low-noise mic intended for use in hearing instruments like hearing aids.

Why Calibrate?  If you calibrate your microphones, your audio processing will be able to look at the in-coming digital data and know what sound level is happening in the real world.  Knowing the true sound level in the different frequency bands allows you to tailor your algorithms (amplification, compression, noise reduction) to better respond to a person's specific hearing loss.

Calibration Approach:  To calibrate these microphones, I'm using the approach shown above.  Here, I put the Tympan in my sound chamber and play known sounds at it.  I record the digital values obtained by the Tympan (recorded via its SD card) and compare them to the "truth" that was simultaneously recorded from my laboratory-grade microphone (Bruel & Kjaer 4191).

Truth Microphone:  It is important that the truth microphone be placed very close to the Tympan microphone during this calibration.  Ideally, they'll see exactly the same sound levels.  Pictures showing my arrangement is shown below.

Connecting the Tympan Microphone:  Each of the three test microphones connects to the Tympan in a different way:

• Sony Mic:  Like many lapel microphones, this mic comes nicely packaged with a 1/8" (3.5 mm) phono plug.  We have a mic jack on the Tympan PCB just for this purpose!  I programmed the Typman to supply a 2.5V bias voltage for this microphone.  
• PCB Mic: This microphone is just a raw element.  We designed the Tympan to have two of these mics right on the circuit board so that they'd be easy to use.  Therefore, this mic is already wired and simply needs to be enabled in the Tympan software.  
• Knowles Mic: This is a raw hearing aid microphone.  I soldered some wires to it and connected the wires to the Tympan's "line in" holes that are on the edge of the Tympan PCB.  I provided the bias voltage from a pair of AA alkaline batteries.

Software:  For this test, I wrote an Arduino sketch that allows me to switch the Tympan between the three different microphones.  The sketch saves the digitized audio to the Teensy's SD card.  Additionally, the sketch steps through different levels of gain on the Tympan input (0 dB to +40 dB) so that I can see its effect.  My Arduino code is on my GitHub here.  You'll also need the Tympan library, which you can get here.

Example Data:  Below is a spectrogram of the audio data that I recorded during one of these calibration tests.  The top plot shows the audio from the truth microphone.  The bottom plot is the signal recorded by the Tympan, in this case for the Sony mic.  You can see that I had a loop of audio that I was playing over-and-over into the room.  The audio loop alternates between white noise and a 1 kHz test tone.  In the bottom plot, you can clearly the see the effect of increasing the gain of the Tympan input.

Analyzing the Truth Data:  I started by analyzing the truth data.  Looking at just the white noise periods, I filtered the audio into 3rd-octave bands from 125 Hz to 16 kHz and assessed the average signal level in each band.  Because this laboratory microphone is itself regularly calibrated, I know its data in units of Pascals, which leads directly to units of sound pressure level (SPL).  The SPL that I measured in each band is shown in the figure below.  This is the truth to which I will compare the Tympan data.

Analyzing the Tympan Data:  I performed the same 3rd-octave band analysis of the Tympan data.  Because this microphone is not calibrated (that's what we're doing here) I can only measure the signal levels relative to digital full scale (ie, relative to the level at which the system starts to digitally clip the data).  The plot below shows the 3rd-octave band levels that I measured for the Tympan for the five different gain settings.  The blue line at the bottom shows the levels when the input gain was set to 0 dB while the green line at the top is for an input gain of +40 dB.

Combining with the Truth:  The last step to computing the scale factor is to combine the Tympan data with the truth data.  By bringing the two sets of data together, I can compute how the Tympan digital values (dBFS) can be scaled to reveal the true, in-the-air sound pressure level (SPL).  The figure below shows the resulting scale factor for the Sony mic for the different Tympan gain settings.  It is good to see that the gain setting only appears to affect the overall sensitivity of the system -- that the shape of the frequency response is basically the same for all gain settings.

Repeat for Other Mics:  When I perform this calibration process for all of the microphones, I get the figure below.  It shows that the Sony mic (blue) has a up-sloping response, which is good for a closely-placed lapel microphone.  The PCB mic (orange) is more flat, but has nearly the same sensitivity as the Sony mic.  The most interesting response is the Knowles mic (yellow) because it is nearly 20 dB more sensitive than the other two.  That is quite a difference!

What Gain Setting to Use?  In my tests, I tried a bunch of different gain settings.  Which is the right one to use?  Well, one approach to picking the right gain is to decide what is loudest sound that is likely to be seen.  Let's assume that 120 dB SPL is the loudest sound that we want the Tympan to handle  Then, we look at our sensitivity numbers and choose a gain setting that permits this maximum SPL without clipping.  Using the data for the graph above, the calculation goes like this:

• Sony Mic at 120 dB SPL yields a digital level of: (120-94) + -48.6 = -22.6 dBFS.  This means that it has 22.6 dB of excess headroom.  I can safely set the gain to +20 dB.
• PCB Mic at 120 dB SPL yields a digital level of: (120-94) + -47.4 = -21.4 dBFS.  This means that it has 21.4 dB of excess headroom.  I can safely set the gain to +20 dB.
• Knowles Mic at 120 dB SPL yields a digital level of; (120-94) + -28.9 = -2.9 dBFS.  This means that it only has 2.9 dB of headroom.  For this mic, I would leave the gain at 0 dB. 

We're Calibrated!  With this testing, we've now calibrated these three microphones with the Tympan.  We know the frequency response and we know how to relate our recorded digital values to real-world SPL numbers.  We also have some guidance as to what gain value we should use for each microphone.  We got a lot done!

Follow-Up:  I've added the raw data and my Matlab analysis files to my GitHub repo.  You can get them here.

Enclosing the Electronics

After designing and building our custom electronics for the Tympan, it was clear that we needed an enclosure to hold it all together.  Using zip ties (my usual go-to solution) just wasn't good enough.  I needed something more proper to wrap around and protect the electronics.  I was kinda nervous about taking this step because, if you've never designed an enclosure before, it is HARD!  Here's a quick picture tour of how I got to a workable enclosure.  Yay pictures!

Making Something Attractive?  Normally, one wants an enclosure to be both functional and attractive.  As I started into this task, though, I very quickly learned that "attractive" is not really part of my skill set.  It's especially challenging  when the electronics (ie, the bulky guts that I'm trying to cover up with my enclosure) have already been designed and built and can't be altered.  This imposes some challenging constraints on the enclosure.  So, I set aside the goal of making it attractive and I focused on simply making it functional.

First Attempt:  Working with a fellow engineer who knows more about CAD than I do, we decided to use a two piece design that would sandwich the electronics and battery.  We'd 3D print the two pieces of the enclosure and hold the whole thing together with four small screws.  As you can see below, our first design ended up being pretty boxy.

Our first design.  Boxy!

Begin the Revisions!  Seeing it on the screen, I could begin to visualize in my brain how I might want to hold and use the device.  I decided that, if I was wear this device, I'd most likely keep it in my pants' pocket.  I decided that the boxiness of its nose would make it hard to slide into and out of my pocket.  So, we revised the design to make the bottom cover slimmer and we put big slope on the top cover.

Revising the design to make it slimmer and easier to slide down into a pocket.

Iterate, Iterate, Iterate!  With the overall configuration taking shape, we began to make lots of little changes to accommodate the battery wires, to give better access to the volume pot, and to expose the LEDs (box #3 below).  At this point, I decided that the sloping nose made it harder to get your fingers around the volume pot, so we we removed the slope (box #4).  This is the design that we used for our first 3D print.

Real-World Experience:  With the real, physical print of design #4, we were able to put it all together for the first time -- electronics plus battery plus top and bottom enclosure.  Almost immediately, I realized that I needed access to the Teensy's reset button, which was covered by our enclosure.  Oops!  So, we added a round hole to expose the reset button (see #5 above).  Then, we further expanded the hole to expose the SD card (#6 above).  Unsure if the hole was big enough, we took our 3D print of design #4 and used a dremel tool to cut the square hole (see picture below).  It wasn't pretty, but the hole worked just fine to access the SD card.

The opening to expose the SD card isn't attractive, but it does work.

Final Design.  After all of these iterations, we got to our final design, which is shown below.  The CAD files for this design are shared on the Tympan GitHub here.

Our final design.  Download it from our GitHub!

SLA vs FDM.  We printed a couple of copies of our final design using SLA.  The quality of the translucent SLA prints was fantastic, especially the surface finish.  But, the SLA prints were expensive -- too expensive to make lots of copies.  Luckily, I got hooked up with the good folks at Tangible Creative, who were able to FDM print 10 sets of enclosures for about $10 a set.  They came out pretty nicely (see picture below).  Sorry about the boring black color.  Next time I'll pick something more fun!

Fitting the Tympan electronics into the FDM-printed enclosure.

Make it Pretty!  So, while I ended up with an enclosure that does the job, it has plenty of room for improved aesthetics.  If you think that you can make it better, take our design files and make something better!  Please!