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|
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 recorded from the Klipsch earbuds during this calibration testing.|
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.|
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.|
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!