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2016 December 28

Headphone impedance with Analog Discovery 2

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 22:41
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In Loudspeaker impedance with Analog Discovery 2, I described measuring the impedance of loudspeakers with the network analyzer function of the Analog Discovery 2. In this post, I looked at some new Panasonic headphones that I got myself for Christmas (Panasonic RP-HJE120-PPK In-Ear Headphone, the best-seller on Amazon and the same model my son has, though in a different color).

I have figured out how to use the Waveforms 2015 software a little better now, so I can compute the magnitude of impedance as an extra column in the output (using the “Custom One” optional calculated channel).  This cuts down slightly on the missing metadata from the data files, though I really wish that they would do a dump of the instrument settings as comments at the beginning of the file.

The headphones had essentially the same curves whether in the ear or not in the ear, so I am just plotting the in-ear electrical characteristics.

The headphones are fairly well fit by a simple model: a resistor in series with an inductor.

The headphones are fairly well fit by a simple model: a resistor in series with an inductor.

Zooming into the audio region shows surprisingly little variation in the impedance over the whole audio range. There is a small resonance peak around 2.6kHz, but it is small and broad, nothing like the resonance peaks of loudspeakers.

Zooming into the audio region shows surprisingly little variation in the impedance over the whole audio range. There is a small resonance peak around 2.6kHz, but it is small and broad, nothing like the resonance peaks of loudspeakers.

I had some problems with repeatability of measurements, with curves jumping 0.5Ω up or down, but preserving their shape. I think that the problem is with poor contacts in the breadboard I was using, as I had the same problems earlier characterizing nFETs. The resonance peak around 2.6kHz corresponds roughly with the peak of information content in speech, so slight enhancement there is probably perceived as improved audio quality over a completely flat spectrum. But the enhancement here is tiny, so it may just be the result of flattening the spectrum as much as feasible.

The noise in the measurements probably reflects the small signal levels—I had an 18Ω resistor in series with the 16Ω headphones, and an amplitude of only 25mV across the pair, which gives me only 8.3mV RMS at the headphones.  That means that only 4.2µW of power is being used to generate the sound.  Panasonic claims a sensitivity of 96dB/mW, so 4.2µW should be about 72dB SPL (remember that dB is 10 \log_{10} of a power ratio, and 10 \log_{10} of an amplitude ratio). The 72dB seems about right for the loudness.  The headphones can supposedly handle 200mW, which would be 119dB—easily loud enough to cause permanent hearing loss.  Perhaps I should have students test their preamplifiers with earbud headphones instead of loudspeakers—the 24mA limit would give 9mW, which would be quite loud in a headphone.

The R+L model does not fit at high frequencies all that well, and the phase relationships are not what one would expect of a pure R+L model:

The phase only gets to 60°, while a true inductor in series with a resistor would have reached 90° and done so somewhat sooner.

The phase only gets to 60°, while a true inductor in series with a resistor would have reached 90° and done so somewhat sooner.

Overall, I’m impressed at how flat the impedance is over the audio range. I don’t know how good the headphones are acoustically (especially as my hearing seems to be really down in the 4kHz–8kHz range—signals seem louder to me at 9kHz than at 5kHz), but I’ve no complaints about them so far.

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