The Inspiration
Traditional heart rate monitoring requires specialized PPG sensors (like an Apple Watch) or ECG straps. However, a fascinating 2023 paper explored using the internal ear canal as a resonance chamber for cardiac sounds.
As a clinician-builder, the question was simple: Can we do this with hardware people already own?
The Prototype Setup
The experiment utilized the transducers in bone conduction earphones (specifically Shokz OpenRun) used in reverse—effectively turning the vibration drivers into highly sensitive contact microphones.
- Transducer: Shokz OpenRun placed firmly against the tragus.
- Processing Unit: iPhone 15 Pro via the Lightning/USB-C audio jack.
- Software: A custom Swift-based audio buffer analyzer to filter frequencies between 20Hz and 100Hz (the range of the S1 and S2 heart sounds).
Bone conduction drivers are essentially magnets and coils. When physical vibrations (like a pulse) hit them, they induce a small voltage. The trick is amplifying that voltage without drowning it in electronic floor noise.
The Reality Gap
In a perfectly silent room, with the subject holding their breath, the "Lub-Dub" signal is clear as day on the spectrogram. However, as soon as the subject moves, speaks, or even blinks, the Signal-to-Noise ratio (SNR) collapses.
Consumer hardware lacks the Active Noise Cancellation (ANC) specialized for bio-signals. While clinical-grade ear-ECGs use shielded cables and high-impedance amplifiers, my "off-the-shelf" approach struggled with:
- Jaw movement: Even small TMJ micro-movements create massive spikes.
- Ambient Resonance: The ear canal acts as a megaphone for room noise.
Future Outlook
This experiment confirmed that the ear canal is a goldmine for physiological data. While consumer Shokz aren't ready to be clinical stethoscopes yet, the next generation of "Hearables" with internal-facing microphones will likely integrate this by 2026.