Stethoscope

This activity allows students to transform their smartphone into a stethoscope to visualize their heart rate. It establishes a fascinating link between digital technology and human physiology.

The stethoscope, invented by French physician René Laennec in 1816, remains the most iconic symbol of medicine. Laennec reportedly invented it after feeling uncomfortable placing his ear directly on a female patient's chest; he rolled a sheet of paper into a tube and discovered that it amplified the heart sounds beautifully. The characteristic lub-dub rhythm of the heartbeat comes from the sequential closing of heart valves during the cardiac cycle. The first sound (lub) is produced by the closure of the mitral and tricuspid valves at the onset of ventricular contraction (systole). The second sound (dub) comes from the aortic and pulmonary valves closing as the ventricles relax (diastole). A smartphone microphone, placed against the chest and combined with FizziQ's signal processing capabilities, can detect these same sounds, allowing students to visualize their own heartbeat as a waveform and calculate their heart rate from the acoustic signal.

Learning objectives:

The student uses their smartphone's microphone as an acoustic sensor to detect heartbeats. By placing the device against their chest and activating signal filtering in FizziQ, the student can record the sound waves produced by their heart, observe their characteristic shape and then calculate their heart rate from the number of peaks identified.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

25

What students will do :

- Detect heart sounds using the smartphone microphone placed against the chest
- Visualize the characteristic waveform pattern of the cardiac cycle
- Identify the two main heart sounds (S1 and S2) in the recorded signal
- Calculate the heart rate from the time interval between consecutive heartbeat cycles
- Understand the physiological origin of each heart sound

Scientific concepts:

- Heart sounds
- Signal processing
- Filtering
- Cardiac cycle
- Systole and diastole

Sensors:

- Microphone (acoustic sensor for heart sound detection)

What is required:

- Smartphone with the FizziQ application
- A calm environment
- FizziQ experience notebook

Experimental procedure:

Find a quiet environment with minimal background noise. Sit comfortably and relax for 2 minutes before starting.
Open FizziQ and select the Microphone sensor (waveform or sound level mode).
Place the smartphone's microphone directly against your bare chest, slightly to the left of the sternum, where the heart sounds are strongest.
Press gently but firmly to ensure good acoustic contact. Hold the phone steady with one hand.
Start recording. You should see periodic clusters of activity in the sound waveform, corresponding to each heartbeat.
If the signal is weak, try activating any signal filtering options in FizziQ to reduce high-frequency noise and enhance the low-frequency heart sounds.
Record for at least 30 seconds to capture approximately 30-50 heartbeat cycles.
Stop recording and examine the waveform. Each heartbeat should appear as a pair of peaks (S1 and S2) separated by a short interval.
Count the number of heartbeat cycles (pairs of peaks) in a 20-second interval.
Calculate your heart rate: HR (bpm) = (number of cycles / 20) × 60.
Try to identify the S1 sound (the larger, longer first peak, the lub) and the S2 sound (the shorter, sharper second peak, the dub).
Compare your result with a manual pulse measurement at the wrist to verify accuracy.

Expected results:

The heart sounds should appear as periodic clusters in the microphone signal, with a typical interval of 0.6-1.0 seconds between cycles (60-100 bpm). The S1 sound is usually the larger and longer-duration signal, while S2 is shorter and higher-pitched. The signal quality depends heavily on the phone's microphone sensitivity, the acoustic coupling with the chest, and the ambient noise level. Some phones produce very clear heart sound recordings, while others may show only faint signals requiring close examination. The calculated heart rate should agree with a manual pulse count within ±3-5 bpm. Students may also observe respiratory modulation: the heart rate increases slightly during inhalation and decreases during exhalation (respiratory sinus arrhythmia), which is a normal physiological phenomenon.

Scientific questions:

- What causes the two distinct heart sounds (lub and dub) in each cardiac cycle?
- Why is the first heart sound (S1) typically louder and longer than the second (S2)?
- What is the interval between S1 and S2 within a single heartbeat? What does it represent physiologically?
- How does a clinical stethoscope improve upon simply placing an ear against the chest?
- What cardiac abnormalities can be detected by listening to heart sounds?
- How does the respiratory cycle affect the heart rate (respiratory sinus arrhythmia)?

Scientific explanations:

Heart sounds, commonly called "lub-dub", are produced by the closing of heart valves during the heart's cycle of contraction and relaxation. The first sound ("lub") is duller and prolonged, caused by the closure of the atrioventricular valves (mitral and tricuspid) during ventricular contraction (systole).

The second sound ("dub") is shorter and higher, produced by the closure of the semilunar valves (aortic and pulmonary) when the pressure in the arteries exceeds that of the ventricles at the end of systole. A smartphone's microphone, although designed to pick up the human voice, has sufficient sensitivity to detect these acoustic vibrations, particularly when placed firmly against the sternum, where sounds are best transmitted through the rib cage.

The main challenge is the signal-to-noise ratio: heartbeats are of low amplitude compared to ambient noise and body movements. This is why filtering is crucial.

The low-pass filter set at 70 Hz in FizziQ attenuates high frequencies (including most breathing and ambient noise) while preserving heart sounds which are primarily between 20 and 150 Hz. To optimize the measurement, it is recommended to: 1) Maintain constant but not excessive pressure of the phone against the chest; 2) Hold your breath briefly to reduce interference; 3) Remain perfectly still while recording.

Signal visualization allows one to observe not only the heart rate, but also the separation between the two characteristic sounds, providing a window into the dynamics of the cardiac cycle. This simple but powerful technique illustrates how everyday devices can be hijacked for rudimentary medical applications.

Extension activities:

Frequently asked questions:

Q: I cannot hear or see any heart sounds in the recording. What should I do?
R: Ensure the microphone is pressed firmly against bare skin on the left side of the chest. Eliminate all background noise (turn off fans, close windows). Try repositioning the phone slightly. Some phone cases muffle the microphone and should be removed.

Q: The signal is dominated by noise rather than heart sounds. How can I improve it?
R: Use FizziQ's filtering options if available. Low-pass filtering (cutting frequencies above 200 Hz) can significantly improve heart sound visibility. Also ensure you are breathing shallowly and sitting very still.

Q: I can see periodic peaks but cannot distinguish S1 from S2. Is this normal?
R: On many smartphones, the microphone resolution may not be sufficient to clearly separate S1 and S2 within a single heartbeat. Focus on counting complete cycles (S1-S2 pairs) for heart rate measurement.

Q: My heart rate seems different from what I feel at my wrist. Which is more accurate?
R: Both methods should give similar results. If the acoustic measurement is unclear, the manual pulse is more reliable. Discrepancies may arise from miscounting faint signals or from the measurement being taken at slightly different times.