4.3 Transducer for Non-Electrical Biosignals

|

111

Fig. 4.19: Self-construction of an electronic stethoscope for connection to the PC sound card.

shielded lead

Plug

Microphon

R1

Fig. 4.20: Wiring diagram for an electret microphone.

bandpass filters in parallel and, filtered in this way, output to the paper recorder as

individual curves. In parallel, the ECG was registered as an independent marker for

cardiac phase. With the advent of imaging techniques, especially echocardiography

using ultrasound, phonocardiography was increasingly displaced as a standard cardi-

ological diagnostic technique. Today, electronic stethoscopes are commercially avail-

able. A microphone integrated in the sound tube converts the sound, which is then

transmitted wirelessly by radio to a PC.

In Figure 4.19 a simple self-construction for the wired connection to a PC sound

card is shown. The sound tube of the stethoscope was cut open and an electret micro-

phone was sunk into it. Electret microphones have a very small size and have uniform

sensitivity over a wide frequency range, which covers the bandwidth of heart sounds

of max. 1 kHz. The microphone capsule has an integrated impedance converter. This is

supplied via an upstream working resistor of 2.2 kΩ. The supply voltage is provided by

the PC sound card via the microphone input (phantom power). Furthermore, a three-

pole cable with jack plug is required. Figure 4.20 shows the connection diagram.

Another form of phonocardiographic self-monitoring by the patient shows Fig-

ure 4.21. This is the functional monitoring of an implanted mechanicalprosthetic heart

valve. Such implants have an increased risk of thrombotic formation, which in turn