Upgraded applications of High Sensitivity Piezo Film Sensor CM-01B in Conventional Medical Earphone

With no amplification function, conventional stethoscopes sound weak and make the ears uncomfortable. They cannot block ambient noise or adjust their frequency. Therefore, Isweek here puts forward a new type of high sensitivity contact microphone (piezoelectric sensor)  used as a pickup part in electronic stethoscopes. It collects human life info with its outstanding matching between its piezoelectric film and human body. Electronic stethoscopes and pulse transducers produced by 3M, heart sound and pulse transducers made by Beijing Yeesong both adopt CM-01B as their collection terminal. Its basic performance parameters are as follows (Figure 1-1).

Parameter Min. Typ. Max Unit
Sensitivity   40   V/mm
Lower limit  frequency (-3dB)   8   Hz
Upper limit frequency (+3dB)   2.2   kHz
Resonant frequency   5   kHz
Spring constant   20   N/m
Electronic noise



  1   mVpk-pk
Supply voltage 4 5 30 V-DC
Suply current   0.1   mA
Operation temperature +5   +60
Storage temperature -20   +85

Figure 1-1

Cased with metallized plastic package, sensor CM-01B is dedicatedly designed for electronic stethoscopes. It possesses advantages of light weight, low cost, high sensitivity of 40V/mm, wide band width of 8~2.2kHz and strong shock resistance, etc. CM-01B’s internal circuit schematic drawing is shown below. (Fig 1-2)


The buffer output signal from the sensor is primarily amplified in the front-end circuit, and then is filtered and amplified (with man-machine dialog KEYS button, the digital signal processing circuit sends different command to DSP for sounds in different frequency band, so as to select the audio signal in a certain frequency band range and independently amplify it), and is recovered to real signal through audio power amplifier driven headset. This can effectively avoid the interference of outside noise. Meanwhile, some dynamic data (heart rate, breath)  and other reference data can be upload to mobile APP via bluetooth module. The DSP terminal can digitize and compress the audio signal that it requires, and store the processed data completely in external storage medium (e.g. SD, HDD) for tracking and replay. For example, for some special cases, doctors can send the corresponding documents through modern medical network system to other experts for remote consultation, which can significantly enhance the veracity and reliability of diagnosis.


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