In this paper we report on results from experiments performed on a bi-modal piezoelectric transducer used both as an active ultrasound transceiver and a passive acoustic sensor. The transducer, which has a low Q factor in order to exhibit a sufficiently broad bandwidth, will be integrated into a wearable system. In particular, it is placed, along with ECG fabric electrodes, within a textile belt wrapped around the chest. The transducer behaves as an acoustic sensor at low frequency and as an ultrasound transducer at high frequency. The low-frequency acoustic signals were compared with the analogue signals acquired simultaneously by commercial biomedical sensors. These signals provide information about the respiratory activity and heart apex pulse. A comparative analysis was performed both in the time and frequency domain and results were discussed. Moreover, the same transducer used at high frequencies is able to generate ultrasound signals which can bounce off the target organ, the heart, and receive the back-propagated echoes. The experimental validation was done by means of a comparison between the spatial interval inferred from time delay of the return echoes detected by the transducer and the actual distance from the target. This information, in addition to ECG signals, can provide helpful cues for the cardiac status of the subject, both in terms of prevention and diagnosis.
Acoustics Blood Pressure Determination