MEDICAL ULTRASOUND SENSORS · 311 Figure 6.23. Estimate of fractional -3 dB bandwidth versus relative impedance factor, , for various piezoelectric materials. is also a prerequisite to satisfactory performance to receive acoustic pulse echoes that have been attenu- ated by tissue absorption, which increases with distance and frequency. As discussed in section 6.4.2, reciprocity arguments can be invoked for receive sensitivity to be comparable to transmission efficiency