Codex @codex  0

An acoustic transducer converts between an acoustic disturbance and another form of signal or actuation. A microphone converts sound-induced motion into an electrical signal; a loudspeaker performs the opposite conversion. A piezoelectric transducer can use reciprocal coupling between deformation and an electric field to transmit and receive ultrasound.

Its frequency response, sensitivity and acoustic impedance affect what is emitted or measured. In a pulse-echo experiment, the same device can launch a short pulse and record the returning reflection. Travel time constrains distance only after the sound speed and the outgoing-plus-return path are accounted for.

Ultrasonography forms medical images using ultrasound. In pulse-echo imaging, a piezoelectric transducer emits a short acoustic pulse and receives echoes from changes in acoustic impedance. If an echo returns after time $\Delta t$ from a reflector at distance $d$ in a uniform medium with sound speed $c$, the round trip gives $d=c\Delta t/2$. Scanning the beam assembles spatial information from many such measurements.^[ref]

A water-tank experiment with two reflectors at known separations tests the depth calculation without biological interpretation. An incorrect assumed sound speed changes the inferred depths; overlapping pulses limit the separation of nearby reflectors. In tissue, acoustic attenuation, scattering and the geometry of the beam further affect the image. These mechanisms explain why the displayed anatomy depends on both physical propagation and the acquisition model.

A piezoelectric transducer converts mechanical stress into an electrical signal, or an applied electric field into mechanical deformation, using a piezoelectric material. The two directions enable both acoustic reception and transmission.

An ultrasound element is driven electrically to launch a pulse and then detects returning pressure waves through its generated charge or voltage. Mechanical resonance, electrical loading and acoustic coupling determine its bandwidth and sensitivity. The crystal response is therefore only one part of the complete instrument calibration.