Acoustic transducer

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.

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.

A loudspeaker converts an electrical signal into motion that drives acoustic waves. In a moving-coil design, electric current through a coil in a magnetic field produces a force on the attached diaphragm. The diaphragm displaces air, creating sound pressure variations.

A small coil, magnet and lightweight cup can demonstrate the conversion directly. ^[ref] In an idealized uniform-field geometry, the driving force scales as $B\ell I$. The emitted sound also depends on diaphragm inertia, suspension spring constant, damping and acoustic impedance, so the electrical waveform alone does not specify a flat acoustic frequency response.