Acoustic Thermometry is an arcane, elegant temperature measurement technique. It utilizes sound’s temperature dependent transit time in a medium to measure temperature. The medium may be a solid, liquid or gas. Acoustic thermometers function in environments that conventional sensors cannot tolerate. Examples include extreme temperatures, applications where the sensor would be subjected to destructive physical abuse and nuclear reactors. Acoustic Thermometer is a device to measure air/water temperature based on the principle that the speed of sound varies as the square root of temperature. For example, Acoustic Thermometry of Ocean Climate (ATOC) is an idea to observe the world’s oceans, and the ocean climate in particular, using Ultrasonic Transducer based acoustic transmissions.

Acoustic Thermometer using Ultrasonic TransducerGas Path Acoustic Thermometers respond very quickly to temperature changes because they have essentially no thermal mass or lag. An acoustic thermometer’s “body” is the measurand. Additionally, an acoustic thermometer’s reported “temperature” represents the total measurement path transit time as opposed to a conventional sensor’s single point determination. As such, an acoustic thermometer is blind to temperature variations within the measurement path. It reports the measurement path’s delay as its “temperature,” whether or not the path is iso-thermal.

A pleasant surprise is that the sonic transit time in a gas path acoustic thermometer is almost entirely insensitive to pressure and humidity, leaving temperature as the sole determinant. Additionally, sonic speed in air varies predictably as the square root of temperature.

A practical acoustic thermometer demonstration begins with selecting a sonic transducer and a dimensionally stable measurement path. A wideband ultrasonic transducer is desirable to promote fast, low jitter, hi-fidelity response free of resonances and other parasitics. The electrostatic type device meets these requirements. A single transducer serves as both transmitter and receiver.

For more details about the construction and operation of Acoustic Thermometer, you can go through the Linear Technologies Application Note.131 “An Introduction to Acoustic Thermometry“, at: http://cds.linear.com/docs/Application%20Note/an131f.pdf