A couplant is a material that serves as a medium for the transmission of sound waves. Usually, couplant gel is a form of water-based substance, or a paste composed of oils or grease-like chemicals. It is placed in physical contact with a transducer that receives audio signals in the air and then coverts them to electrical impulses for transmission. Microphones and sound test equipment use couplant gel or dry couplants to facilitate this.
Ultrasonic testing of materials can also employ complete immersion of the transducer in a couplant-like water, or just a thin film of glycerin or oil between the transducer and medium being studied. Acoustic couplant is important for ultrasonic testing because air is a fairly poor medium for the transmission of sound waves in general as compared to solids. The level of energy that ultrasonic frequencies carry falls off dramatically when transmitted through air, so these materials are meant to minimize this loss.
Many conventional substances can be used as a couplant, including motor oil or even hair gel. This is because even a very thin layer of air between a transducer and sound specimen will have strong attenuation effects, and nearly any solid placed between them will reduce this. Electrical components can generate a lot of heat, however, so specially formulated couplants are designed to accommodate this.
Ultrasonic couplants in the nuclear and medical equipment industry go a step further by requiring materials that are low in halogen or sulfur compounds at less than 50 parts per million (ppm). Propylene glycol, the same material used in automotive antifreeze, is another specialty compound used. It is chemically nonreactive and can withstand temperatures of 200° Fahrenheit (90° Celsius) before undergoing thermal breakdown. Optical couplant fills another unique need. Often referred to as index matching gel, it is used in the splicing of fiber optic cable to minimize variations in the index of refraction that occur where the fibers meet, which can degrade signal transmission.
The main properties looked for in a good couplant material are its acoustic properties, corrosion inhibition, and surface wetting so that it binds well. The length of time it stays wet, known as drying time, is also important, as well as the temperature levels it can endure and its uniformity. A unique aspect of some couplant gel is that it contains a fluorescent tracer dye that glows in the ultraviolet band, which is used to monitor coverage levels.