In the wireless audio industry, two types of intermodulation interference exist: transmitter produced and receiver produced.
Transmitter produced intermodulation is caused by one or more transmitters that generate a signal in the output circuitry of another transmitter. The presence of additional frequencies (intermodulation products) are either the sum or the difference of the transmitted frequencies. The intermodulation product frequency is then re-transmitted from the transmitter's antenna.
Receiver produced intermodulation interference is the result of two or more transmitter signals mixing in a receiver's amplifier. The resultant intermodulation product frequency can interfere with other signals (of similar frequencies) that pass through the receiver. As a result, the intermodulation distortion can degrade the quality of the desired signal being amplified.
Of all the harmonics and intermodulation components produced, we are often only interested in those which fall in the pass band of our equipment and, in the case of the intermodulation components, those which happen to be closest to our fundamental frequencies. The third order components are the closest and also usually the highest in amplitude followed by the 5th order.
Because of this, they are usually the products of most concern and are those which are commonly measured by manufacturers of wireless audio systems to determine where intermodulation might occur in multi-systems and are defined in transmitter and receiver performance specifications.