One of the consequences of operating electrical and electronic equipment is the possibility of disturbing, or interfering with, nearby items of electronic equipment. The term given to this type of disturbance is electromagnetic interference (EMI). Placing a portable radio receiver close to a computer and tuning through the radio's wavebands can illustrate this effect. The computer will radiate electromagnetic energy; this is received by the radio and heard as 'noise'. Radio equipment is designed to receive electromagnetic energy; however, the noise in this simple experiment is unwanted. Electrical or electronic products will both radiate and be susceptible to the effects of EMI. This is a paradox since many principles of electrical engineering are based on electromagnetic waves coupling with conductors to produce electrical energy and vice versa (generators and motors). Furthermore, systems are specifically designed to transmit and receive electromagnetic energy, i.e. radio equipment. The problem facing aircraft electrical and electronic systems is the unwanted noise; in the case of the computer/radio experiment, this unwanted noise is no more than a nuisance. In complex avionic systems, the consequences of EMI can be more serious. The ability of an item of equipment to operate alongside other items of equipment without causing EMI is electromagnetic compatibility (EMC).