In electronics, motorboating is a type of low frequency parasitic oscillation (unwanted cyclic variation of the output voltage) that sometimes occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put", in audio output from speakers or earphones.
The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output.
Although low frequency parasitic oscillations in audio equipment may be due to a range of causes, there are a few types of equipment in which it is frequently seen: As with all electronic oscillation, motorboating occurs when some of the output energy from an amplifying device like a transistor or vacuum tube gets coupled back into the input circuit of the device (or possibly into an earlier stage of the amplifier circuit) with the proper phase for positive feedback.
Since most amplifying devices, transistors and tubes, are inverting, with the output signal 180° opposite in phase from the input, the feedback path must contribute the other 180° of shift.
The output (plate) impedance of the diode vacuum tube rectifiers in the power supply is high, so the increasing impedance of the filter capacitors at low frequencies can mean that low frequency swings in the current drawn by output stages can cause voltage swings in the power supply voltage which feed back to earlier stages,[2][6][4] making the system a subaudio oscillator.
If the decoupling capacitors which bypass radio noise from the power supply wires are missing or inadequate, or the long power leads pick up excessive RF from the antenna then it is possible for the RF transmitter signal to enter the radio's receiving circuits through the supply wires.