![Magnetic envelope, sine wave](https://cdn.statically.io/img/static.wikia.nocookie.net/memoryalpha/images/3/30/Magnetic_envelope%2C_sine_wave.jpg/revision/latest/scale-to-width-down/180?cb=20051112081610&path-prefix=en)
A magnetic envelope and a sine wave (right) before and after
A sine wave was an oscillating waveform.
In 2286, on his memory test, Spock was asked by the computer to adjust the sine wave of a magnetic envelope so that anti-neutrons could pass through but anti-gravitons couldn't. His response was to change the shape of the envelope. (Star Trek IV: The Voyage Home)