enter image description here

On the sine wave above, there are four obvious sections, each a quarter of the length of the wave.

Each half of the wave is (to my knowledge) describable as a pulse. But what I'm looking for is a word to describe the quarter sections. In particular the sections [0, pi/2] and [pi, 3pi/2] in which the wave goes from zero amplitude, to the maximum absolute amplitude it will achieve before going back to zero again.

If no specific word exists, any help coming up with one to use would also be nice.

I did find this page (http://zonalandeducation.com/mstm/physics/waves/partsOfAWave/waveParts.htm) which seems like it would have had the answer, though I did not find one.

  • [0,π/2] would be the increasing concave down portion of the curve, while [π, 3π/2] would be the decreasing concave up portion of the curve. I don't know if there is a single word name, but referring to them by their interval is probably sufficient.
    – jxh
    Jan 23, 2019 at 21:53
  • Unfortunately, I need to be terse with this. I'm going to be using this word quite a bit. I've been toying with the words "expansion" and "contraction" to describe the first and second half of each phase. Assuming zero is fully contracted.
    – Seph Reed
    Jan 23, 2019 at 23:20
  • I think you should ask this question in a forum that specializes in the particular field you are writing about. You need the jargon term used in that field.
    – Hot Licks
    Jan 24, 2019 at 3:45

2 Answers 2


Since the cycles match quadrants in a circle, you could refer to [0, π/2] as the first quarter and [π, 3π/2] as the third quarter.

Alternatively, you could borrow terminology from a four-stroke engine, and refer to the portions of the cycle as intake, and power.

enter image description here

Four-stroke cycle used in gasoline/petrol engines. 1 = Intake, 2 = Compression, 3 = Power, 4 = Exhaust. The right blue side is the intake port and the left brown side is the exhaust port. The cylinder wall is a thin sleeve surrounding the piston head which creates a space for the combustion of fuel and the genesis of mechanical energy.

  • This is really, really neat thinking.
    – Seph Reed
    Jan 24, 2019 at 1:44

This segment is the positive, monotonically increasing quarter period (which also happens to be the first quarter period of the sine wave starting at the origin, which you might term QP1 for maximum brevity).

You can cycle between positive/negative and monotonically increasing/decreasing to unambiguously describe the other three quarter periods.

  • I think that is the right approach. I think that monotonically is unnecessary though. You could just say "positive rising quarter period", "positive falling quarter period", etc. (Also I'm using rising/falling instead of increasing/decreasing just as an alternative.)
    – dangph
    Jan 24, 2019 at 4:07

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