C=C stretching vibrations

The C=C stretch vibration appears between 1670 and 1640. Frequency increases as substituents are added to the double bond.

Monosubstituted ones are about 1640
Disubstituted about 1650
Tri and tetra absorb around 1670

Trans substituents absorb about 10 higher than cis.

The C=C is typically a weak absorption. It may be completely obscured by a shifted C=O stretch. It won't absorb at all if the attached groups are symmetric. Cis alkenes have less symmetry than trans, so will have a more intense absorption, despite being at a lower frequency.

Rings and terminal alkenes typically have high and low symmetry respectively, so we would expect terminal alkenes to have a higher absorption than cyclic alkenes.

Conjugation with another C=C or a C=O will increase single bond character, decreasing the frequency.

Conjugated systems often have two closely spaced peaks, resulting from possible conformations.