Average shock spacing and average standing wavelength are estimated from computed near-field of axisymmetric underexpanded screeching jets at low supersonic Mach numbers. These are used to predict screech frequencies based on existing formulations which rely either on shock spacing or on standing wavelength. Results obtained suggest that shock spacing and standing wavelength may respectively be dominant length scales for A1 and A2 axisymmetric screech modes observed for axisymmetric jets at low supersonic Mach numbers. This implies that presence of two axisymmetric screech modes in supersonic jet screech could be attributed to the two co-existing length scales in the jet near-field.
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