These transmission loss groupings effectively define the expected level of ambient noise for fully developed turbulent flow, apart from other in-pipe noise or structural borne noise generation. Turbulent flow noise will propagate with the velocity of the fluid and will decay. Alternatively, acoustic fields and noise created from acoustic resonances will propagate at the speed of sound and decay less with distance, compared to turbulent flow noise.
Other guidelines such as the International Electrotechnical Commission (IEC) for control valve noise have shown a dependency on the cutoff frequency and first coincidence pipe frequency. The IEC Technical Monograph 41 describes a transmission loss model where very low frequencies below the cutoff frequency are not even considered due to high transmission loss. This is contradictory to some of the data taken in the GMRC research at reciprocating compressor stations where a significant contribution is made by the low acoustic excitation of wavelengths in the axial direction of the pipe. However, the IEC technical monograph also shows a low point for transmission loss (high noise transmission) around the first coincidence of the pipe frequency. Control valve noise is typically above the ring frequency of the pipe and assumed to be the most significant noise source, calculated in the IEC standard by the strouhal correlation and valve diameter.