Compressible gas flows tend to have much “louder” piping systems, due to the high potential for broadband noise and the relatively good conversion of mechanical to acoustic energy. This relationship between the cutoff frequency and the ring frequency effectively defines a frequency band of high noise transmission, where the transmission loss through the pipe will be relatively low.
The Norton method of 1997 recommends two equations for the transmission loss, above and below the pipe ring frequency. These equations are provided below as Equation 2-3 and 2-4. The Norton method does not show a low point for high noise transmission around the cutoff frequency, but the severe negative slope for low frequencies is still consistent with the NASA guideline. The NASA model and the Norton method are illustrated in Figure 2-3 for a typical 10-inch diameter steel pipe.