In one company, an engine test cell had been generating high noise levels in maintenance areas near the exhaust discharge. The engine exhaust stack ran from ground level to the test cell roof where it ended in a pair of reactive and absorptive silencers.
Preliminary measurements revealed that the strongest tonal components appeared at 289 Hz with a smaller peak at 144 Hz. The major noise frequency was found to correspond with the engine firing frequency. For the straight six cylinder engine under test, at 5800 rpm, the fundamental firing frequency could be calculated at 290 Hz, with a subharmonic at 145 Hz.
As the tone fell between the optimum performance of the reactive and absorptive silencers, a custom-designed silencer was required. A tuned side-branch (Helmholtz) resonator, 'teed' off the main outlet, was chosen. The length of the side branch (29 cm) was one quarter of the wavelength at the critical frequency. The length of the silencer could be changed in situ to compensate either for manufacturing tolerances or for any slight shifts in the tonal frequency.
The silencer was made in-house from a section of 3 in (76 mm) mild steel steam pipe. The internal surface of the pipe was left plain as a particular tone was to be reduced - it was expected that an absorbent lining would have broadened the range of the silencer's performance, but lowered its attenuation at the specific frequency of interest.
The trace shows the frequency analysis of the exhaust noise before and after the addition of the silencer, measured close to the exhaust.
Information supplied by Coventry City Council and Brico Engineering Limited.