Effect of exit spacing in a multiple-jet nozzle on noise levels at audible frequencies

This work investigated the effect of exit spacing on audible noise levels generated by a multiple-jet nozzle, which replaces the single exit in a common nozzle with multiple smaller exits to reduce the risk of hearing damage. Sound measurements and 1/3-octave band analyses were performed for multiple-jet nozzles with varying exit diameters and exit spacings. Results show that the effect of exit spacing is evident below a certain frequency range, which falls in the relatively low frequency part of the sound spectrum. The frequency range affected by exit spacing was found to extend to higher frequencies if the exits were more closely distributed. It was found that decreasing the exit spacing will shift the sound spectrum toward the low frequency side, which has been shifted toward the high frequency side by the use of smaller exits, and will increase the sound pressure levels at lower frequencies. When the exits were confined in a limited area, to maintain a constant total exit area, decreasing the exit diameters will increase the number of exits and reduce the exit spacing. Smaller exit diameters will increase the sound frequency and shift the sound spectrum toward the high frequency side, while decreased exit spacing has the opposite effect of shifting the spectrum toward the low frequency side. Due to these two opposing effects, for a multiple-jet nozzle with a limited size, there is an optimum exit diameter that, depending on the method used to evaluate the overall sound pressure level, generates the lowest sound pressure levels in the audible frequency range. If there is no limit to its physical size, a multiple-jet nozzle should avoid using an exit spacing that is too small. The minimum exit spacing required to avoid the adverse effects of a small exit spacing requires more extensive studies.