The next step was to use these formulas to design everything from the softest flutes to whistles which resembled those on the old steam locomotives. The result was that it was possible to design whistles of high output which did not require the customary high pressures associated with steam locomotives and steamboats. I was able to design whistles which resembled steam calliope whistles which would produce outputs as high as 110 dB at 100 ft (140 dB at 1 meter) operating on as little as 15 PSI, rather than the usual 150 PSI.
I found that the acoustical output of a whistle is much more dependant upon the flow rate rather than the actual operating pressure, thus if you use a relatively large air slot and large diameter inlet, you could get the required flow rate at relatively low pressure. Thus, the operating efficiency of a whistle could be greatly increased over that of the traditional high pressure steam whistle.
It makes sense! The whistles designed to be used on locomotives and steamboats HAD to be designed to accommodate the high pressures, since the engines required high pressure in order to be able to produce a useful amount of output. The whistles did not, but would overblow unless they were designed to accommodate the high pressure.