Research on the Acoustic Air-Jet Generator

Abstract

This paper describes the principle design and performance of a new type of static siren operating in both the sonic and ultrasonic ranges. The siren called the “Multiwhistle R. B.” consists essentially of a combination of air-jet whistles operating within an acoustic chamber. With a suitably designed chamber, it is possible to greatly increase the acoustic emission. It is also possible, with a single apparatus, to produce different kinds of acoustic spectrum either simultaneously or successively by exchanging one set of a type of jet whistle for another set or by mixing the various types together. The industrial type of “Multiwhistle R. B.” hereafter described has the advantage of an extremely low maintenance cost and its energy consumption compares favorably with the usual dynamic rotating sirens. For example, a “Multiwhistle R. B.” working in the supersonic range at 33 kc produces an acoustic emission higher than 135 decibels for an air discharge as low as 1.4 g/sec per jet whistle. The “Multiwhistle R. B.” with its exponential horn has an over-all size of 270-mm diameter and is 350 mm high. It can be operated in any position. The emission frequency of this apparatus is quite independent of the power, this last factor being dependent only on the number of whistles in operation. In the investigation of the mechanism of the production of ultrasonic waves it was found possible to produce a high intensity level and a powerful emission whilst operating with low air pressure. The main factors governing the efficiency of this static generator are the characteristics of the jet-whistles and acoustic chamber. Operating with jet-whistle pressures between 1 and 6 kg/cm2 the “Multiwhistle R. B.” produces acoustic waves of frequencies between 3 and 50 kc. In certain cases it is even possible to create maxima complex spectrum both in the sonic and supersonic range, this is of interest for applications such as fog dispersal or industrial aerosol agglomeration.