Measuring and testing – Vibration – By mechanical waves
Patent
1978-01-03
1979-03-20
Kreitman, Stephen A.
Measuring and testing
Vibration
By mechanical waves
73649, G01N 2902
Patent
active
041447689
ABSTRACT:
A system for resolving a complex acoustic field, resulting when acoustic energy propagates through a duct, into component unit spatial pressure patterns or modes is disclosed. In each disclosed system arrangement, an acoustic filter, which includes one or more annular cavities, circumferentially surrounds the duct through which the acoustic energy propagates and sound is coupled from the duct into one end of each annular filter cavity via a series of openings which exhibit a relatively high acoustic impedance and a relatively high resistance to fluid flow. Each filter cavity is dimensioned such that it is excited into resonance when the circumferential distance between adjacent nodes of a modal component of the pressure pattern within the duct is mathematically related to the velocity of sound within a filter cavity by a predetermined factor. When a cavity resonates, maximum acoustic pressure transfer occurs between the duct and two or more microphones that are circumferentially spaced apart from one another along the second end of each annular filter cavity. A gas mixture is introduced into the annular cavities to control the velocity of sound within the cavities and the composition of the gas is controlled such that cavity resonance occurs at each pressure mode of interest. Various measurements are facilitated by the system such as slowly varying the composition of the gas and processing the microphone signals to directly obtain the modal amplitude distribution of a relatively time invariant sound field or adjusting the gas composition for cavity resonance at each mode of interest and recording transient phenomena of the acoustic energy propagating through the duct.
REFERENCES:
patent: 1528586 (1925-03-01), Tate
patent: 3009104 (1961-11-01), Brown
patent: 3434334 (1969-03-01), Vandenbussche
patent: 3533485 (1970-10-01), Buffum et al.
patent: 3781782 (1973-12-01), Scott et al.
patent: 3824842 (1974-07-01), Wiat et al.
Andersson Anders O.
Purves Robert B.
Kreitman Stephen A.
The Boeing Company
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