Apparatus for analyzing complex acoustic fields within a duct
First Claim
1. An acoustic measurement system for determining the modal structure of an acoustic pressure field within a duct comprising:
- an acoustic filter including at least one acoustic cavity, said acoustic filter in acoustic communication with said duct to couple sound energy from said duct to each of said acoustic cavities, each of said acoustic cavities being dimensioned and arranged to exhibit resonance in response to a modal component of said acoustic pressure pattern within said duct when the velocity of sound within each of said cavities is established at a predetermined value;
gas supply means for supplying a mixture of gasses to each of said acoustic cavities of said acoustic filter, said gas supply means including means for varying the composition of said gas mixture supplied to said acoustic cavities to selectively tune said filter for resonance at a plurality of modal components of said acoustic pressure field; and
acoustic detector means mounted in acoustic communication with said acoustic filter, said acoustic detector means supplying a signal representative of the acoustic pressure within said acoustic cavities to provide an indication of the amplitude of the modal component of said pressure field which excites said cavities into resonance.
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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.
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Citations
6 Claims
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1. An acoustic measurement system for determining the modal structure of an acoustic pressure field within a duct comprising:
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an acoustic filter including at least one acoustic cavity, said acoustic filter in acoustic communication with said duct to couple sound energy from said duct to each of said acoustic cavities, each of said acoustic cavities being dimensioned and arranged to exhibit resonance in response to a modal component of said acoustic pressure pattern within said duct when the velocity of sound within each of said cavities is established at a predetermined value; gas supply means for supplying a mixture of gasses to each of said acoustic cavities of said acoustic filter, said gas supply means including means for varying the composition of said gas mixture supplied to said acoustic cavities to selectively tune said filter for resonance at a plurality of modal components of said acoustic pressure field; and acoustic detector means mounted in acoustic communication with said acoustic filter, said acoustic detector means supplying a signal representative of the acoustic pressure within said acoustic cavities to provide an indication of the amplitude of the modal component of said pressure field which excites said cavities into resonance. - View Dependent Claims (2, 3, 4, 5, 6)
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Specification