Northrop Grumman Systems Corporation

Jonathon J Linch

Scott R Sorbel

Kyle M Rahrig

An exemplary apparatus attenuates sound in a frequency range, e.g. audible frequencies. A resilient planar membrane has first and second honeycombs with unit cells attached to opposing sides of the planar membrane. Each unit cell has rigid walls extending perpendicular to the planar membrane. Buttresses are mounted inside the unit cells where dimensions of the buttresses result in an alteration of an attenuation versus frequency characteristic in the audible or other frequencies relative to an attenuation versus frequency characteristic without the buttresses.

Honeycomb unit cell acoustic metamaterial with in situ buttresses for tuned acoustic frequency attenuation

A propeller includes a hub coaxially surrounding a longitudinal axis. A ring shroud coaxially surrounds the longitudinal axis and is spaced radially from the hub. At least one propeller blade is fixedly attached to both the hub and ring shroud and extends radially therebetween for mutual rotation therewith. At least one stub blade has a first stub end radially spaced from a second stub end. The first stub end is fixedly attached to a selected one of the hub and ring shroud. The second stub end is cantilevered from the first stub end and is radially interposed between the first stub end and the selected one of the hub and ring shroud.

Propeller

An exemplary apparatus minimizes an aircraft propeller noise having an associated frequency and a corresponding wavelength. A resonant tone produced on the propeller blade elicits a sound with substantially the same frequency as the associated frequency of the offending source. The resonant tone is powered by an inflow of air in or across an aperture of a resonant structure during the rotation of the blade and the aperture has an opening from which the sound is emitted. The resonant structure is dimensioned to generate a sound with substantially the same frequency as the offending source. The aperture is located on the blade with the opening a distance from a target origin of the noise that is substantially one half of the wavelength of the noise so that the sound arrives at the target origin of the noise substantially 180 degrees out of phase with the noise.

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Los Angeles, CA, United States of America

Jonathon J Linch