SITIS Archives - Topic Details
Program:  SBIR
Topic Num:  AF071-023 (AirForce)
Title:  Near-field acoustic holography system
Research & Technical Areas:  Air Platform, Space Platforms, Human Systems

  Objective:  Design, develop, and demonstrate a practical 3-dimensional acoustic holography system capable of measuring and describing jet plumes from full scale high performance military jet aircraft engines and high efficiency commercial/transport jet engines.
  Description:  The acoustic source characteristics of jet plumes from high performance military fighter engines as well as jet plumes from high bypass commercial transport engines are not well defined. This is due to the difficulty in making a complete set of descriptive acoustic measurements characterizing the size, intensity, directivity, and distribution of the acoustic source/jet plume . High performance military engines may have an apparent time varying distributed acoustic source which is at least 40’ long and 15’ high and wide. Acoustic near-field acoustic holography concepts have been proposed but have not been practically realized for full-scale jet engines. A 3-D microphone measurement array to fully characterize the acoustic near-field around the aircraft and jet plume would require thousands of individual microphones. The concept of the near-field acoustic holography system is to be able to measure and characterize the 3-D acoustic near-field with a reasonable number (<150) of microphones and innovative signal processing/signature analysis techniques implemented in software. The technical challenges include identifying the acoustic source frequency dependent spatial distribution, identifying the time varying acoustic shock structure in the plume, collecting accurate acoustic data with high ambient temperatures and wind, and identifying and quantifying the major acoustic radiation angles and strength. The current state-of-the-art is near-field acoustic holography of cold, model scale (1”-2” throat diameter) jets in laboratory conditions. The full scale near-field acoustic holography system should be at least semi-portable, i.e. be capable of being set up at different air field test sites with not more than 1-2 days set up time by not more than 3 technicians. The system should be able to handle overall sound pressure levels up to 160 dB. The system should have a calibrated bandwidth from at least 5 Hz to not less than 30 kHz. The data from the near-field acoustic holography system will be instrumental in conceiving and defining jet source noise reduction technologies and quantifying their noise performance benefits for both high performance military jet engines and high efficiency commercial jet engines.

  PHASE I: The Phase I products should include as a minimum a hardware, software, and analysis design for a full scale acoustic holography system with data display concept. Risk areas should be defined and potential risk reduction experiments/demonstrations should be conducted and reported.
  
  PHASE II: The Phase II products should include as a minimum a working integrated near-field acoustic holography system with hardware and software, calibration procedures and data, and a demonstration of a measurement and analysis of a military high performance aircraft/engine 3-D acoustic near-field.

  DUAL USE COMMERCIALIZATION: Military application: Applications include personnel noise field and noise exposure calculations and identification and performance measurement of possible techniques for jet noise reduction. Commercial application: Commercial applications include identifying techniques for jet noise reduction from commercial jet aircraft engines and defining noise hazard areas for commercial jet aircraft ground operations.

  References:  1. "Nearfield Acoustic Holography (NAH) I - Theory of Generalized Holography and the Development of NAH," by JD Maynard, E.G. Williams and Y. Lee. 2. "Nearfield Acoustic Holography (NAH) II - Holographic Reconstruction Algorithms and Computer Implementation," by W.A. Veronesi and J.D. Maynard.

Keywords:  acoustics, near-field noise, near-field acoustic holography, noise, noise exposure

Additional Information, Corrections, References, etc:
Ref #1: The Journal of the Acoustical Society of America, Volume 78, Issue 4, October 1985, pp.1395-1413
Available through online document delivery services.
Ref #1: The Journal of the Acoustical Society of America, Volume 78, Issue 4, October 1985, pp.1395-1413
Available through online document delivery services.
Ref #2: Acoustical Society of America, Journal (ISSN 0001-4966), vol. 81, May 1987, p. 1307-1322.
Available through online document delivery services.
Ref #2: Acoustical Society of America, Journal (ISSN 0001-4966), vol. 81, May 1987, p. 1307-1322.
Available through online document delivery services.

Questions and Answers:
Q: Any requirements about the spatial resolution and measurement accuracy?
A: The goals for the near-field acousstic holograph system are as follows:
Spatial Resolution - 3 ft
Angular Resolution - 5 degrees
Measurement Accuratcy +- 1 dB

These are meant to be ambitious goals. The requirements are as follows:
Spatial Resolution - 10 ft
Angular Resolution - 30 degrees
Measurement Accuracy +-3 dB
Q: Any requirements about the spatial resolution and measurement accuracy?
A: The goals for the near-field acousstic holograph system are as follows:
Spatial Resolution - 3 ft
Angular Resolution - 5 degrees
Measurement Accuratcy +- 1 dB

These are meant to be ambitious goals. The requirements are as follows:
Spatial Resolution - 10 ft
Angular Resolution - 30 degrees
Measurement Accuracy +-3 dB

Record: of