1. Acoustic Holography
Sean Douglass

2. Traditional Acoustic Imaging
If medium is opaque, light cannot be used
Ultrasound yields somewhat blurry 2D images
Sonar is great for detecting presence and location, not as great for determining identity/details of an object
Top: airplane on ocean floor
Bottom: detecting schools of herring from a fishing boat
Acoustic holography fills the gaps

3. Optical holography
Reference beam and object beam create interference fringes on holographic film
Illuminating film with laser creates a 3D image
Changing angle of observation changes the angle of the object

4. Conventional Acoustic Holography
Borrows principles of optical holography
Monofrequency sound pressure field is used as the wave
Array of microphones (or one microphone scanning a plane) is used as the hologram surface
Reference pressure waves from tweeter interfere with reflected object waves at the plane of the microphone array
Diffraction pattern is created from interference, pressure intensity mapped onto photographic film (brighter spot = higher intensity)
Laser used to illuminate photographic film, and a hologram is visible
Image can be reconstructed from hologram on another photographic plate

5. Conventional Acoustic Holography

6. Conventional Acoustic Holography Difficulties
Principles taken straight from optical holography → same constraints exist
Resolution can be no greater than the wavelength
Measurements taken in the farfield, exponentially decaying waves not measured
Objects may emit wavelengths longer than the crucial details of the object itself
Monofrequency source used → monochromatic image created
Pressure amplitude measured → only pressure amplitude field can be recreated
Very time consuming – one measurement may take hours
These issues can be resolved with generalized holography (theory) and nearfield acoustic holography, or NAH (practice)

7. Generalized Holography, NAH
If evanescent waves are also measured, much more data is available
Complicated math (Fourier transforms, Green’s function, boundary conditions, etc.) allow the unique determination of the acoustic field ψ emitted by the object of interest
Pressure field, acoustic vector intensity field, directivity, particle velocity, field gradient, and total power radiated can all be determined
NAH uses an array of microphones to capture decaying wave information
Object which radiates sound is placed directly
underneath microphone grid, signal of each
microphone is processed to obtain results

8. NAH Results Surface velocity of (4,2) vibrating plate
Noise source identification
Noise transmission localization/quantification
Imaging vibrational modes of a guitar
NAH vs conventional AH acoustic intensity for two closely spaced point sources

9. Further Developments
Using arbitrary images to compute phase map and create hologram

10. Bibliography
A. Metherell, H. El-Sum, J. Dreher, L. Lamore. Introduction to Acoustical Holography. The Journal of the Acoustical Society of America, issue 42 (1967).
J. Maynard, E. Williams, Y. Lee. Nearfield acoustic holography: I. Theory of generalized holography and the development of NAH. The Journal of the Acoustical Society of America, issue 78 (1987).
R. MacAnally, C. Yeh. Acoustic Imaging by Holography. The Office of Naval Research (1969).