History of Acoustic Photography

More for fun, we tried first acoustic experiments using simple electret-microphones with adapters to our self-made EEG-amplifier. To check the Interference Transformation (HIT) in acoustic spaces, first we pasted the mic's on the ceiling. Later we used cardboards. The task was, to detect sources of excitement only by inspecting the channel data. First, we had only eight channels.

To boost the selectivity, in the early beginning a neuron-like method was applied using a non-linear (exponential) transfer- (or threshold-) function for the "neurons" - the pixels of the bitmap. To get a non-mirrored image, the first implementation of PSI-Tools only had the ability to calculate reconstructions. (To calculate projections it was necessary to mirror the time axis of channel data before the calculation of an image.)

In this years beamforming (for ultrasonic imaging, Radar, Sonar etc.) used delay elements in hardware. My idea was, to demonstrate, that faster computers in future allow more precise software solutions. Software has a much better ability and higher accuracy to handle delays between each microphone and each pixle. With hardware delays, the delays could not be accurate for each pixle. Only a simple approximation - a "beam-forming" was practical known. With software, a new, pixle-precise way to calculate interference images - my "interference transformation" came in sight. This way, the image pixel size could be encreased in a variable ways - depending only of calculation times. And the space properties could now vary between infinite different cases, especcially for 3-dimensional sources.

For a 400 by 400 pixle interference image (integration time unknown) a Intel-PC needed 17 hours, see project report Neuro3d. So the hope in 1993 was, that the Intel-PCs are fast enough if we finish the project in some years. We started software development on a 16-node Parsytec Power'Xplorer. In addition I used a ISA-bus 8-node transputer card T805 in my PC.

Indoor Experiments

Using 8 Microphones

First acoustic image from August 23, 1994 (Gerd Heinz, software: Sabine Höfs). A single impulse was applied parallel to eight loudspeakers. For the record a sample rate of 20 kS per channel was used. The record was done in a room without any sound-absorbing materials. Microphones paste directly at the ceiling in a height of 2.60 meter, speakers were faced direct on the floor. The image showed not more but the possibility, to start further works in the field of interference transformation and acoustic photography.

Eight channel record of a single loudspeaker using a 20 kHz sample rate again. The speaker got originally a single dirac-impulse.

Reconstruction of the excitement map of a single speaker. The speaker layed central to the 8 microphones (August 17, 1995).

Using 16 Microphones

Channel data of the stereo-radio. 16 channel microphon array.

Arrangement of microphones and two speakers with the detected layer. The microphone array is located in a distance of 2.4 meter to the speakers. The microphones paste directly at the wall to avoid short reflections.

Interference integral with a reconstrution of 16 microphones channel data. Reconstruction layer is located exactly in the distance of the loudspeakers. A closer look behind the arrangement shows, that small errors occured reconstructing the generator space. Black circles show the theoretical position of the speakers. It is possible, that the used cheep electret microphons had some parameter errors
(march 15th, 1996)

First Acoustic Movies

In this first movies the excitements came from very short, very high interferences. (In pseudo-wavefield movies it is in mostly not possible for the eyes to find the source locations.)

Parameters: v = 333m/s, picture_rate = sample_rate = 50 kHz, integration_interval = 1 sample (no integration)