OVERVIEW

As part of an outreach effort for NPRB 1218, I spent three days at Blatchely Middle School in Sitka, AK, (April 22-24) presenting the concepts of array beam forming to five periods of Stacy Golden’s science class. My motivation was to use NPRB 1218 as an opportunity to explain to students some fundamental physics and engineering concepts that extend way beyond underwater acoustics.

CLASSROOM PRESENTATION

One day was spent talking in the classroom and conducting a demonstration on how our ears localize sounds. The presentation is included below, and I used class iPads and headphones to allow five classes of 20-30 students each to listen to the sounds in detail. (The demo stereo sound played on the iPads is on slide 13).

Here is a link to the presentation.

HYDROPHONE DEPLOYMENT

On April 23 I deployed two hydrophones simultaneously in Sitka Harbor, separated by 5 feet, and demonstrated to classes how adjusting the separation between the phones influences one’s ability to determine the direction of various sounds. Below are some sound snippets of various vessels detected and “tracked” in Thompson Harbor. Both hydrophones were deployed at 1 m depth off the side of the floating dock, with “broadside” facing south and right “end fire” facing Sitka Channel. The snippets feed the sound from one hydrophone (the one closer to the harbor parking lot) into the left ear , and the sound from the hydrophone closer to Sitka Channel into the right ear. There is interference from a radio station that can be heard on some samples. The samples have been decimated to 8 kHz, and channels normalized to equal “loudness”.

The Thompson harbor two-element array deployment. The “left” hydrophone cable is visible to the left of the table. The “right” hydrophone is behind the table. Thus, vessels entering or exiting the channel are heard on the “right” audio channel. Some directional hydrophone assemblies are also visible.

A close up of the (relatively antiquated) real-time monitoring and recording setup.

A group of us listening to a vessel go by. The sounds from this vessel would appear to come from the “right” on the recordings.

(1): A seaplane takes off from Sitka Channel, passing from left to right across the array, eventually passing right “end fire”. This sound contains a fine example of harmonics, and how the volume of the propellor increases when the plane passes to the side. The sound attenuates once the plane lifts off the water:

Here is a spectrogram of the data:

Note the harmonics of the plane. The engine rpm times the number of blades yields frequency of the lowest harmonic.

(2): An unknown vessel makes a U-turn while “end fire” to the right of the array and then accelerates when entering the Sitka Sound Channel.

(3): The Thompson Harbor harbormaster passes “broadside” to the array from left to right, revving his outboard motor just as he reaches his closest point of approach:

Here is a spectrogram of the data:

Harmonics from the outboard can be seen, along with cavitation bubbles and interference fringes from the “Lloyd’s Mirror Effect”.

(4): A vessel passes to the right “end fire,” while Blatchley Middle School teacher Stacy Golden bangs a metal rod to the left “end fire” of the array:

Here is a spectrogram of the banging:

Banging a metal rod can actually produce ultrasonic sounds!

(5): The F/V Venus passes “broadside” from left to right, while a Bay boat passes “broadside” from right to left:

Here is a spectrogram:

(6): The F/V Venus starts up its GenSet (generators) to the left “end fire” of the two-element hydrophone array.