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.
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).
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”.
(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:
(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:
(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:
(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.
Here are a selection of photos and figures related to the 2014 field experiment. Photo credits: E. Jimenez, P. Martinez and J.C. Salinas/PRIMMA-UABCS
When not used for long periods, the batteries should be disconnected inside the decoy to prevent full discharging, because if the battery pack voltage falls below 1.5 V, the charger IC board will not permit charging. However, here are some steps to “resuscitate” a dead battery pack. Warning: you will need a 12 V power supply with alligator clips and a screwdriver.
Ideally you should have a power supply like this, set to 12 V output and current limited to 0.5 A.
Remove the lid of the decoy. Make sure you keep track of the six screws and six plastic sleeves for the screws, which tend to fall out when the lid is upside down. Find a box so that the lid can be flipped upside down while remaining connected to the charging board. Check that all connectors are connected–perhaps that is your problem! In particular, the flat 8-wire connector needs to be together for the charger to work.
When storing for long periods, disconnect the two connectors next to the screwdriver (both white and black/red connectors).