Friday, April 9, 2010

What do our source and a trombone have in common?

The picture below shows one of our acoustic sources being moved into the warehouse in Kaohsiung. This type of source is similar to a trombone. To change the pitch on the trombone, the trombonist moves the slide in or out, which changes the length of the instrument and alters the pitch (frequency). To change the pitch our source is playing, we adjust its length by moving part of the black tube.

Our source produces sound in the band of frequencies between 225 and 325 Hz. For reference, 261 Hz is the C below middle C on the Western musical scale. A baritone singer could easily sing our source signal (though we haven't found any singers eager to be lowered to a depth of 1000 meters in the ocean just to sing for a year!).

Our source plays a very simple type of "music". The figure below shows a picture of the signal we recorded during one of our source tests this week. If you click on the picture, you'll hear what the source sounds like.


The source signal is the humming sound that increases in pitch over time. The picture below (called a spectrogram) shows how the frequency of the source signal changes with time. The source signal corresponds to the dark red line. From this picture you can see that the source starts at a frequency of 225 Hz and slowly increases to 325 Hz over a period of 135 seconds.

While this signal may not make any of the Top 40 music charts, it is very useful to acoustic tomographers. We'll show you more about this in a later post when we discuss how signal processors (like Kathleen) earn their paychecks!