guitars are easily
recognized buy their large hollow resonating chamber. This
chamber is a key part of the acoustic guitar because it is how the
guitar amplifies the sound making it easier to hear. Unlike
electric guitars, acoustic guitars use no magnets or electronics to
help in creating their full-bodied sound so they rely on the properties
waves and the design of the guitar.
The sound properties of acoustic guitars can get very complicated so I'll try my best to explain the basics. If you are interested in learning more there are a few excellent web sites dedicated to explaining the nitty gritty of resonating chambers (http://www.phys.unsw.edu.au/music/guitaracoustics/modes.html). The strings on a guitar are connected to the tuning pins on the head and on the bridge plate on the resonating chamber; when a string vibrates it also causes the bridge plate to vibrate. As the bridge plate vibrates, it forces the top board on the guitar to vibrate also, then the top board makes the air inside vibrate. When the air starts to vibrate it exits out the hole under the strings causing a louder sound because more air particles are vibrating compared to how many vibrate from the the string alone. This increase in vibrating air causes a louder sound. The way the waves travel inside the resonating chamber also help amplify the sound.
Image from http://www.phys.unsw.edu.au/music/guitar/anatomy.html
As a surface vibrates it creates nodes, these are places where there are no vibrations. As shown in the picture below the black rod is vibrating back and forth from the blue dotted line. The red spots show places where the rod does not move.
The top boards on guitars also have these nodes (many pictures of these can be seen at: http://www.phys.unsw.edu.au/music/guitar/patterns_engl.html, the black lines are black sand that was sprinkled on the guitar before it was vibrated. The sand then settled in the nodes. These pictures show just how variant and complicated the sound in the resonating chamber can be). These nodes are useful when tuning a finished guitar so the guitar can be tuned to be most efficient. By being tuned I'm talking about the guitar itself, not the strings. When a guitar is tuned it makes the guitar vibrate at the same frequency as the strings so they are not working against each other, this will let all the energy go towards amplifying the sound waves and not fighting against different frequencies.
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