51_columns - Lesson 51: Resonating Air Columns Many musical...

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Lesson 51: Resonating Air Columns Many musical instruments depend on the musician in some way moving air through the instrument. This includes brass and woodwind instruments, as well as instruments like pipe organs. The air that is moving through these columns (also called pipes or tubes ) resonates as it passes through, creating the sound of the notes we hear. The column can be any tube, even if it has been bent into different shapes or has holes cut into it. All instruments like this can be divided into two categories, open ended or closed ended . An open ended instrument has both ends open to the air. An example would be an instrument like a trumpet. You blow in through one end and the sound comes out the other end of the pipe. The keys on the trumpet allow the air to move through the "pipe" in different ways so that different notes can be played. A closed ended instrument has one end closed off, and the other end open. An example would be an instrument like some organ pipes (although in some designs they are open), or a flute. Although you blow in through the mouth piece of a flute, the opening you’re blowing into isn’t at the end of the column, it’s along the side of the flute. The end of the column is closed off near the mouth piece. The frequencies of sounds made by these two types of instruments are different because of the different ways that air will move at a closed or open end of the column. The diagrams that I will be drawing are based on the way the air will move as a wave. Although the sound waves actually travel through the columns as longitudinal waves, I will be drawing transverse waves. This is just because they are easier to draw and recognize in the diagrams. Identifying Fractions of Wavelengths Before we look at the diagrams of the columns, let’s make sure that you know what fractions of a wave look like. This will be important in the way you interpret the diagrams later. 8/16/2007 © studyphysics.ca Page 1 of 8 / Section 8.3 Illustration 1: A trumpet is open ended. Illustration 2: A flute is closed ended.
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8/16/2007 © studyphysics.ca Page 2 of 8 / Section 8.3 Illustration 3: One wavelength (λ). Illustration 4: Three quarters of a wavelength (3/4 λ) Illustration 5: One half of a wavelength (1/2 λ).
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These wave fractions might appear upside down, flipped over, turned around, etc. in the diagrams you will see, but they will still represent the same portions of a wave. When we are talking about the sounds that columns can make, what we are really concerned with is how much of the wave we can fit into the column. Different amounts of a wavelength in a column will result in a different frequency being heard.
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This note was uploaded on 12/02/2011 for the course PHYSICS 235 taught by Professor Staff during the Fall '08 term at Rutgers.

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51_columns - Lesson 51: Resonating Air Columns Many musical...

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