Pipe Instruments (Ex. Flutes, trumpets, pipe organs) If we have a tube with sound waves going through it we can also set up a standing wave as well. There are two types of pipes used musically. open pipes (both ends are open, and antinodes). closed pipes (one end is open [antinode] the other is closed [node]). If both ends are closed it can only be struck not blown into.
Pipe instruments Closed pipe Open Pipes Fundamental Fundamental Antinode Antinode Again for each harmonic just multiply by the number of the harmonic Node Antinode Node fundamental f = v/(4L) fundamental f = v/(2L) *v is the speed of sound in air = 331.5 + .6C
Closed pipes only have the odd harmonics present Each harmonic would double, triple, quadruple etc. of the fundamental Fundamental 3rd harmonic 5th harmonic 2nd harmonic 4th harmonic An open end will not support a node
Changing the notes on pipes Same with a string, you can either change the length of the pipe or force nodes at certain points. Trombone is the most obvious at changing the length of the pipe. Pipes normally come with keys. These keys can increase the length of the pipe (they give the air an alternate path to follow). Some keys force nodes at certain points.
Sound waves interfere with each other the same as other waves. You can either get constructive or destructive interference. Constructive interference is commonly used in megaphones (not electric type) Most sound waves are forced the same direction and they all constructively interfere with one another. That gives you a larger amplitude and volume.
Destructive interference Different uses are becoming common in very noisy environments. Canceling out very complex sounds (like speech) is very difficult. However if the same frequency is constantly heard it is very easy. The same frequency is pretty much always heard from most engines. So pilots normally wear noise canceling earphones.
Interference does not have to be permanent Waves go right through each other unaffected, by the collision. At the point the two waves meet they will be canceled or amplified, but all other points they will return to normal. The only way it lasts is if the waves are going in the same direction.
Beats If the waves are only slightly off one another you will hear destructive then constructive then destructive etc. This fluctuation is called beats. # of beats = f1 – f2