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Name: Joanne
Status: student
Grade: 9-12
Country: Canada
Date: Winter 2012-2013


Question:
I am a flautist with the local Youth Symphony Orchestra. I always wondered, how is it possible for a woodwind instrument to reach high notes in the upper octave? Unlike a violin (or other string instruments), since the process is less visual, I have trouble understanding it.

Replies:
It is harder to visualize how a flute works than how a violin works. Maybe this will help:

Imagine a bathtub full of water, with a two-by-four board floating across one end. If you push the board down and up quickly, you will make a little wave that will travel to the far end of the bathtub and be reflected back toward the near end. That reflected wave is really important. It is what allows the flute to select one frequency among all possible frequencies to be the one that people will hear.

Imagine pushing the board down and up repeatedly, at a fixed frequency. If you choose the right frequency, the waves reflected back from the far end will arrive just in time to agree with the position of the board at the time they arrive. The wave reflected back and arriving at the near end will be high at exactly the time that the board is up, and low at exactly the time that the board is down. If you were to achieve this, you would have produced a standing wave on the surface of the water in the tub. The reflected wave would actually help to push the board up at just the time it needed to go up, and you would have the condition physicists call "resonance". You would be "in tune" with the length of the bathtub.

Now imagine putting a cork in the water, right at the position of the board. It will be going up and down right along with the board. Now move that cork to the far end of the bathtub. Again it will be going up and down in time with the board, but its motion will be delayed by the time it takes a wave to travel from the board to the far end of the tub.

Now put the cork in the middle of the tub. It /should/ be going up and down, right? If there were no reflected wave, it /would/ be going up and down. But you have set up a standing wave, so the high of the wave coming from the board is exactly cancelled by the low of the reflected wave, and the cork sits dead still in the water. It is at what physicists call a "node" in the standing wave. The forward and reflected waves cancel each other at this point.

Now suppose you were to move the board up and down twice as quickly as before. Now you will have four nodes, one at a quarter of the distance from the board to the far end, one - as before - right in the middle, and one at three quarters of the distance. Again you will have a standing wave, but one of twice the frequency of the wave you had before.

If you were to slow the board's up and down motion by a little, you would just get a mess. The reflected wave would be out of time with the board's motion, so it would not reinforce the board's motion, but instead act against it. That frequency would only work if you had a little bit longer bathtub. Similarly, if you were to speed up the board's motion by a little bit, you get waves that would only reinforce each other if you had a little bit shorter bathtub.

Alright, enough with bathtubs. Flutes actually have a pressure wave travelling in them, and a reflected pressure wave travelling in the opposite direction. Instead of wave height, you have wave pressure. You already know what a pressure wave is, because that is what you have been listening to all your life. A pressure wave is what we call "sound". If you want to see a pressure wave, you can do it by playing with a spring. A "Slinky" is a particularly nice sort of spring with which to visualize pressure waves, because it has a low enough wave-propagation speed that you can see the wave travelling down it and being reflected back.

Tim Mooney


Joanne

Wikipedia at the following URL:

http://en.wikipedia.org/wiki/Acoustic_resonance

It is a good article that you should look at for more details.

To answer your question, please note: The center acoustic wave is one complete cycle from 0 at the start of the positive part (assigning the convention of: positive part on right, negative to left) to 0 at the end of the negative part of the cycle. That is one complete cycle. The signal on the right shows a one-and-one-half signal (acoustic wave), the signal on the left is one-half of a signal.

On violins, the musician varies the length of the string by pressing their fingers on the fingerboard, effectively shortening the length of the string. A low number of vibration cycles on a string results in a low musical pitch, a high number of vibration cycles on a string results in higher pitches.

Woodwinds produce music as the musician blows air into the air chamber by vibrating the air at resonant frequencies through air chambers, whose lengths are varied by use of the valves on the woodwind instrument.

So, in summary, the resonant frequency the air chamber of the woodwind determines the pitch of the sound that is produced.

Sincere regards, Mike Stewart


Hi Joanne,

Thanks for the question. It is possible for a woodwind instrument to obtain higher notes (called higher harmonics). In fact, the richness of tone is due to these higher harmonics being present in the sound output. In short, the blowing of air into a woodwind instrument sets up vibrations in the air inside the instrument. For example, these vibrations primarily occur at a frequency, say 500 Hertz (Hz). However, there is some 1000 Hz and 1500Hz vibrations occurring too. The 1000 Hz and 1500 Hz vibrations are the higher harmonics.

I hope this helps. Please let me know if you have any additional questions.

Thanks Jeff Grell


Hi Joanne,

For a violin, the base vibration is provided by the hair on the bow sliding over the string or by plucking of the string. The wavelength and the note are determined by the length of the string and the friction. The violinist selects the lengths required by the musical piece by pushing down on the string against the wood and thus the string is shorter. Higher notes are shorter pieces of string and/or greater speed of friction placed on the string via the hair.

For a woodwind, the base vibration is provided for by the pursed lips of the flautist forcing air at a certain speed over an opening. The opening allows for the vibrational constant to be formed and a resonance to take up residence-- we perceive that wavelength as a note. Resonance is determined by the length of the chamber(the tube) as the wave vibrates through it. Higher notes are shorter pieces of resonating chamber and/or greater speed of forced air.

So it is actually a combination of speed, friction and resonant material that the master of the art utilizes to provide the complex resonances that we so much enjoy.

Thank you for a fascinating question! Keep on resonating! Peter E. Hughes, Ph.D. Milford, NH


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